Kyushu University Academic Staff Educational and Research Activities Database
List of Papers
Onda Ken Last modified date:2018.06.19

Professor / Department of Chemistry / Faculty of Sciences


Papers
1. Kazuhide Koike, David C. Grills, Yusuke Tamaki, Etsuko Fujita, Kei Okubo, Yasuomi Yamazaki, Masaki Saigo, Tatsuhiko Mukuta, Ken Onda, Osamu Ishitani, Investigation of excited state, reductive quenching, and intramolecular electron transfer of Ru(II)-Re(i) supramolecular photocatalysts for CO2 reduction using time-resolved IR measurements, Chemical Science, https://doi.org/10.1039/c7sc05338j, 9, 11, 2961-2974, 2018.01, Supramolecular photocatalysts in which Ru(ii) photosensitizer and Re(i) catalyst units are connected to each other by an ethylene linker are among the best known, most effective and durable photocatalytic systems for CO2 reduction. In this paper we report, for the first time, time-resolved infrared (TRIR) spectra of three of these binuclear complexes to uncover why the catalysts function so efficiently. Selective excitation of the Ru unit with a 532 nm laser pulse induces slow intramolecular electron transfer from the 3MLCT excited state of the Ru unit to the Re unit, with rate constants of (1.0-1.1) × 104 s-1 as a major component and (3.5-4.3) × 106 s-1 as a minor component, in acetonitrile. The produced charge-separated state has a long lifetime, with charge recombination rate constants of only (6.5-8.4) × 104 s-1. Thus, although it has a large driving force (-ΔG0CR ∼ 2.6 eV), this process is in the Marcus inverted region. On the other hand, in the presence of 1-benzyl-1,4-dihydronicotinamide (BNAH), reductive quenching of the excited Ru unit proceeds much faster (kq[BNAH (0.2 M)] = (3.5-3.8) × 106 s-1) than the abovementioned intramolecular oxidative quenching, producing the one-electron-reduced species (OERS) of the Ru unit. Nanosecond TRIR data clearly show that intramolecular electron transfer from the OERS of the Ru unit to the Re unit (kET > 2 × 107 s-1) is much faster than from the excited state of the Ru unit, and that it is also faster than the reductive quenching process of the excited Ru unit by BNAH. To measure the exact value of kET, picosecond TRIR spectroscopy and a stronger reductant were used. Thus, in the case of the binuclear complex with tri(p-fluorophenyl)phosphine ligands (RuRe(FPh)), for which intramolecular electron transfer is expected to be the fastest among the three binuclear complexes, in the presence of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH), kET was measured as kET = (1.4 ± 0.1) × 109 s-1. This clearly shows that intramolecular electron transfer in these RuRe binuclear supramolecular photocatalysts is not the rate-determining process in the photocatalytic reduction of CO2, which is one of the main reasons why they work so efficiently..
2. Masaki Hada, Shohei Saito, Sei'Ichi Tanaka, Ryuma Sato, Masahiko Yoshimura, Kazuhiro Mouri, Kyohei Matsuo, Shigehiro Yamaguchi, Mitsuo Hara, Yasuhiko Hayashi, Fynn Röhricht, Rainer Herges, Yasuteru Shigeta, Ken Onda, R. J.Dwayne Miller, Structural Monitoring of the Onset of Excited-State Aromaticity in a Liquid Crystal Phase, Journal of the American Chemical Society, https://doi.org/10.1021/jacs.7b08021, 139, 44, 15792-15800, 2017.11, Aromaticity of photoexcited molecules is an important concept in organic chemistry. Its theory, Baird's rule for triplet aromaticity since 1972 gives the rationale of photoinduced conformational changes and photochemical reactivities of cyclic π-conjugated systems. However, it is still challenging to monitor the dynamic structural change induced by the excited-state aromaticity, particularly in condensed materials. Here we report direct structural observation of a molecular motion and a subsequent packing deformation accompanied by the excited-state aromaticity. Photoactive liquid crystal (LC) molecules featuring a π-expanded cyclooctatetraene core unit are orientationally ordered but loosely packed in a columnar LC phase, and therefore a photoinduced conformational planarization by the excited-state aromaticity has been successfully observed by time-resolved electron diffractometry and vibrational spectroscopy. The structural change took place in the vicinity of excited molecules, producing a twisted stacking structure. A nanoscale torque driven by the excited-state aromaticity can be used as the working mechanism of new photoresponsive materials..
3. Keiki Fukumoto, Mohamed Boutchich, Hakim Arezki, Ken Sakurai, Daniela Di Felice, Yannick J. Dappe, Ken Onda, Shin ya Koshihara, Ultrafast electron dynamics in twisted graphene by femtosecond photoemission electron microscopy, Carbon, https://doi.org/10.1016/j.carbon.2017.08.032, 124, 49-56, 2017.11, Twisted multilayer graphene (tMLG) present electronic properties that depend on the relative misalignment and interaction between layers. These interactions affect the band structures and the carrier dynamics upon photonic excitation. These structures are being under scrutiny and recent work high-lighted the strong potential they offer for optoelectronic devices. However, the ultrafast carrier dynamics is still at an early stage, often due to the instrumental limitations. Here, we investigated the carrier dynamics by femtosecond photoemission electron microscopy of chemical vapor deposited (CVD) twisted graphene super lattices presenting different interlayer rotation angles. The photo-generated carrier lifetimes in these selected regions show a longer lifetime compared to monolayer graphene (1 ML). This observation is assigned to the presence of band gap and sub bands in the trilayer graphene and has been supported by DFT calculations..
4. Tatsuhiko Mukuta, Peter V. Simpson, Jamila G. Vaughan, Brian W. Skelton, Stefano Stagni, Massimiliano Massi, Kazuhide Koike, Osamu Ishitani, Ken Onda, Photochemical Processes in a Rhenium(I) Tricarbonyl N-Heterocyclic Carbene Complex Studied by Time-Resolved Measurements, Inorganic Chemistry, https://doi.org/10.1021/acs.inorgchem.6b02936, 56, 6, 3404-3413, 2017.03, We carried out time-resolved infrared (TR-IR) and emission lifetime measurements on a Re(I) carbonyl complex having an N-heterocyclic carbene ligand, namely, fac-[Re(CO)3(PyImPh)Br], under photochemically reactive (in solution in acetonitrile) and nonreactive (in solution in dichloromethane) conditions to investigate the mechanism of photochemical ligand substitution reactions. The TR-IR measurements revealed that no reaction occurs on a picosecond time scale and the cationic product, namely, fac-[Re(CO)3(PyImPh)(MeCN)]+, is produced on a nanosecond time scale only in solution in acetonitrile, which indicates that the reaction proceeds thermally from the excited state. Because no other products were observed by TR-IR, we concluded that this cationic product is an intermediate species for further reactions. The measurements of the temperature-dependent emission lifetime and analysis using transition-state theory revealed that the photochemical substitution reaction proceeds from a metal-to-ligand charge transfer excited state, the structure of which allows the potential coordination of a solvent molecule. Thus, the coordinating capacity of the solvent determines whether the reaction proceeds or not. This mechanism is different from those of photochemical reactions of other types of Re(I) carbonyl complexes owing to the unique characteristics of the carbene ligand..
5. Mitsuyuki Kaneta, Takumu Honda, Ken Onda, Mina Han, Repeated photoswitching performance of azobenzenes adsorbed on gold surfaces
a balance between space, intermolecular interactions, and phase separation, New Journal of Chemistry, https://doi.org/10.1039/c6nj03121h, 41, 4, 1827-1833, 2017.01, Single-component azobenzene thiol (Me-SH) and two-component azobenzene dodecyl disulfide (Me-SS) have been used to understand how “space” necessary for trans ↔ cis photoisomerization, “intermolecular interactions” and “phase separation” affects the repeated photoswitching ability of azobenzene units on flat gold surfaces. Me-SH self-assembled monolayers (SAMs) with available space and π-π stacking interactions between photoswitchable azobenzene units showed ∼51% of the first trans-to-cis photoisomerization and good photoswitching performance in response to alternating irradiation with UV and visible light. On the other hand, our absorption spectral data and AFM observations suggest that each chemical component of Me-SS did not move continuously on a flat gold surface and was almost fixed after partially phase-separated SAM formation. The first trans-to-cis photoconversion of the Me-SS SAMs with additional free space arising from phase separation reached as high as 75 ± 3%. Repeated irradiation of the Me-SS SAMs with UV and visible light seems to cause (i) out-of-plane (homeotropic and/or oblique) reorientation and/or (ii) random arrangements of mixed trans- and cis-azobenzenes. Consequently, the initial excellent photoswitching ability declined gradually as the number of repeated irradiations increased. The observations indicate that a balance between space, intermolecular interactions, and phase separation plays an important role in forming repeatedly photoswitchable monolayers..
6. Sei'ichi Tanaka, Yoshitaka Matsubara, Tsuyoshi Asatani, Tatsuki Morimoto, Osamu Ishitani, Ken Onda, Structural deformation of a ring-shaped Re(I) diimine dinuclear complex in the excited state, Chemical Physics Letters, https://doi.org/10.1016/j.cplett.2016.09.006, 662, 120-126, 2016.10, We investigated the excited state of a ring-shaped dinuclear Re(I) diimine bis-carbonyl complex that shows promise with regard to efficient photosensitized reactions and multi-electron storage, using time-resolved infrared (TR-IR) vibrational spectroscopy and quantum chemical calculations. Anomalous peaks in the CO stretching region of the TR-IR spectrum were well reproduced by the calculations. It was found that metal-to-ligand charge transfer occurs only in one of the Re complex units, and that the excited state geometry is deformed relative to the ground state in order to relax the associated steric tension. This structural deformation generates the unique photophysical properties of the complex..
7. Masaki Hada, Katsura Norimatsu, Sei′ichi Tanaka, Sercan Keskin, Tetsuya Tsuruta, Kyushiro Igarashi, Tadahiko Ishikawa, Yosuke Kayanuma, R. J.Dwayne Miller, Ken Onda, Takao Sasagawa, Shin Ya Koshihara, Kazutaka G. Nakamura, Bandgap modulation in photoexcited topological insulator Bi2Te3 via atomic displacements, Journal of Chemical Physics, https://doi.org/10.1063/1.4955188, 145, 2, 2016.07, The atomic and electronic dynamics in the topological insulator (TI) Bi2Te3 under strong photoexcitation were characterized with time-resolved electron diffraction and time-resolved mid-infrared spectroscopy. Three-dimensional TIs characterized as bulk insulators with an electronic conduction surface band have shown a variety of exotic responses in terms of electronic transport when observed under conditions of applied pressure, magnetic field, or circularly polarized light. However, the atomic motions and their correlation between electronic systems in TIs under strong photoexcitation have not been explored. The artificial and transient modification of the electronic structures in TIs via photoinduced atomic motions represents a novel mechanism for providing a comparable level of bandgap control. The results of time-domain crystallography indicate that photoexcitation induces two-step atomic motions: first bismuth and then tellurium center-symmetric displacements. These atomic motions in Bi2Te3 trigger 10% bulk bandgap narrowing, which is consistent with the time-resolved mid-infrared spectroscopy results..
8. Masaki Hada, Shohei Saito, Sei'ichi Tanaka, Ryuma Sato, Kyohei Matsuo, Mitsuo Hara, Yasuhiko Hayashi, Kazuhiro Mouri, Masahiko Yoshimura, Yasuteru Shigeta, Shigehiro Yamaguchi, Ken Onda, R. J.Dwayne Miller, Ultrafast snapshots of the molecules twisting in liquid crystal state, International Conference on Ultrafast Phenomena, UP 2016 International Conference on Ultrafast Phenomena, UP 2016, https://doi.org/10.1364/UP.2016.UTh2B.2, 2016.07, We demonstrated time-resolved electron diffraction and vibrational spectroscopy to characterize the structure and dynamics of molecules in liquid crystal state. Our findings present the ultrafast local deformation triggering helical twisting motion in molecules..
9. Tadahiko Ishikawa, Ryosuke Hosoda, Yoichi Okimoto, Sei'Ichi Tanaka, Ken Onda, Shinya Koshihara, Reiji Kumai, Direct observations of the photoinduced change in dimerization in K-TCNQ, Physical Review B, https://doi.org/10.1103/PhysRevB.93.195130, 93, 19, 2016.05, The photoinduced dynamics of a potassium-tetracyanoquinodimethane (K-TCNQ) single crystal in the generalized Peierls phase are evaluated via time-resolved vibrational spectroscopy. The transient reflectivity spectrum of the photoinduced state in the mid-IR range shows a decrease in the height and width of the reflectivity band because of the electron-molecular-vibration-coupled CN stretching mode at approximately 2180 cm-1. This spectral change suggests that the photoexcitation of the charge transfer in TCNQ molecules induces melting of the dimerization of the molecules. From detailed analysis of the spectral evolution, the relaxation time constant from the photoinduced state to the dimerized state is estimated to be approximately 0.6 ps. Even after the recovery of the dimerization, a fluctuation is still observed, probably because of a domain-wall soliton. The fluctuation gradually dissipates with a time constant of approximately 2.3 ps. Direct observation of the dimerization process reveals the true dynamics of the photoinduced cooperative phenomenon within this system..
10. Tatsuhiko Mukuta, Sei'ichi Tanaka, Akiko Inagaki, Shin Ya Koshihara, Ken Onda, Direct Observation of the Triplet Metal-Centered State in [Ru(bpy)3]2+ Using Time-Resolved Infrared Spectroscopy, ChemistrySelect, https://doi.org/10.1002/slct.201600747, 1, 11, 2802-2807, 2016.01, [Ru(bpy)3]2+ is well-known as a prototype for the Ru(II) complexes used in a wide variety of photofunctional materials. The triplet metal-centered (3MC) state is important in this complex, since it dominates the phosphorescence lifetime and photoreaction processes. Despite this, the 3MC state has not yet been observed by spectroscopic methods. In the present study, we demonstrated that time-resolved infrared vibrational spectroscopy enables observations of the 3MC state. A vibrational band at 1599 cm−1 was found to exhibit unique temporal behavior that differed from that of other bands assignable to the triplet metal-to-ligand charge-transfer (3MLCT) state. This unique behavior was assessed under various experimental conditions and it was concluded that the band arises from the short-term population (∼23 ps) of the 3MC state during relaxation to the bottom of the 3MLCT state. These results agree with [Fe(bpy)3]2+ spectra, which show that the 5MC state is the most stable excited state..
11. Sei'Ichi Tanaka, Kosuke Takahashi, Masanari Hirahara, Masayuki Yagi, Ken Onda, Characterization of the excited states of distal- and proximal-[Ru(tpy)(pynp)OH2]2+ in aqueous solution using time-resolved infrared spectroscopy, Journal of Photochemistry and Photobiology A: Chemistry, https://doi.org/10.1016/j.jphotochem.2015.06.018, 313, 87-98, 2015.12, The quasi-stable excited states of distal- and proximal-[Ru(tpy)(pynp)OH2]2+ (tpy = 2,2′,6′,2″-terpyridine, pynp = 2-(2-pyridyl)-1,8-naphthyridine), which exhibits unique irreversible photoisomerization, were assessed in aqueous solution using time-resolved infrared vibrational spectroscopy and quantum chemical calculations. Transient vibrational spectra from 1300 to 1650 cm-1 were acquired 120 and 2 ps after photoexcitation with a 400 nm pulse for the distal- and proximal-isomers, respectively. The calculated vibrational spectra assuming the lowest triplet states were in good agreement with the experimentally observed spectra, and the vibrational bands are assigned to normal vibrational modes obtained from calculations. The lifetimes of the excited state were determined to be 9.7 ns and 6.4 ps for the distal- and proximal-isomers, respectively. Based on these results, we determined various characteristic features of these excited states. Despite the large difference in lifetime, both isomers were assigned to the triplet metal-to-ligand charge transfer (3MLCT) state and there was almost no difference in the positions of the atoms directly coordinated to the Ru atom. The dissimilarity between the isomers is that the pynp ligands are significantly distorted in the case of the distal-isomer while there is hydrogen bonding between the pynp and OH2 ligands in the proximal-isomer. The strong interaction between the low lying 3MLCT state and the ground state presumably causes the very short lifetime and the forbidden photoisomerization of the proximal-isomer, whereas photoisomerization occurs via the thermally populated metal centered (3MC) state from the high-lying 3MLCT state in the case of the distal-isomer..
12. Tadahiko Ishikawa, Stuart A. Hayes, Sercan Keskin, Gastón Corthey, Masaki Hada, Kostyantyn Pichugin, Alexander Marx, Julian Hirscht, Kenta Shionuma, Ken Onda, Yoichi Okimoto, Shin Ya Koshihara, Takashi Yamamoto, Hengbo Cui, Mitsushiro Nomura, Yugo Oshima, Majed Abdel-Jawad, Reizo Kato, R. J. Dwayne Miller, Direct observation of collective modes coupled to molecular orbital-driven charge transfer, Science, https://doi.org/10.1126/science.aab3480, 350, 6267, 1501-1505, 2015.12, Correlated electron systems can undergo ultrafast photoinduced phase transitions involving concerted transformations of electronic and lattice structure. Understanding these phenomena requires identifying the key structural modes that couple to the electronic states.We report the ultrafast photoresponse of the molecular crystal Me4P[Pt(dmit)2]2, which exhibits a photoinduced charge transfer similar to transitions between thermally accessible states, and demonstrate how femtosecond electron diffraction can be applied to directly observe the associated molecularmotions. Even for such a complex system, the key large-amplitudemodes can be identified by eye and involve a dimer expansion and a librational mode.The dynamics are consistent with the time-resolved optical study, revealing how the electronic, molecular, and lattice structures together facilitate ultrafast switching of the state..
13. Keiki Fukumoto, Yuki Yamada, Shin Ya Koshihara, Ken Onda, Lifetimes of photogenerated electrons on a GaAs surface affected by nanostructural defects, Applied Physics Express, https://doi.org/10.7567/APEX.8.101201, 8, 10, 2015.10, The recombination dynamics of photogenerated carriers on temporal and spatial scales of 100 fs and 100 nm, respectively, on an inhomogeneous GaAs surface with structural defects was investigated using time-resolved photoemission electron microscopy with femtosecond laser pulses. The observed photocarrier lifetimes (τ) ranged from subpicoseconds to picoseconds at isolated structural defects and were inversely proportional to the photoemission intensity at each defect. We concluded that τ corresponds to the carrier trapping time to midgap defect states and estimated the density of the midgap states at each defect on the basis of the relation between τ and the photoemission intensity..
14. R. Fukaya, Y. Okimoto, M. Kunitomo, Ken Onda, T. Ishikawa, S. Koshihara, H. Hashimoto, S. Ishihara, A. Isayama, H. Yui, T. Sasagawa, Ultrafast electronic state conversion at room temperature utilizing hidden state in cuprate ladder system, Nature Communications, https://doi.org/10.1038/ncomms9519, 6, 2015.10, Photo-control of material properties on femto-(10-15) and pico-(10-12) second timescales at room temperature has been a long-sought goal of materials science. Here we demonstrate a unique ultrafast conversion between the metallic and insulating state and the emergence of a hidden insulating state by tuning the carrier coherence in a wide temperature range in the two-leg ladder superconductor Sr14-xCaxCu24O41 through femtosecond time-resolved reflection spectroscopy. We also propose a theoretical scenario that can explain the experimental results. The calculations indicate that the holes injected by the ultrashort light reduce the coherence among the inherent hole pairs and result in suppression of conductivity, which is opposite to the conventional photocarrier-doping mechanism. By using trains of ultrashort laser pulses, we successively tune the carrier coherence to within 1 picosecond. Control of hole-pair coherence is shown to be a realistic strategy for tuning the electronic state on ultrafast timescales at room temperature..
15. Tsugumi Umanodan, Se I.Ichi Tanaka, Suguru Naruse, Tadahiko Ishikawa, Ken Onda, Shin Ya Koshihara, Sachio Horiuchi, Yoichi Okimoto, Different time-scale relaxation dynamics in organic supramolecular ferroelectrics studied by linear and nonlinear spectroscopy, Journal of the Physical Society of Japan, https://doi.org/10.7566/JPSJ.84.073707, 84, 7, 2015.07, Time-resolved linear and nonlinear optical responses were investigated in an organic supramolecular ferroelectric material composed of protonated 2,3-di(2-pyridinyl)pyrazine (H-dppz) and deprotonated chloranilic acid (Hca). We irradiated nanosecond laser pulses (λ = 532 nm) on the crystal, pumped the intramolecular excitation of the Hca molecule, and observed a clear redshift of the molecular vibrational modes of C=O and C-O- just after the photoexcitation. Each softened mode gradually relaxed on different time scales, indicating that the electrons of the Hca molecules were redistributed after the photoexcitation. By the same excitation, a large suppression of the second-harmonic (SH) intensity was observed, driven by the macroscopic disordering of the transferred protons. The decay time of the SH intensity was longer than those of the vibrational modes, suggesting that the microscopic vibrations and macroscopic ferroelectricity have dynamics on different time scales..
16. K. Fukumoto, Y. Yamada, T. Matsuki, Ken Onda, T. Noguchi, R. Mizokuchi, S. Oda, S. Koshihara, Visualization of ultrafast electron dynamics using time-resolved photoemission electron microscopy, 19th International Conference on Ultrafast Phenomena, 2014 Ultrafast Phenomena XIX - Proceedings of the 19th International Conference, https://doi.org/10.1007/978-3-319-13242-6_82, 162, 337-340, 2015.01, We constructed a TR-PEEM which can directly image the photo-generated electron dynamics in semiconductor on nm and fs scales. Carrier transport properties relating to device performance, carrier lifetime, drift velocity and mobility, are investigated..
17. Ken Onda, Tatsuhiko Mukuta, Sei’Ichi Tanaka, Kei Murata, Akiko Inagaki, Observation of the dark state in ruthenium complexes using femtosecond infrared vibrational spectroscopy, 19th International Conference on Ultrafast Phenomena, 2014 Ultrafast Phenomena XIX - Proceedings of the 19th International Conference, https://doi.org/10.1007/978-3-319-13242-6_116, 162, 475-478, 2015, Comprehensive analyses of the excited states of prototypical ruthenium complexes using time-resolved infrared vibrational spectroscopy reveal a peak that can be assigned to the dark 3MC (metal centered) state, which plays an important role for their photofunctions..
18. Ken Onda, Observation of ultrafast photoinduced dynamics in strongly correlated organic materials, Springer Series in Chemical Physics, https://doi.org/10.1007/978-3-319-23657-5_9, 112, 179-201, 2015, Complex photoinduced dynamics in organic charge transfer complexes, which have strong electron correlation and low-dimensional electronic systems, have been investigated using various ultrafast analytical techniques. A short-lived hidden photoinduced phase was determined from the transient reflectivity spectrum using a 120 fs pulse over a wide photon energy range from infrared to visible. The initial process of the photoinduced dynamics and its electronic coherence were revealed using a pulse compression technique down to 12 fs. The molecular and lattice structures were found to change in a different way from the electronic states, based on observations using two types of time-resolved vibrational spectroscopy and femtosecond electron diffraction. Time-resolved photoemission electron microscopy was developed for observing transient domain structures during photoinduced phase transitions..
19. Yoshitaka Matsubara, Sho Ogihara, Jiro Itatani, Nobuya Maeshima, Kenji Yonemitsu, Tadahiko Ishikawa, Yoichi Okimoto, Shin Ya Koshihara, Takaaki Hiramatsu, Yoshiaki Nakano, Hideki Yamochi, Gunzi Saito, Ken Onda, Coherent dynamics of photoinduced phase formation in a strongly correlated organic crystal, Physical Review B - Condensed Matter and Materials Physics, https://doi.org/10.1103/PhysRevB.89.161102, 89, 16, 2014.04, The photoinduced phase formation in a strongly correlated crystal (EDO-TTF)2PF6 (EDO-TTF: 4,5-ethylenedioxytetrathiafulvalene) is investigated using a 12 fs laser pulse. The formation time is determined as 40 fs with observation of coherence of electron-phonon coupled excited states prior to formation. The temperature-independent dephasing time is determined as ∼22 fs up to 180 K and the frequency of phonon oscillation is ∼38 THz, corresponding to the intramolecular vibrations in EDO-TTF. The phase formation is coherently controlled by relative-phase-controlled two-pulse excitation..
20. Tatsuhiko Mukuta, Naoto Fukazawa, Kei Murata, Akiko Inagaki, Munetaka Akita, Sei'Ichi Tanaka, Shin Ya Koshihara, Ken Onda, Infrared vibrational spectroscopy of [Ru(bpy)2(bpm)] 2+ and [Ru(bpy)3]2+ in the excited triplet state, Inorganic Chemistry, https://doi.org/10.1021/ic402474t, 53, 5, 2481-2490, 2014.03, This work involved a detailed investigation into the infrared vibrational spectra of ruthenium polypyridyl complexes, specifically heteroleptic [Ru(bpy)2(bpm)]2+ (bpy = 2,2′-bipyridine and bpm = 2,2′-bipyrimidine) and homoleptic [Ru(bpy)3]2+, in the excited triplet state. Transient spectra were acquired 500 ps after photoexcitation, corresponding to the vibrational ground state of the excited triplet state, using time-resolved infrared spectroscopy. We assigned the observed bands to specific ligands in [Ru(bpy)2(bpm)]2+ based on the results of deuterium substitution and identified the corresponding normal vibrational modes using quantum-chemical calculations. Through this process, the more complex vibrational bands of [Ru(bpy)3] 2+ were assigned to normal vibrational modes. The results are in good agreement with the model in which excited electrons are localized on a single ligand. We also found that the vibrational bands of both complexes associated with the ligands on which electrons are little localized appear at approximately 1317 and 1608 cm-1. These assignments should allow the study of the reaction dynamics of various photofunctional systems including ruthenium polypyridyl complexes..
21. Ken Onda, Hideki Yamochi, Shin Ya Koshihara, Diverse photoinduced dynamics in an organic charge-transfer complex having strong electron-phonon interactions, Accounts of Chemical Research, https://doi.org/10.1021/ar500257b, 47, 12, 3494-3503, 2014.01, Phenomena that occur in nonequilibrium states created by photoexcitation differ qualitatively from those that occur at thermal equilibrium, and various physical theories developed for thermal equilibrium states can hardly be applied to such phenomena. Recently it has been realized that understanding phenomena in nonequilibrium states in solids is important for photoenergy usage and ultrafast computing. Consequently, much effort has been devoted to revealing such phenomena by developing various ultrafast observation techniques and theories applicable to nonequilibrium states. This Account describes our recent studies of diverse photoinduced dynamics in a strongly correlated organic solid using various ultrafast techniques. Solids in which the electronic behavior is affected by Coulomb interactions between electrons are designated as strongly correlated materials and are known to exhibit unique physical properties even at thermal equilibrium. Among them, many organic charge-transfer (CT) complexes have low dimensionality and flexibility in addition to strong correlations; thus, their physical properties change sensitively in response to changes in pressure or electric field. Photoexcitation is also expected to drastically change their physical properties and would be useful for ultrafast photoswitching devices. However, in nonequilibrium states, the complicated dynamics due to these characteristics prevents us from understanding and using these materials for photonic devices. The CT complex (EDO-TTF)2PF6 (EDO-TTF = 4,5-ethylenedioxytetrathiafulvalene) exhibits unique photoinduced dynamics due to strong electron -electron and electron-phonon interactions. We have performed detailed studies of the dynamics of this complex using transient electronic spectroscopy at the 10 and 100 fs time scales. These studies include transient vibrational spectroscopy, which is sensitive to the charges and structures of constituent molecules, and transient electron diffraction, which provides direct information on the crystal structure. Photoexcitation of the charge-ordered low-temperature phase of (EDO-TTF)2PF6 creates a new photoinduced phase over 40 fs via the Franck-Condon state, in which electrons and vibrations are coherently and strongly coupled. This new photoinduced phase is assigned to an insulator-like state in which the charge order differs from that of the initial state. In the photoinduced phase, translations of component molecules proceed before the rearrangements of intramolecular conformations. Subsequently, the charge order and structure gradually approach those of the high-temperature phase over 100 ps. This unusual two-step photoinduced phase transition presumably originates from steric effects due to the bent EDO-TTF as well as strong electron-lattice interactions. (Chemical Equation Presented)..
22. Keiki Fukumoto, Yuki Yamada, Ken Onda, Shin Ya Koshihara, Direct imaging of electron recombination and transport on a semiconductor surface by femtosecond time-resolved photoemission electron microscopy, Applied Physics Letters, https://doi.org/10.1063/1.4864279, 104, 5, 2014, Much effort has been devoted to the development of techniques to probe carrier dynamics, which govern many semiconductor device characteristics. We report direct imaging of electron dynamics on semiconductor surfaces by time-resolved photoemission electron microscopy using femtosecond laser pulses. The experiments utilized a variable-repetition-rate femtosecond laser system to suppress sample charging problems. The recombination of photogenerated electrons and the lateral motion of the electrons driven by an external electric field on a GaAs surface were visualized. The mobility was estimated from a linear relationship between the drift velocity and the potential gradient..
23. Keiki Fukumoto, Ken Onda, Yuki Yamada, Takashi Matsuki, Tatsuhiko Mukuta, Sei Ichi Tanaka, Shin Ya Koshihara, Femtosecond time-resolved photoemission electron microscopy for spatiotemporal imaging of photogenerated carrier dynamics in semiconductors, Review of Scientific Instruments, https://doi.org/10.1063/1.4893484, 85, 8, 2014, We constructed an instrument for time-resolved photoemission electron microscopy (TR-PEEM) utilizing femtosecond (fs) laser pulses to visualize the dynamics of photogenerated electrons in semiconductors on ultrasmall and ultrafast scales. The spatial distribution of the excited electrons and their relaxation and/or recombination processes were imaged by the proposed TR-PEEM method with a spatial resolution about 100 nm and an ultrafast temporal resolution defined by the cross-correlation of the fs laser pulses (240 fs). A direct observation of the dynamical behavior of electrons on higher resistivity samples, such as semiconductors, by TR-PEEM has still been facing difficulties because of space and/or sample charging effects originating from the high photon flux of the ultrashort pulsed laser utilized for the photoemission process. Here, a regenerative amplified fs laser with a widely tunable repetition rate has been utilized, and with careful optimization of laser parameters, such as fluence and repetition rate, and consideration for carrier lifetimes, the electron dynamics in semiconductors were visualized. For demonstrating our newly developed TR-PEEM method, the photogenerated carrier lifetimes around a nanoscale defect on a GaAs surface were observed. The obtained lifetimes were on a sub-picosecond time scale, which is much shorter than the lifetimes of carriers observed in the non-defective surrounding regions. Our findings are consistent with the fact that structural defects induce mid-gap states in the forbidden band, and that the electrons captured in these states promptly relax into the ground state..
24. S. A. Hayes, T. Ishikawa, S. Keskin, M. Hada, A. Marx, G. Corthey, K. Pichugin, M. Nomura, R. Kato, Ken Onda, Y. Okimoto, S. Koshiharab, R. J.D. Miller, Observation of the photoinduced phase transition in Me4P[Pt(dmit)2]2 by femtosecond electron diffraction, 19th International Conference on Ultrafast Phenomena, UP 2014 19th International Conference on Ultrafast Phenomena, UP 2014, 2014, Femtosecond electron diffraction has been used to probe the photoinduced phase transition in the strongly-correlated system, Me4P[Pt(dmit)2]2, revealing molecular motions involved in this process and breaking new ground in terms of chemical complexity..
25. Hirokazu Oda, Ken Onda, Masaru Nakagawa, Photochemical grafting reactions of a benzophenone-containing alkanethiol monolayer on au with deuterated polystyrene, Bulletin of the Chemical Society of Japan, https://doi.org/10.1246/bcsj.20130137, 86, 9, 1035-1040, 2013.09, Photochemical reactions of 4-[(10-sulfanyldecyl)oxy]benzophenone with deuterated polystyrene (PS-d8) were studied using Fourier-transform infrared (FT-IR) spectroscopy and vibrational sum frequency generation (VSFG) spectroscopy. The FT-IR spectral changes of PS thin films indicated that exposure to ultraviolet (UV) light caused photochemical consumption of the benzophenone moiety by a hydrogen abstraction reaction of the carbonyl group; the reaction was almost complete at an exposure energy of 2.0 J cm-2 at 254 nm. The VSFG spectral changes of the adsorbed monolayer on Au on UV exposure revealed that the carbonyl groups oriented parallel to the Au substrate surface were consumed at more exposure energies. The exposure-energy-dependent formation of PS-grafted layers was confirmed by atomic force microscopy observations of Au surfaces modified with a photoreactive monolayer..
26. Ryo Fukaya, Yoichi Okimoto, Miyako Kunitomo, Tadahiko Ishikawa, Ken Onda, Shin Ya Koshihara, Akira Isayama, Takao Sasagawa, Unconventional photonic change of charge-density-wave phase in two-leg ladder cuprate Sr14Cu24O41, Journal of the Physical Society of Japan, https://doi.org/10.7566/JPSJ.82.083707, 82, 8, 2013.08, The photoinduced transition from the charge-density-wave (CDW) phase to the metallic phase in low-dimensional ladder-type cuprate Sr14Cu 24O41 was investigated in terms of femtosecond time-resolved reflection spectroscopy. Following the melting of the CDW order just after photoexcitation, a Drude-like metallic state was formed with a large optical response and maintained for more than 50 ps. The Drude weight increased with increasing fluence with threshold behavior as a result of the cooperative interactions in the CDW domains and reached the value of the Drude weight in the hole-doped metallic compound of Sr4Ca10Cu 24O41. These results indicate the photoinduced formation of a similar metallic state driven by hole doping..
27. Yoichi Okimoto, Tsugumi Egawa, Ryo Fukaya, Yoshitaka Matsubara, Yuki Yamada, Nao Yamaya, Tadahiko Ishikawa, Ken Onda, Shin Ya Koshihara, Hiroki Taniguchi, Mitsuru Itoh, Akira Isayama, Takao Sasagawa, Ultrafast real space dynamics of photoexcited state in a layered perovskite-type spin crossover oxide La1.5Sr0.5CoO 4, Journal of the Physical Society of Japan, https://doi.org/10.7566/JPSJ.82.074721, 82, 7, 2013.07, Ultrafast optical responses were investigated using a fs laser system on a layered perovskite-type cobalt oxide, La1.5Sr0.5CoO 4 with a checkerboard-type charge ordering. To investigate the spatial variation of the photoexcited state, we measured the transient changes of three independent optical quantities, i.e., transmittance, reflectivity, and backside reflectivity, as well as their fluence dependence at 0.25 eV. Using the numerical calculations based on Maxwell's equations, we calculated not only the dielectric constant but also the decay length of the photoexcited region (L) and analyzed the real-space dynamics of the photoexcited area in terms of the fluence and time dependence of L. With the above information on the time dependence of L and Kramers-Kronig analysis, we numerically calculated the transient optical conductivity spectra, which showed instantaneous formation of a polaron-like absorption peak in the midinfrared region. A 10 fs pump-probe reflection measurement revealed that the photoexcited state occurs within the time resolution (≈10 fs). The time profile of the reflectivity is accompanied with an oscillation assigned as a breathing phonon mode in the CoO2 layer observed in Raman spectroscopy. Analyses of the decay time and the frequency revealed that the photoirradiation melted the charge ordering in La1.5Sr0.5CoO4..
28. Naoto Fukazawa, Takahiro Tanaka, Tadahiko Ishikawa, Yoichi Okimoto, Shin Ya Koshihara, Takashi Yamamoto, Masafumi Tamura, Reizo Kato, Ken Onda, Time-resolved infrared vibrational spectroscopy of the photoinduced phase transition of Pd(dmit)2 salts having different orders of phase transition, Journal of Physical Chemistry C, https://doi.org/10.1021/jp403193a, 117, 25, 13187-13196, 2013.06, To clarify the mechanism of the later process of photoinduced phase transition (PIPT) in organic charge-transfer complexes, we examined by time-resolved infrared vibrational spectroscopy two dimeric anion radical salts, Et2Me2Sb[Pd(dmit)2]2 (Et 2Me2Sb salt) and Cs[Pd(dmit)2]2 (Cs salt) (Et, Me, and dmit are C2H5, CH3, and 1,3-dithiol-2-thione-4,5-dithiolate, respectively), having similar characteristics except for the order of their phase transitions at thermal equilibrium. The phase transition is first order for the Et2Me 2Sb salt and second order for the Cs salt at thermal equilibrium. Although both salts exhibit a high-temperature phase at later delay times (>100 ps) after the photoexcitation of the low-temperature phase, the time required for the emergence of the high-temperature phase was significantly different: 70 ps for the Et2Me2Sb salt and <0.1 ps for the Cs salt. The slow emergence of the high-temperature phase in the PIPT of the Et2Me2Sb salt presumably has an origin similar to that recognized for the first-order thermal phase transition, that is, steric effects of the Et2Me2Sb cation when the phase transitions occur..
29. Meng Gao, Cheng Lu, Hubert Jean-Ruel, Lai Chung Liu, Alexander Marx, Ken Onda, Shin Ya Koshihara, Yoshiaki Nakano, Xiangfeng Shao, Takaaki Hiramatsu, Gunzi Saito, Hideki Yamochi, Ryan R. Cooney, Gustavo Moriena, Germán Sciaini, R. J.Dwayne Miller, Mapping molecular motions leading to charge delocalization with ultrabright electrons, Nature, https://doi.org/10.1038/nature12044, 496, 7445, 343-346, 2013.04, Ultrafast processes can now be studied with the combined atomic spatial resolution of diffraction methods and the temporal resolution of femtosecond optical spectroscopy by using femtosecond pulses of electrons or hard X-rays as structural probes. However, it is challenging to apply these methods to organic materials, which have weak scattering centres, thermal lability, and poor heat conduction. These characteristics mean that the source needs to be extremely bright to enable us to obtain high-quality diffraction data before cumulative heating effects from the laser excitation either degrade the sample or mask the structural dynamics. Here we show that a recently developed, ultrabright femtosecond electron source makes it possible to monitor the molecular motions in the organic salt (EDO-TTF) 2 PF 6 as it undergoes its photo-induced insulator-to-metal phase transition. After the ultrafast laser excitation, we record time-delayed diffraction patterns that allow us to identify hundreds of Bragg reflections with which to map the structural evolution of the system. The data and supporting model calculations indicate the formation of a transient intermediate structure in the early stage of charge delocalization (less than five picoseconds), and reveal that the molecular motions driving its formation are distinct from those that, assisted by thermal relaxation, convert the system into a metallic state on the hundred-picosecond timescale. These findings establish the potential of ultrabright femtosecond electron sources for probing the primary processes governing structural dynamics with atomic resolution in labile systems relevant to chemistry and biology..
30. Ken Onda, Yoshitaka Matsubara, Tadahiko Ishikawa, Yoichi Okimoto, Shin Ya Koshihara, Takaaki Hiramatsu, Gunzi Saito, Yoshiaki Nakano, Hideki Yamochi, The earliest stage of photoinduced phase transition in a strongly correlated organic system using a 10-fs Pulse, 18th International Conference on Ultrafast Phenomena, UP 2012 18th International Conference on Ultrafast Phenomena, UP 2012, https://doi.org/10.1051/epjconf/20134103001, 41, 2013.04, We examined the initial excited state before photoinduced phase transition in a charge transfer complex (EDO-TTF)2PF6 and revealed the conversion process to the photoinduced phase and the electronic coherence at the excited state..
31. Shunsuke Sato, Yasuo Matubara, Kazuhide Koike, Magnus Falkenström, Tetsuro Katayama, Yukihide Ishibashi, Hiroshi Miyasaka, Seiji Taniguchi, Haik Chosrowjan, Noboru Mataga, Naoto Fukazawa, Shinya Koshihara, Ken Onda, Osamu Ishitani, Photochemistry of fac-[Re(bpy)(CO)3Cl], Chemistry - A European Journal, https://doi.org/10.1002/chem.201202734, 18, 49, 15722-15734, 2012.12, The photochemistry of fac-[Re(bpy)(CO)3Cl] (1 a; bpy=2,2'-bipyridine) initiated by irradiation using <330 nm light has been investigated. Isomerization proceeded in THF to give the corresponding mer-isomer 1 b. However, in the presence of a small amount of MeCN, the main product was the CO-ligand-substituted complex (OC-6-24)-[Re(bpy)(CO) 2Cl(MeCN)] (2 c; bpy=2,2'-bipyridine). In MeCN, two isomers, 2 c and its (OC-6-34) form (2 a), were produced. Only 2 c thermally isomerized to produce the (OC-6-44) form 2 b. A detailed investigation led to the conclusion that both 1 b and 2 c are produced by a dissociative mechanism, whereas 2 a forms by an associative mechanism. A comparison of the ultrafast transient UV-visible absorption, emission, and IR spectra of 1 a acquired by excitation using higher-energy light (e.g., 270 nm) and lower-energy light (e.g., 400 nm) gave detailed information about the excited states, intermediates, and kinetics of the photochemical reactions and photophysical processes of 1 a. Irradiation of 1 a using the higher-energy light resulted in the generation of the higher singlet excited state with τ
32. Tadahiko Ishikawa, Ken Onda, Shin Ya Koshihara, Photoinduced phase transition in strongly electron-lattice and electron-electron correlated molecular crystals, Crystals, https://doi.org/10.3390/cryst2031067, 2, 3, 1067-1083, 2012.07, Strongly electron-lattice- and electron-electron-correlated molecular crystals, such as charge transfer (CT) complexes, are often sensitive to external stimuli, e.g., photoexcitation, due to the cooperative or competitive correlation of various interactions present in the crystals. These crystals are thus productive targets for studying photoinduced phase transitions (PIPTs). Recent advancements in research on the PIPT of CT complexes, especially Et2Me2Sb[Pd(dmit)2]2 and (EDO-TTF)2PF6, are reviewed in this report. The former exhibits a photoinduced insulator-to-insulator phase transition with clearly assigned spectral change. We demonstrate how to find the dynamics of PIPT using this system. The latter exhibits a photoinduced hidden state as an initial PIPT process. Wide energy ranged time-resolved spectroscopy can probe many kinds of photo-absorption processes, i.e., intra-molecular and inter-molecular electron excitations and intramolecular and electron-molecular vibrations. The photoinduced spectral changes in these photo-absorption processes reveal various aspects of the dynamics of PIPT, including electronic structural changes, lattice structural changes, and molecular deformations. The complexities of the dynamics of the latter system were revealed by our measurements..
33. T. Ishikawa, M. Kitayama, A. Chono, Ken Onda, Y. Okimoto, S. Koshihara, Y. Nakano, H. Yamochi, T. Morikawa, T. Shirahata, Y. Misaki, Probing the metalinsulator phase transition in the (DMEDO-EBDT) 2PF 6 single crystal by optical measurements, Journal of Physics Condensed Matter, https://doi.org/10.1088/0953-8984/24/19/195501, 24, 19, 2012.05, The temperature and polarization dependence of the optical reflectivity spectra of a quasi-one-dimensional 1/4-filled band system, (DMEDO-EBDT) 2PF 6, have been investigated. We observed clear anisotropy in the electronic structures corresponding to the anisotropic transport properties. The appearance of a charge gap (E g>0.1eV) and transfer of the spectral weight accompanied by the metalinsulator phase transition were clearly observed. In addition, a split of the intramolecular vibrational modes was observed, which strongly suggested the existence of charge disproportionation in the low temperature phase. We also observed a photoinduced reflectivity change, which implied the occurrence of a photoinduced phase transition from the low temperature insulating phase to the high temperature metallic phase..
34. Naoto Fukazawa, Minoru Shimizu, Tadahiko Ishikawa, Yoichi Okimoto, Shin Ya Koshihara, Takaaki Hiramatsu, Yoshiaki Nakano, Hideki Yamochi, Gunzi Saito, Ken Onda, Charge and structural dynamics in photoinduced phase transition of (EDO-TTF) 2PF 6 examined by picosecond time-resolved vibrational spectroscopy, Journal of Physical Chemistry C, https://doi.org/10.1021/jp210708q, 116, 9, 5892-5899, 2012.03, Using time-resolved near-infrared reflectance spectroscopy and time-resolved mid-infrared vibrational spectroscopy, we studied photoinduced phase transition of the charge-ordered insulating phase in a charge-transfer complex (EDO-TTF) 2PF 6 (EDO-TTF: ethylenedioxy- tetrathiafulvalene) in the hundred picosecond range after photoexcitation. The temporal profiles at 0.83-1.03 eV, which are a characteristic of the photoinduced charge-disproportionate phase immediately after photoexcitation, suggested the formation of a new metastable phase in the hundred picosecond range. Time-resolved vibrational spectra at 1300-1700 cm -1, where charge- and structure-sensitive C=C stretching vibrational modes are located, elucidated that the nature of the new phase is very close to that of the high-temperature metallic phase and it takes about 100 ps for the new phase to emerge accompanied with charge and structure fluctuation..
35. K. Fukumoto, N. Fukazawa, T. Ishikawa, S. Koshihara, H. M. Yamamoto, R. Kato, Ken Onda, Photo-induced structural changes at a surface of organic single crystals observed by vibrational sum frequency generation spectroscopy, Acta Physica Polonica A, 121, 2, 313-315, 2012.02, Phase transitions from insulator to metal induced by thermal excitation and by photo-irradiation at the surface of β′-(BEDT-TTF)(TCNQ) are investigated using vibrational sum frequency generation spectroscopy. The results are compared with linear reflectivity measurements which detect the information mainly from bulk. IR absorption spectra of a g vibrational modes of TCNQ induced by the dimerization are observed using vibrational sum frequency generation spectroscopy, and its intensity is found to become smaller by thermal excitation indicating the transition to the metal phase. Time-dependent degree of dimerization is also observed by pump-probe experiments with almost the same statistics as the static measurements. On the other hand, in the reflectivity experiments, the reduction of dimerization is also observed by elevating the temperature, however, the spectral shapes are deformed for the time-resolved measurements. These results imply that the different dynamics occur at the surface and in the bulk..
36. T. Egawa, Y. Okimoto, R. Fukaya, Ken Onda, T. Ishikawa, S. Y. Koshihara, A. Isayama, T. Sasagawa, Femtosecond reflection spectroscopy in La1.5Sr 0.5CoO4, Acta Physica Polonica A, https://doi.org/10.12693/APhysPolA.121.307, 121, 2, 307-309, 2012.01, Ultrafast optical response has been investigated using fs laser system on a perovskite-type cobalt oxide, La1.5Sr0.5CoO4. After the photoirradiation at room temperature, the time profile of relative change of reflectance (ΔR/R) shows a sudden change within the pulse duration (≈ 150 fs) and decays with a lifetime of ≈ 330 fs. The sign of ΔR/R after the photoexcitation is positive in the mid-infrared region (at 0.50 eV) while negative in the visible energy region (at 2.0 eV), implying photoinduced change of the electronic structure after the photoexcitation..
37. Y. Matsubara, T. Yoshida, T. Ishikawa, Y. Okimoto, S. Y. Koshihara, Ken Onda, Photoinduced ionic to neutral phase transition in TTF-CA studied by time-resolved infrared vibrational spectroscopy, Acta Physica Polonica A, https://doi.org/10.12693/APhysPolA.121.340, 121, 2, 340-342, 2012.01, We studied the photoinduced ionic to neutral phase transition in tetrathiafulvalene-p-chloranil (TTF-CA) using time-resolved infrared vibrational spectroscopy. By monitoring the charge sensitive b1uυ 10 band of CA after photoexcitation by a 1.55 eV pulse, we found that the photoinduced N-phase has the same charge of the N-phase in thermal equilibrium and there is no charge variation during the photoinduced phase transition. We also found that the photoinduced N-phase grows through at least two steps over 300 ps..
38. K. Seko, Y. Okimoto, M. Kurashima, R. Fukaya, T. Egawa, T. Ishikawa, Ken Onda, S. Y. Koshihara, T. Kyomen, M. Itoh, Photoinduced phase transition in Pr0.5Ca0.5CoO 3 studied by sequential pulse excitations, Acta Physica Polonica A, https://doi.org/10.12693/APhysPolA.121.369, 121, 2, 369-371, 2012.01, A perovskite-type cobalt oxide, Pr0.5Ca0.5CoO 3 (PCCO), shows photoinduced phase transition. In this study, we successively irradiated two laser pulses with different intensities to PCCO and probed the transient change of the reflection at 2.0 eV. Assuming propagation of the two different photoinduced metallic states, we could reproduce the time profiles as well as the magnitude in the reflectance change, indicating the fabrication of the photoinduced multilayered thin film in Pr 0.5Ca0.5CoO3..
39. S. Koshihara, Ken Onda, Y. Okimoto, T. Ishikawa, Search for the photo-induced hidden phase in inorganic and organic systems, Acta Physica Polonica A, https://doi.org/10.12693/APhysPolA.121.328, 121, 2, 328-331, 2012.01, We review the dynamical behavior of strongly electron-lattice (orbital) coupled system induced by photo-excitation of fs laser pulse. By virtue of the femtosecond (fs) spectroscopy and picosecond (ps) time-resolved X-ray diffraction measurements, we demonstrate that the photo-irradiation creates new ordered state so called as "hidden phase" which can never be realized under thermo-equilibrium condition. This exotic phase commonly plays the key role for realizing the gigantic photo-induced changes in optical properties of inorganic and organic electron-lattice strongly coupled systems..
40. T. Ishikawa, T. Tanaka, N. Fukazawa, Y. Matsubara, Y. Okimoto, Ken Onda, S. Koshihara, M. Tamura, R. Kato, Slow dynamics of the photoinduced phase transition in Pd(dmit)2 salts (dmit = 1,3-dithiol-2-thione-4,5-dithiolate), Acta Physica Polonica A, https://doi.org/10.12693/APhysPolA.121.316, 121, 2, 316-318, 2012.01, Slow dynamics of the initially photoinduced state has been observed by the pump-probe type time-resolved reflection spectroscopy in the charge separated phase of the half-filled strong dimer system, Et2Me 2Sb[Pd(dmit)2]2 (dmit = 1,3-dithiol-2-thione-4, 5-dithiolate). We have succeeded to reproduce the probe photon energy dependence of the time profile qualitatively in the time delay range from 10 ps to 1 ns assuming the dynamical expansion of the domain of the photo-induced dimer-Mott insulating phase in the host charge-separated one..
41. R. Fukaya, M. Kunitomo, Y. Okimoto, T. Ishikawa, Ken Onda, S. Koshihara, H. Yui, T. Sasagawa, Ultrafast dynamics of photoinduced electronic phase modulation in ladder cuprate of Sr14-xCaxCu24O41, Acta Physica Polonica A, https://doi.org/10.12693/APhysPolA.121.310, 121, 2, 310-312, 2012.01, Ultrafast carrier dynamics caused by photoexcitation in quasi-one-dimensional two-leg ladder cuprate Sr4Ca 10Cu24O41 was investigated by the femtosecond reflection spectroscopy. After the photoexcitation along the leg direction, the transient reflectivity changes (ΔR/R) in the mid-infrared region showed instant decrease within 150 fs. The suppressed ΔR/R increased so rapidly on the picosecond time scale that the reflectivity value finally became larger than that in the initial state. Such a successive response in ΔR/R, which was also observed in other photon energy regions, is discussed in terms of ultrafast variation of the Drude weight in the ladder system by the photoirradiation..
42. Shin Ya Koshihara, Shinichi Adachi, Yoichi Okimoto, Tadahiko Ishikawa, Ryo Fukaya, Keiki Fukumoto, Manabu Hoshino, Ken Onda, Photoinduced phase transitions in strongly correlated electron systems, Quantum Electronics and Laser Science Conference, QELS 2012 Quantum Electronics and Laser Science Conference, QELS 2012, 2012, The search for materials that show a phase transition triggered by weak external stimulation of light is an important and attractive target for photonic and materials science today. My talk reviews experimental evidence indicating that photo-injected local excitation can really trigger an electron-lattice coupled cooperative phenomenon named as photo-induced phase transition (PIPT: Photo-domino effect) in wide class of materials based on the dynamical x-ray technique combined with the ultrafast spectroscopy. Obtained results also demonstrate that 'hidden phase' in strongly correlated systems which never appears in the phase diagram under thermo-equilibrium condition plays an essential role for the gigantic photo-response of the strongly correlated electron systems..
43. Yoshitaka Matsubara, Yoichi Okimoto, Tatsushi Yoshida, Tadahiko Ishikawa, Shin Ya Koshihara, Ken Onda, Photoinduced neutral-to-ionic phase transition in tetrathiafulvalene-p- chloranil studied by time-resolved vibrational spectroscopy, Journal of the Physical Society of Japan, https://doi.org/10.1143/JPSJ.80.124711, 80, 12, 2011.12, We studied the photoinduced neutral-to-ionic phase transition in tetrathiafulvalene-p-chloranil (TTF-CA) using time-resolved infrared (IR) vibrational spectroscopy with a broadband femtosecond IR laser pulse and a linear IR detector array. After photoexcitation, the strength of the TTF a gν3 band increased, indicating that dimerization is activated within ~1 ps. In contrast, the strength of the CA b 1uν10 band, whose frequency is proportional to the degree of charge transfer (ρ), decreased, and a broad weak band appeared in the lower-frequency region. This implies that large fluctuations in charge transfer continued until about 20 ps after the photoexcitation. These results indicate a large difference between the electronic structures of the ionic phases generated by thermalexcitation and photoexcitation, although a dimerized structure was observed in both cases..
44. Y. Okimoto, T. Miyata, M. S. Endo, M. Kurashima, Ken Onda, T. Ishikawa, S. Koshihara, M. Lorenc, E. Collet, H. Cailleau, T. Arima, Ultrafast spectral weight transfer in RBaCo2O 6-δ (R=Sm, Gd, and Tb)
Role of electronic correlation in a photoinduced phase transition, Physical Review B - Condensed Matter and Materials Physics, https://doi.org/10.1103/PhysRevB.84.121102, 84, 12, 2011.09, We performed femtosecond reflection spectroscopy on a series of A-site ordered perovskite-type cobalt oxide RBaCo2O6-δ (R=Sm, Gd, and Tb) crystals, in which the electronic transfer was controlled by R. The transient reflectivity and the optical conductivity [σPI(ω)] obtained by a Kramers-Kronig analysis showed an ultrafast change within a time resolution (150 fs) at room temperature and the appearance of signals of a hidden state that were different from the high-temperature metallic state. The transferred spectral weight in σPI(ω) upon photoexcitation sensitively depended on the R species, indicating an important role of electronic correlation in the photoexcited state..
45. Y. Okimoto, M. Kurashima, K. Seko, T. Ishikawa, Ken Onda, S. Koshihara, T. Kyomen, M. Itoh, Acceleration of domain wall movement by photoirradiation in perovskite-type cobaltite, Physical Review B - Condensed Matter and Materials Physics, https://doi.org/10.1103/PhysRevB.83.161101, 83, 16, 2011.04, Femtosecond reflection spectroscopy was performed on a perovskite-type cobalt oxide, namely, Pr0.5Ca0.5CoO3, that undergoes a photoinduced spin-state transition. After photoirradiation at 30 K, the time profile of the reflectance change shows a broad peak reflecting the propagation of the photodomain (about 60 Co sites per one photon). Analysis of the peak position indicates the sudden increase of the velocity of the propagation with increasing the excitation intensity. Such acceleration with increase in fluence originates from an abrupt sound velocity change driven by a cooperative photoinduced structural transition..
46. Mina Han, Yasuo Norikane, Ken Onda, Yoko Matsuzawa, Masaru Yoshida, Masahiko Hara, Light-driven modulation of fluorescence color from azobenzene derivatives containing electron-donating and electron-withdrawing groups, New Journal of Chemistry, https://doi.org/10.1039/c0nj00353k, 34, 12, 2892-2896, 2010.12, We report a simple preparation and color-tunable fluorescence of a series of azobenzene derivatives. The introduction of an electron-withdrawing or electron-donating group at the X position of azobenzenes (1-8) containing a biphenyl unit makes it possible to modulate the fluorescence color of the UV-exposed azobenzene solutions from blue to yellow, which correlates with the electron-donating abilities of the respective substituents. Theoretical calculations suggest that changes in both the dihedral angles between two phenyl rings of the biphenyl unit and the dipole moments between the trans and cis forms depending on the substituents seem to be important factors in determining the photochemical properties of chromophores..
47. Ken Onda, Sho Ogihara, Jiro Itatani, Tadahiko Ishikawa, Yoichi Okimoto, Shinya Koshihara, Xiangfeng Shao, Yoshiaki Nakano, Hideki Yamochi, Gunzi Saito, Photoinduced dynamics of a quasi-1D organic conductor over a range from 10 fs to 100 ps, International Conference on Ultrafast Phenomena, UP 2010 International Conference on Ultrafast Phenomena, UP 2010, 2010.12, We studied ultrafast photoinduced phase transition in the organic conductor (EDO-TTF)2PF6 using a 10-fs broadband pulse and a picosecond narrowband pulse, and revealed the roles of coherent molecular vibrations and the charge melting process..
48. Ken Onda, M. Shimizu, F. Sakaguchi, S. Ogihara, T. Ishikawa, Y. Okimoto, S. Koshihara, X. F. Shaod, Y. Nakano, H. Yamochi, G. Saito, Ultrafast and large reflectivity change by ultraviolet excitation of the metallic phase in the organic conductor (EDO-TTF) 2PF 6, Physica B: Condensed Matter, https://doi.org/10.1016/j.physb.2009.11.054, 405, 11 SUPPL., 2010.06, We examined the ultrafast response of the metallic high-temperature phase in the conducting charge transfer complex (EDO-TTF) 2PF 6. A large reflectivity change of approximately 10% was observed when the intra-molecular band was excited by a weak 3.1eV ultraviolet light pulse. The lifetimes of the photo-induced states were 0.2 and 0.7ps in the Drude-like band and the intra-molecular band, respectively. Measurement of the photo-induced spectrum just after photo-excitation and simulation using the Drude model revealed that the electronically excited EDO-TTF molecules shortened the relaxation time of conduction electrons and reduced the reflectivity of the Drude-like band..
49. Wen can Zhou, Zheng cao Li, Zheng jun Zhang, Ken Onda, Sho Ogihara, Yoichi Okimoto, Shin ya Koshihara, Nanostructured TiO2 photocatalyst and pump-probe spectroscopic study, Frontiers of Materials Science, https://doi.org/10.1007/s11706-009-0055-7, 3, 4, 403-408, 2009.11, Titanium dioxide (TiO2), with its large band gap, has attracted much attention due to its excellent photocatalytic activity. TiO2 ball-shaped nano-particles were deposited on silicon substrates by a thermal oxidation approach. With an increase in the annealing temperature the surface morphology and the structure of TiO2 remained stable, exhibiting good heat stability; meanwhile, the hydrogen production rate also increased. The femtosecond pump-probe spectroscopic study showed that the lifetime of carriers of the samples as- deposited and post-annealed at different temperatures were longer than 20 ps..
50. T. Ishikawa, N. Fukazawa, Y. Matsubara, R. Nakajima, Ken Onda, Y. Okimoto, S. Koshihara, M. Lorenc, E. Collet, M. Tamura, R. Kato, Large and ultrafast photoinduced reflectivity change in the charge separated phase of Et2 Me2 Sb [Pd (1,3-dithiol-2-thione-4,5-dithiolate) 2] 2, Physical Review B - Condensed Matter and Materials Physics, https://doi.org/10.1103/PhysRevB.80.115108, 80, 11, 2009.09, A large photoinduced change in reflectivity has been observed in the low-temperature charge separated (CS) phase of a dimeric radical anion salt, Et2 Me2 Sb [Pd (dmit) 2] 2 (dmit=1,3 -dithiol-2-thione-4,5-dithiolate). Just after the photoexcitation, the reflectivity abruptly changed reflecting the appearance of a photoinduced metastable state, indicating occurrence of recrystallizing of the CS phase by intradimer photoexcitation within a picosecond. Quantitative analysis considering the linear combination of the dielectric functions of the CS and the dimer-Mott state suggests a rather high efficiency of the photoinduced phase transition. One photon can change the valence of about five dimers. This photoinduced metastable state relaxed to the initial CS state via two successive types of relaxation processes, a fast and a slow one. The relaxation time (τ) and the reflectivity of the fast process showed a clear excitation intensity and temperature dependence. In particular, τ and the estimated domain size were enhanced up to the transition temperature (Tc) with increasing temperature. This phenomenon, a sort of critical slowing down around Tc, suggests that the density of the photoinduced state as well as the external temperature plays an important role in determining the relaxation dynamics of the photoinduced state. The results obtained indicate that this photoinduced phenomenon can be classified as a tuning of the charge in crystals via cooperative interaction between the degrees of freedom of "charge" and "molecular orbital" of the constituents, i.e., as a type of photoinduced phase transition..
51. Y. Okimoto, X. Peng, M. Tamura, T. Morita, Ken Onda, T. Ishikawa, S. Koshihara, N. Todoroki, T. Kyomen, M. Itoh, Ultrasonic propagation of a metallic domain in Pr0.5Ca0.5CoO3 undergoing a photoinduced insulator-metal transition, Physical Review Letters, https://doi.org/10.1103/PhysRevLett.103.027402, 103, 2, 2009.08, Femtosecond optical response was investigated on a perovskite-type cobalt oxide, Pr0.5Ca0.5CoO3 during an insulator-metal (I-M) transition accompanied with the change in spin configuration. After photoirradiation at 30 K, the reflectivity showed a sudden and large increase with subsequent variation depending on the observed photon energy. An exact calculation of Maxwell's equations for the Pr0.5Ca0.5CoO3 after the photoirradiation enabled us to observe the ultrafast dynamics of I-M phase transition and the motion of the photonically created metallic domain at the velocity of ultrasonic wave..
52. Ken Onda, Sho Ogihara, Tadahiko Ishikawa, Yoichi Okimoto, Xiangfeng Shao, Yoshiaki Nakano, Hideki Yamochi, Gunzi Saito, Shin Ya Koshihara, Anomalous photo-induced response by double-pulse excitation in the organic conductor (EDO-TTF)2PF6, Journal of Physics: Conference Series, https://doi.org/10.1088/1742-6596/148/1/012002, 148, 2009.04, We measured ultrafast reflectivity changes induced by double-pulse excitation in the organic conductor (EDO-TTF)2PF6. Using double-pulse excitation with a relatively high intensity, the sign of reflectivity change became reversed at around 0.8 ps and subsequently the reflectivity change reverted to that of the normal photo-induced state after about 1 ps. Using a optically phase-locked double-pulse with low intensity, we found that the temporal profile excited by an in-phase double-pulse differs from that by an out-of-phase double-pulse despite the time difference between the double-pulses being only 1.31 fs. This was true even when there is almost no overlap between each pulse in the double-pulse. These results indicate that the photo-response in this material to double-pulse excitation differs greatly from the linear sum of the responses to single pulses..
53. S. Ogihara, Ken Onda, M. Shimizu, T. Ishikawa, Y. Okimoto, X. F. Shao, Y. Nakano, H. Yamochi, G. Saito, S. Koshihara, Excitation photon energy dependence of photo-induced phase transition in (EDO-TTF) 2PF 6, Journal of Physics: Conference Series, https://doi.org/10.1088/1742-6596/148/1/012008, 148, 2009.04, The conducting charge transfer complex (EDO-TTF) 2PF 6 has two types of charge transfer bands in the low temperature insulator phase: CT1 at 0.56 eV and CT2 at 1.38 eV. We excited these bands independently with a tunable ultrashort pulse laser and studied the difference of the photo-induced phases by measuring change in reflectivity spectrum over a wide photon energy range. As a result, we found that both the photo-induced phases by CT1 and CT2 excitation are the same except for their photo-conversion efficiencies.
54. Xianzhou Peng, Yoichi Okimoto, Masato Endo, Ken Sato, Takeshi Aizawa, Ken Onda, Tadahiko Ishikawa, Shin Ya Koshihara, Taka Hisa Arima, Femtosecond time-resolved reflection spectroscopy in Pr 1-xCaxMnO3 (x=0.37), Physica Status Solidi (C) Current Topics in Solid State Physics, https://doi.org/10.1002/pssc.200879884, 6, 1, 252-255, 2009, The ultrafast dynamics of photoinduced phase transition (PIPT) in a crystal of Pr1-xCaxMnO3 (x 0.37) stud been studied by means of femtosecond time-resolved spectroscopy. Just after photoirradiation of 1.58 eV light at 100 K, ultrafast change of reflectivity (ΔR/R) was observed witiiin ~1 ps utilizing the probe light of 0.5 eV due to the ferromagnetic metallic phase transition. While the fast component of ΔR/R gradually increases with increasing the excitation intensity, the slow component observed at the delay time of ∼1 ps shows unusual dependence on the excitation intensity. The signature of ΔR/R changes from negative to positive with the excitaiton power, which cannot be explained by the photothermal heating effect. In addition, we investigated the temperature dependence of the ΔR/R at 0.5 eV and observed the sudden change of ΔR/R around TN, which indicates that the efficiency of PIPT is closely correlated with the ordering of spin and extra electron in Mn4+..
55. Yoichi Okimoto, Xianzhou Peng, Mieko Tamura, Takaaki Morita, Ken Onda, Tadahiko Ishikawa, Shin Ya Koshihara, Naoyuki Todoroki, Toru Kyomen, Mitsuru Itoh, Photoinduced insulator-metal transition in Pr 0.5Ca 0.5CoO 3 as studied by femtosecond spectroscopy, Journal of Physics: Conference Series, https://doi.org/10.1088/1742-6596/148/1/012019, 148, 2009, Ultrafast optical response has been investigated using fs laser system on a perovskite-type cobalt oxide, Pr 0.5Ca 0.5CoO 3 which undergoes the insulator-metal transition at 89 K. After the photo-irradiation at 30 K, the reflectivity shows a sudden increase. The ultrafast reflectance changes are quantitatively reproduced by the numerical calculation considering the linear combination of the dielectric functions of the initial insulating and the final metallic state, indicating ultrafast phase transition on the time scale of femtosecond..
56. N. Fukazawa, T. Ishikawa, R. Nakajima, Y. Matsubara, M. Kitayama, S. Koshihara, Y. Okimoto, Ken Onda, M. Lorenc, E. Collet, M. Tamura, R. Kato, Probing the photoinduced phase transition in (C 2H 5) 2(CH 3) 2Sb[Pd(dmit) 2] 2, Journal of Physics: Conference Series, https://doi.org/10.1088/1742-6596/148/1/012003, 148, 2009, The dynamics of the photoinduced effect in the low temperature phase of (C 2H 5) 2(CH 3) 2Sb[Pd(dmit) 2] 2 (Et 2Me 2Sb[Pd(dmit) 2] 2) salt was studied by using pump-probe time resolved reflection spectroscopy with the time resolution of about 120 fs. The observed photoinduced reflectivity change was highly efficient and spectral shape of the photoinduced state was similar to that of dimer Mott insulator. In addition, oscillatory structure in photoinduced signal due to coherent phonon was observed reflecting the structural modulation coupling with valence change of the dimer state ([Pd(dmit) 2] 2). We discuss the primary dynamics of the photoinduced phenomena in this salt at low temperature phase..
57. Tadahiko Ishikawa, Naoto Fukazawa, Yoshitaka Matsubara, Ryohei Nakajima, Ken Onda, Yoichi Okimoto, Shin Ya Koshihara, Masafumi Tamura, Reizo Kato, Maciej Lorenc, Eric Collet, The photo-induced phase transition in Et2Me 2Sb[Pd(dmit)2]2, Physica Status Solidi (C) Current Topics in Solid State Physics, https://doi.org/10.1002/pssc.200879835, 6, 1, 112-115, 2009, The large photo-induced change of reflectivity has been observed in the unique charge separated phase of the Et2Me2Sb[Pd(dmit) 2]2 crystal We observe a clear spectral change due to the disappearance of the charge separated phase by femtosecond photo-excitation in the [Pa(dmit)2] dimer. The dynamics of the photo-excited state depends on the excitation intensity. Obtained results strongly suggest that cooperative effect plays a key role in the photo-excited state of this system, We tentatively classify the observed phenomena as a photo-induced phase transition due to the melting of the charge separation..
58. J. Itatani, M. Rini, A. Cavalleri, Ken Onda, T. Ishikawa, S. Ogihara, S. Koshihara, X. Shao, H. Yamochi, G. Saito, R. W. Schoenlein, Ultrafast gigantic photo-response in charge-ordered organic salt (EDO-TTF)2PF6 on 10-fs time scales, Conference on and Quantum Electronics and Laser Science Conference Lasers and Electro-Optics, QELS 2008 2008 Conference on and Quantum Electronics and Laser Science Conference Lasers and Electro-Optics, QELS, https://doi.org/10.1109/QELS.2008.4553098, 2008.09, The initial dynamics of photo-induced phase transition in (EDO-TTF) 2PF6 was investigated using 10-fs laser pulses. We observed sub-20-fs gigantic photo-responses (|ΔR/R|>100%) and a clear signature of a structural bottleneck (∼60 fs) for the first time..
59. Ken Onda, Sho Ogihara, Kenji Yonemitsu, Nobuya Maeshima, Tadahiko Ishikawa, Yoichi Okimoto, Xiangfeng Shao, Yoshiaki Nakano, Hideki Yamochi, Gunzi Saito, Shin Ya Koshihara, Photoinduced change in the charge order pattern in the quarter-filled organic conductor (EDO-TTF)2PF6 with a strong electron-phonon interaction, Physical Review Letters, https://doi.org/10.1103/PhysRevLett.101.067403, 101, 6, 2008.08, The quasistable state in the photoinduced phase transition for the quasi-one-dimensional quarter-filled organic conductor (EDO-TTF)2PF6 has been examined by ultrafast reflective measurements and time-dependent model calculations incorporating both electron-electron and electron-phonon interactions. The transient optical conductivity spectrum over a wide probe photon-energy range revealed that photoexcitation induced a new type of charge-disproportionate state. Additionally, coherent and incoherent oscillations dependent on probe photon energies were found, as predicted by the calculation..
60. Ken Onda, Sho Ogihara, Tadahiko Ishikawa, Yoichi Okimoto, Xiangfeng Shao, Hideki Yamochi, Gunzi Saito, Shin Ya Koshihara, The photo-induced phase and coherent phonon in the organic conductor (EDO-TTF)2PF6, Journal of Physics Condensed Matter, https://doi.org/10.1088/0953-8984/20/22/224018, 20, 22, 2008.06, We have investigated the nature of the photo-induced state and coherent phonon in the conducting charge transfer complex (EDO-TTF)2PF 6 by measuring the ultrafast reflectivity change over a wide photon energy range from 0.069 eV (18 ν m) to 2.1 eV (580 nm). The photo-induced spectra just after photo-excitation indicate that the photo-induced phase is similar to but clearly different from that in a thermally induced metal phase though they are the same for general photo-induced phase transition. The temporal profiles at each probe photon energy are accompanied by extraordinarily large amplitude oscillation originating from coherent phonon generation. The period of the coherent phonon depends discretely on the probe photon energy, while the generation efficiency of both the photo-induced phase and coherent photon reaches a maximum when the peak top of a charge transfer band is excited. These results are probably attributable to the strong electron-vibration coupling in this complex..
61. Bin Li, Jin Zhao, Min Feng, Ken Onda, Interferometric two-photon photoemission correlation technique and femtosecond wet-electron dynamics at the TiO2(110) surface, Frontiers of Physics in China, https://doi.org/10.1007/s11467-008-0001-7, 3, 1, 26-40, 2007.10, The femtosecond time-resolved two-photon photoemission (TR-2PP) and the ultra high vacuum (UHV) surface science techniques are integrated to investigate the electronic structures and the interfacial electron transfer dynamics at the atomically ordered adsorbate overlayers on TiO2 singlecrystalline surfaces. Our research into the CH3OH/TiO2 system exhibits complex dynamics, providing abundant information with regard to electron transport and solvation processes in the interfacial solvent structures. These represent the fundamentally physical, photochemical, and photocatalytic reactions of protic chemicals covered with metal-oxides..
62. J. Itatani, M. Rini, A. Cavalleri, Ken Onda, T. Ishikawa, S. Koshihara, X. Shao, H. Yamochi, G. Saito, R. W. Schoenlein, Ultrafast gigantic photo-response in organic salt (EDO-TTF)2PF6 initiated by 20-fs laser pulses, Frontiers in Optics, FiO 2007 Frontiers in Optics, FiO 2007, 2007.01, The early dynamics of photo-induced phase transition in charge-ordered organic salt (EDO-TTF)2PF6 was investigated using ultrashort near-infrared laser pulses. The gigantic photo-response (DR/R>100 %) characteristic to the insulator-to-metallic phase transition was observed <50-fs after excitation..
63. Jin Zhao, Ken Onda, Bin Li, Hrvoje Petek, Ultrafast proton-coupled electron transfer in heterogeneous photocatalysis, Physical Chemistry of Interfaces and Nanomaterials V Physical Chemistry of Interfaces and Nanomaterials V, https://doi.org/10.1117/12.678279, 6325, 2006.11, At metal-oxide/protic-solvent interfaces, partially hydrated or "wet electron" states represent the lowest energy pathway for electron transfer. Here we study the photoinduced charge transfer at the H 2O/TiO 2(110) interface by means of time-resolved two-photon photoemission spectroscopy and electronic structure theory. At ∼1 monolayer coverage of H 2O on partially hydroxylated TiO 2 surfaces we find an unoccupied electronic state 2.4±0.1 eV above the Fermi level. Density functional theory shows this to be a two-dimensional "wet electron" state, which is distinct from hydrated electrons observed on water-covered metal surfaces. The decay of electrons from the wet electron state by the resonant charge transfer to the conduction band of TiO 2 occurs in ≤15 femtoseconds. Similar unoccupied electronic structure is observed for CH 3OH covered TiO 2(110) surfaces; however, the electron dynamics are considerably more complex. The wet electron state dynamics of CH 3OH/TiO 2 exhibit both energy and population decay. The excited state lifetime is strongly coverage dependent increasing to >100 fs range above 1 ML CH 3OH coverage. Significantly, a pronounced deuterium isotope effect (CH 3OD) indicates a strong correlation between the interfacial electron transfer and the motion of protons in the molecular overlayer..
64. Jin Zhao, Bin Li, Ken Onda, Min Feng, Hrvoje Petek, Solvated electrons on metal oxide surfaces, Chemical Reviews, https://doi.org/10.1021/cr050173c, 106, 10, 4402-4427, 2006.10, Solvated electrons can be introduced into protic solvents through the dissolution of highly reducing species such as alkali metals or through the exposure of liquids to ionizing radiation. Molecular cluster anions provide a versatile environment where electron solvation can be studied through cluster-size selection over a range from the single molecule to the near-bulk solvent limits. TR-2PP has been used to study electron solvation for a variety of solvent molecule overlayers on metal surfaces including alkanes, alcohols, nitriles and water. On metal surfaces, the solvation dynamics are governed by the degree of crystallinity of ice and the electronic structure of the substrate. The two-dimensional solvated electrons at solvent surfaces may be particularly reactive metastable reagents that can participate in a variety of interfacial reactions..
65. Bin Li, Jin Zhao, Ken Onda, Kennath D. Jordan, Jinlong Yang, Hrvoje Petek, Ultrafast interfacial proton-coupled electron transfer, Science, https://doi.org/10.1126/science.1122190, 311, 5766, 1436-1440, 2006.03, The coupling of electron and nuclear motions in ultrafast charge transfer at molecule-semiconductor interfaces is central to many phenomena, including catalysis, photocatalysis, and molecular electronics. By using femtosecond laser excitation, we transferred electrons from a rutile titanium dioxide (110) surface into a CH3OH overlayer state that is 2.3 ± 0.2 electron volts above the Fermi level. The redistributed charge was stabilized within 30 femtoseconds by the inertial motion of substrate ions (polaron formation) and, more slowly, by adsorbate molecules (solvation). According to a pronounced deuterium isotope effect (CH3OD), this motion of heavy atoms transforms the reverse charge transfer from a purely electronic process (nonadiabatic) to a correlated response of electrons and protons..
66. Jiro Itatani, Matteo Rini, Andrea Cavalleri, Ken Onda, Tadahiko Ishikawa, Shin ya Koshihara, Xiangfeng Shao, Hideki Yamochi, Gunzi Saito, Robert W. Shoenlein, Ultrafast gigantic photo-response in (EDO-TTF)2PF6 initiated by 10-fs laser pulses, International Conference on Ultrafast Phenomena, UP 2006 International Conference on Ultrafast Phenomena, UP 2006, 2006.01, We photoexited a charge-ordered organic salt (EDO-TTF)2PF6 with sub-10-fs optical pulses. The photo-induced metallic phase appeared within 50-fs after pumping, characterized by large changes in reflectivity (ΔR/R~0.8) followed by strong coherent phonon modulation..
67. Ken Onda, Bin Li, Jin Zhao, Hrvoje Petek, The electronic structure of methanol covered TiO2(110) surfaces, Surface Science, https://doi.org/10.1016/j.susc.2005.06.044, 593, 1-3, 32-37, 2005.11, We report on the electronic structure rutile TiO2(1 1 0) surfaces following the exposure to methanol. Two-photon photoemission (2PP) spectra reveal a strong resonance at 2.2-2.4 eV above Fermi level, which gains maximum intensity at ∼1 ML coverage of methanol. From the coverage, temperature and emission angle dependence of the methanol induced spectral features and similarities to a related state on water covered TiO2(1 1 0) surfaces, we assign the resonance to charge transfer excitation from the reduced five-coordinate Ti5c+4-δ ions to hydroxyl groups on the bridging oxygen rows..
68. Atsushi Kubo, Ken Onda, Hrvoje Petek, Zhijun Sun, Yun S. Jung, Hong Koo Kim, Femtosecond imaging of surface plasmon dynamics in a nanostructured silver film, Nano Letters, https://doi.org/10.1021/nl0506655, 5, 6, 1123-1127, 2005.06, Light interacting with nanostructured metals excites the collective charge density fluctuations known as surface plasmons (SP). Through excitation of the localized SP eigenmodes incident light is trapped on the nanometer spatial and femtosecond temporal scales and its field is enhanced. Here we demonstrate the imaging and quantum control of SP dynamics in a nanostructured silver film. By inducing and imaging the nonlinear two-photon photoemission from the sample with a pair of identical 10-fs laser pulses while scanning the pulse delay, we record a movie of SP fields at a rate of 330-attoseconds/frame..
69. Ken Onda, Bin Li, Jin Zhao, Kenneth D. Jordan, Jinlong Yang, Hrvoje Petek, Wet electrons at the H2O/TiO2(110) surface, Science, https://doi.org/10.1126/science.1109366, 308, 5725, 1154-1158, 2005.05, At interfaces of metal oxide and water, partially hydrated or "wet-electron" states represent the lowest energy pathway for electron transfer. We studied the photoinduced electron transfer at the H 2O/TiO2(110) interface by means of time-resolved two-photon photoemission spectroscopy and electronic structure theory. At ∼1-monolayer coverage of water on partially hydroxylated TiO2 surfaces, we found an unoccupied electronic state 2.4 electron volts above the Fermi level. Density functional theory shows this to be a wet-electron state analogous to that reported in water clusters and which is distinct from hydrated electrons observed on water-covered metal surfaces. The decay of electrons from the wet-electron state to the conduction band of TiO2 occurs in ≤15 femtoseconds..
70. L. Guérin, D. Glijer, N. Moisan, M. Lorenc, M. Buron Le Cointe, E. Collet, H. Cailleau, A. Ota, G. Saito, X. Shao, H. Yamochi, M. Chollet, Ken Onda, T. Ishikawa, S. Koshihara, Phase transition in (EDO-TTF) 2PF 6
Domain growth in the thermal hysteresis and ultra-fast photoinduced effects, Journal of Physics: Conference Series, https://doi.org/10.1088/1742-6596/21/1/024, 21, 1, 149-154, 2005.01, The first order phase transition between the metal (M) and insulating (I) phases of the molecular compound (EDO-TTF) 2PF 6 is investigated by single crystal x-ray diffraction. The coexistence of the insulating and metallic phases and the growth of the domains in the thermal hysteresis are clearly observed during the phase transition. We also present ultra-fast optical experiments using a nitrogen gas flow cryostat. A change of the reflectivity in the photoinduced phase was observed just after excitation. We will also discuss the influence of the excitation light polarisation on the efficiency of the photo-induced I-M phase transition..
71. Ken Onda, Tadahiko Ishikawa, Matthieu Chollet, Xiangfeng Shao, Hideki Yamochi, Gunzi Saito, Shin Ya Koshihara, Ultrafast infrared spectroscopic study of the photo-induced phase transition in (EDO-TTF)2PF6, Journal of Physics: Conference Series, https://doi.org/10.1088/1742-6596/21/1/036, 21, 1, 216-220, 2005.01, We have measured ultrafast reflectivity changes of (EDO-TTF) 2PF6 in the infrared region (0.51 eV - 1.03 eV) after excitation with a 1.58 eV photon in order to reveal the mechanism of ultrafast photo-induced insulator-to-metal phase transition. We found that the temporal profiles of reflectivity change have two components: a faster component within 3 ps associated with a 1-ps or 0.5-ps period oscillation and a slower component lasting over hundreds of a picosecond. In addition, we found that the sign of a reflectivity change is often different from that expected from the thermally induced phase transition. These results indicate that the PIPT takes place via a complicated process due to strong electron and phonon coupling..
72. Ken Onda, Tadahiko Ishikawa, Matthieu Chollet, Shao Xiangfeng, Hideki Yamohchi, Gunzi Saito, Shin Ya Koshihara, Ultrafast infrared spectroscopic study of the photo-induced phase transition in (EDO-TTF)2PF6, International Quantum Electronics Conference 2005, https://doi.org/10.1109/IQEC.2005.1560823, 2005, 8-9, 2005, We have measured ultrafast reflectivity change of (EDO-TTF) 2PF6 in the infrared region (0.51 eV - 1.03 eV) after excitation with 1.58 eV photon in order to reveal mechanism of ultra-fast photo-induced insulator-to-metal phase transition..
73. Ken Onda, Li Bin, Zhao Jin, Kenneth D. Jordan, Yang Jinlong, Hrvoje Petek, Ultrafast photo-induced dynamics at H2O/TiO2(110) and CH3OH/TiO2(110) interfaces, International Quantum Electronics Conference 2005, https://doi.org/10.1109/IQEC.2005.1560866, 2005, 222-223, 2005, We observe ultrafast photo-induced electron dynamics at well-characterized H2O/TiO2(110) and CH3OH/TiO2(110) interfaces by time-resolved two-photon photoemission (TR-2PP) spectroscopy. We find short lifetime wet-electron state at H2O/TiO2(110) and slow solvation process at CH3OH/TiO2(110)..
74. T. Okada, I. Otake, R. Mizoguchi, Ken Onda, S. S. Kano, A. Wada, Optical control of two-photon excitation efficiency of α-perylene crystal by pulse shaping, Journal of Chemical Physics, https://doi.org/10.1063/1.1787490, 121, 13, 6386-6391, 2004.10, An optimized pulse shaping technique was used to examine optical control of two-photon excitation efficiency of an α-perylene crystal in the temperature region from 30 to 290 K. It was observed that a pulse train with a pulse interval of 90 fs and an alternately reversing phase relation increased the excitation efficiency by a factor of 2 for the whole temperature region. A sliding-window Fourier transform of the pulse shape was used to discuss the mechanism of the efficiency increase. It was concluded on the basis of double pulse experimental results that a pulse interval of 90 fs and an alternately reversing phase relation play important roles in the efficiency increase..
75. Ken Onda, Bin Li, Hrvoje Petek, Two-photon photoemission spectroscopy of TiO2(110) surfaces modified by defects and O2 or H2O adsorbates, Physical Review B - Condensed Matter and Materials Physics, https://doi.org/10.1103/PhysRevB.70.045415, 70, 4, 2004.07, Two-photon photoemission (2PP) spectra of TiO2(110) surfaces are measured for the nearly perfect surface, and surfaces modified by introduction of defects and adsorbed molecules. Defects are generated on nearly perfect surfaces by three methods: electron irradiation, annealing in vacuum, and Ar+ sputtering. Nearly perfect or damaged surfaces can be further modified by adsorption of O2 or H2O molecules. 2PP spectroscopy is used to systematically investigate the work function change due to the presence of defects or adsorbates. 2PP spectroscopy detects both surface and bulk oxygen vacancy defects. We find from the results on oxygen adsorption that oxygen vacancies created by electron irradiation, are localized on the surface and may be removed by O2 adsorption at 100 K. The surface defects are substantially different from those created by annealing or by ion sputtering where vacancies in the subsurface region are proposed. We find that O2 acts as an acceptor molecule on surface defect states whereas H2O acts as a donor molecule. From simulation of the work function change as a function of dosage, the dipole moment of H2O adsorbed on TiO2 surface is derived to be 0.5 D positive outward. We also find an unoccupied electronic state 2.45 eV above the Fermi level that appears at submonolayer coverage of H2O, which we tentatively assign to charge transfer from surface titanium ions to the surface-adsorbed H2O molecules or OH ligands..
76. Atsushi Kubo, Ken Onda, Hrvoje Petek, Zhijun Sun, Yun Suk Jung, Hong Koo Kim, Imaging of localized silver plasmon dynamics with sub-fs time and nano-meter spatial resolution, Springer Series in Chemical Physics, 79, 645-649, 2004, Images of local plasmons on a silver grating excited by 400-nm, 10-fs pulses are obtained with 50-as time and 50-nm spatial resolution. The technique is based on interferometric time-resolved two-photon photoemission and photoelectron emission microscopy..
77. Ken Onda, Bin Li, Hrvoje Petek, Surface femtochemistry
Photocatalytic reaction dynamics of methanol/TiO2(110), Springer Series in Chemical Physics, 79, 416-418, 2004, We observe a strong charge-transfer resonance for the methanol/TiO 2(110) surface by time-resolved two-photon photoemission spectroscopy. The resonance decays in a two-component process corresponding to the charge transfer induced wave packet motion followed by solvation by the surrounding molecules..
78. Ken Onda, Kinuka Tanabe, Hidenori Noguchi, Kazunari Domen, Akihide Wada, Dynamic processes of olefins adsorbed on hydroxyl groups of DM20 zeolite excited by picosecond infrared pulses
Vibrational relaxation, short-lived species and desorption, Journal of Physical Chemistry B, 107, 41, 11391-11396, 2003.10, We have investigated olefins adsorbed on Brønsted acidic hydroxyl groups (OD) of DM20 zeolite by transient infrared absorption spectra after excitation of the OD stretching mode using picosecond infrared lasers. The olefins studied were ethylene, deuterated ethylene, propylene, and isobutene, and only isobutene was adsorbed on OD at its methyl group while the other olefins were adsorbed at their double bonds. For all the olefins except isobutene, simple vibrational relaxation of the ν = 1 state was observed and the lifetimes were 2-4 ps. These lifetimes are an order faster than that of isolated OD. For isobutene, a long lifetime component lasting more than 500 ps was observed in addition to vibrational relaxation of 2 ps. The transient absorption spectra for isobutene-adsorbed zeolite at various time delays and frequencies indicated the existence of a short-lived species and a slow desorption process compared with vibrational relaxation..
79. Satoshi Katano, Shinsaku Dobashi, Jun Kubota, Ken Onda, Akihide Wada, Satoru S. Kano, Kazunari Domen, Structural change of CO adsorbed on Pt(111) by laser heating
Time-resolved sum-frequency generation study, Chemical Physics Letters, https://doi.org/10.1016/S0009-2614(03)01194-1, 377, 5-6, 601-606, 2003.08, The IR-visible sum-frequency generation (IVSFG) method was used to perform time-resolved vibrational measurements of CO/Pt(111). Irradiation of intense picosecond laser pulses (35 ps, 1064 nm) resulted in a drop in intensity, redshift, and broadening of the on-top CO stretching peak, followed by recovery to the original peak shape on the order of hundreds of picoseconds. The transient behavior of the CO stretching signal was found to depend on the CO coverage. The transient spectra at saturation coverage consisted of a single peak, whereas at low coverage (0.25 ML) the line shape became asymmetric and each transient peak could be deconvoluted to the multiple components. The origin of the remarkable change in the transient spectral features at the 0.25 ML surface is discussed..
80. R. Mizoguchi, Ken Onda, S. S. Kano, A. Wada, Thinning-out in optimized pulse shaping method using genetic algorithm, Review of Scientific Instruments, https://doi.org/10.1063/1.1569398, 74, 5, 2670-2674, 2003.05, A study was performed to improve an optical pulse shaping system using a spatial light modulator (SLM) controlled by genetic algorithm (GA) optimization. It was demonstrated that the time for finding the optimal condition could be shortened by a proposed 'thinning out' method. The influence of the thinning out on the pulse shape was discussed..
81. H. Noguchi, T. Okada, Ken Onda, S. S. Kano, A. Wada, K. Domen, Time-resolved SFG study of formate on a Ni( 1 1 1 ) surface under irradiation of picosecond laser pulses, Surface Science, https://doi.org/10.1016/S0039-6028(02)02630-4, 528, 1-3, 183-188, 2003.03, Time-resolved sum-frequency generation spectroscopy was carried out on a deuterated formate (DCOO) adsorbed on Ni(1 1 1) surface to investigate the surface reaction dynamics under instantaneous surface temperature jump induced by the irradiation by picosecond laser pulses. The irradiation of pump pulse (800 nm) caused the rapid intensity decrease of both CD and OCO stretching modes of bridged formate on Ni(1 1 1). Different temporal behaviors of intensity recovery between these two vibrational modes were observed, i.e., CD stretching mode recovered faster than OCO. This is the first result to show that the dynamics of adsorbates on metals strongly depends on the observed vibrational mode. From the results of temperature and pump fluence dependence, we concluded that the observed intensity change was not due to the decomposition or desorption, but was induced by a non-thermal process..
82. Ken Onda, Yasuhiro Iwasawa, Akihide Wada, Vibrational relaxation of adsorbate and adsorbent in the CO-adsorbed DM-20 zeolite system, Chemical Physics Letters, https://doi.org/10.1016/S0009-2614(03)00071-X, 370, 3-4, 437-442, 2003.03, We made the first observation of transient vibrational spectra of both adsorbate and adsorbent at molecule-adsorbed surface using a two-color infrared picosecond laser system. The transient measurements were carried out on the CO-adsorbed surface hydroxyl group (OD) of DM-20 zeolite by pumping the CO stretching mode (2175 cm-1) or OD stretching mode (2470 cm-1) and probing over the CO and OD stretching region (2000-2700 cm-1). The T1 lifetime for OD was 5.8 ps and that for CO was 540 ps. By comparison with other CO-adsorbed systems, vibrational relaxation mechanisms of CO on various surfaces are discussed. We also found evidence of energy flows from the vibrational mode of the adsorbent (OD stretching) to that of the adsorbate (CO stretching) via van der Waals interaction..
83. H. Ishida, R. Mizoguchi, Ken Onda, C. Hirose, S. S. Kano, A. Wada, Second harmonic observation of Cu(1 1 1) surface
In situ measurements during molecular adsorption, Surface Science, https://doi.org/10.1016/S0039-6028(02)02651-1, 526, 1-2, 201-207, 2003.02, Second harmonic generation (SHG) spectroscopy using a tunable femtosecond laser has been demonstrated as an effective and a practical in situ monitor of surface electronic states during adsorption processes. We have successfully shown this technique to be suitable for the study of surface electronic states, not only those induced by adsorbed molecules but also those associated with clean surfaces. By observing the change in the SH signals from a Cu(1 1 1) surface during exposure to benzene or CO molecules, we discovered new resonances of the clean Cu surface that could not be ascribed to the well-known surface state (SS)-image state (IS) transition. One of these was from a surface site that is less likely to adsorb benzene, where the SH signal intensity was kept constant until the Cu surface was covered by the sub-monolayer..
84. Athula Bandara, Satoru S. Kano, Ken Onda, Satoshi Katano, Jun Kubota, Kazunari Domen, Chiaki Hirose, Akihide Wada, SFG spectroscopy of CO/Ni(111)
UV pumping and the transient hot band transition of adsorbed CO, Bulletin of the Chemical Society of Japan, https://doi.org/10.1246/bcsj.75.1125, 75, 5, 1125-1132, 2002.05, A UV excitation by a picosecond pulse at 266 nm induced an unusual shoulder on the v CO = 1 ← 0 resonance peak of CO/Ni(111) monitored by sum-frequency generation (SFG) of visible and IR pulses. The observed line shape was reproduced by the use of a dipole-dipole interaction model with the coherent potential approximation (CPA) where the hot band transition with a population ratio of 0.3 to 0.7 (v = 1 to v = 0) was assumed. Neither the transition to the two-phonon bound state nor the coupling with the low-frequency phonon modes explained the observed changes. The shoulder appeared only during the UV excitation, which indicated that the electronically driven excitation, presumably by the hot electrons generated by the irradiation, dominated the process. As possible mechanisms, the involvement of an intermediate negative ion resonance state and/or the non-adiabatic coupling of electronic states with C-O stretching mode were considered..
85. Ken Onda, Masaru Nakagawa, Toshiaki Asakai, Reiko Watase, Akihide Wada, Kunihiro Ichimura, Chiaki Hirose, Controlling packing structure of hydrophobic alkyl tails of monolayered films of ion-paired macrocyclic amphiphiles as studied by sum-frequency generation spectroscopy, Journal of Physical Chemistry B, https://doi.org/10.1021/jp013939v, 106, 15, 3855-3859, 2002.04, Two kinds of monolayered films of ion-paired macrocyclic amphiphiles formed from anionic surfactant 1,2-bis[(dodecyloxy)carbonyl]ethane-1-sulfonate and tetracationic macrocycles with different molecular sizes were prepared on hydrophilic silica plates by the Langmuir-Blodgett technique. Vibrational sum-frequency generation (SFG) spectroscopy was applied to investigate the packing structure of the alkyl chains of the films. The SF intensity ratio of the CH symmetric stretching mode of the methylene group to that of methyl group in the alkyl chains increased when the occupied area of the macrocycle was larger than the sum of the cross-sectional areas of the hydrophobic alkyl tails. The appearance of a methylene CH band in the SFG spectra implies the presence of gauche conformation in the alkyl chains. This indicates that the molecular size of the cyclic tetracation hydrophilic head exclusively governs the packing structure of the alkyl group hydrophobic tails in the monolayer state. The results show that monolayer fabrication using ion-paired macrocyclic amphiphiles has advantages in allowing control over the packing structure of the outermost alkyl chains of solid surfaces through the choice of the size of the tetracation..
86. Ken Onda, Akihide Wada, Kazunari Domen, Chiaki Hirose, Realtime observation of the desorption process of isobutene from zeolite using picosecond infrared lasers, Surface Science, https://doi.org/10.1016/S0039-6028(01)01970-7, 502-503, 319-324, 2002.04, The transient absorption spectra of OD stretching mode of isobutene-adsorbed hydroxyl groups of DM20 zeolite were measured by picosecond infrared-infrared pump-probe experiment. The vibrational relaxation time (T1) of the ν = 1 state was estimated to be 12±2 ps from the temporal profile of the bleaching and hot bands of the OD stretching mode. We found a band indicating the production of a short-lived species at 40 ps after excitation. Moreover we observed the course of isobutene-desorption in the time region from 40 to 100 ps. This result implies that this desorption does not occur immediately but occurs after local vibrational relaxation..
87. K. Kusafuka, H. Noguchi, Ken Onda, J. Kubota, K. Domen, C. Hirose, A. Wada, Time-resolved study of formate on Ni(1 1 1) by picosecond SFG spectroscopy, Surface Science, https://doi.org/10.1016/S0039-6028(01)01969-0, 502-503, 313-318, 2002.04, Time-resolved vibrational measurements were carried out on formate (HCOO) adsorbed on Ni(1 1 1) surface by combining the sum-frequency generation method and picosecond laser system (time resolution of 6 ps). Rapid intensity decrease (within the time resolution) followed by intensity recovery (time constant of several tens of ps) of CH stretching signal was observed when picosecond 800 nm pulse was irradiated on the sample surface. From the results of temperature and pump fluence dependences of temporal behaviour of signal intensity, we concluded that the observed intensity change was induced by non-thermal process. Mechanism of the temporal intensity change was discussed..
88. Ken Onda, Kinuka Tanabe, Hidenori Noguchi, Akihide Wada, Takafumi Shido, Aritomo Yamaguchi, Yasuhiro Iwasawa, Studies of the surface deuterioxyl group and adsorbed D2O on γ-Al2O3 using picosecond infrared pump probe spectroscopy, Journal of Physical Chemistry B, https://doi.org/10.1021/jp0118625, 105, 46, 11456-11461, 2001.11, Picosecond infrared-infrared pump-probe experiments in the OD stretching region were carried out on dehydrated and multilayer water-adsorbed γ-Al2O3. For the dehydrated γ-Al2O3, transient bands assigned to the bleaching and hot bands of the OD stretching mode of isolated surface hydroxyl groups were observed, and the population lifetime (T1 of the vibrational excited state (v = 1) of the mode was 200 ± 10 ps at 293 K. The characteristic temperature dependence of T1 is indicative of a seven-phonon process. For the water-adsorbed γ-Al2O3, the transient bleaching and transient hot bands with a lifetime of 10-12 ps were observed by excitation of the absorption peak at 2630 cm-1. In addition to these bands, weak transient bands were observed in the low-frequency region. These results are considered evidence of the existence of isolated water molecules on the γ-Al2O3 surface. Pure dephasing is also discussed based on the bandwidth of the observed transient bands..
89. K. Domen, K. Kusafuka, A. Bandara, M. Hara, J. N. Kondo, J. Kubota, Ken Onda, A. Wada, C. Hirose, Observation of unstable reaction intermediate by picosecond tunable infrared laser pulses, Studies in Surface Science and Catalysis, 130 A, 365-370, 2000.12, The picosecond tunable infrared laser pulses enabled us to carry out the vibrational spectroscopic measurements of the species adsorbed on surfaces with picosecond time-resolution. The irradiation of near-infrared pump pulses causes thermal excitation of the surface, and the change of adsorbates was probed by sum-frequency generation (SFG) spectroscopy. We applied this method to the direct observation of unstable species of formate on NiO(111) surface. The stable bidentate formate on NiO(111) was transformed to monodentate by the irradiation of pump pulses, and the dynamic behavior of the newly observed monodentate formate was followed as a precursor for the decomposition of formate. The application of the present method has extended to the study of methoxy species on Ni(111)..
90. Athula Bandara, Jim Kubota, Ken Onda, Akihide Wada, Kazunari Domen, Chiaki Hirose, Short-lived reactive formate species on NiO(111) observed by picosecond temperature jump, Surface Science, https://doi.org/10.1016/S0039-6028(99)00076-X, 433, 83-87, 1999.08, The combination of an instantaneous temperature jump induced by the irradiation of picosecond laser pulses and the subsequent observation by sum-frequency generation spectroscopy enabled the identification of a short-lived reactive intermediate in the decomposition of formate on NiO(111) surface. The irradiation of 1064 nm laser pulses caused the vibrational peak at 2160 cm-1, which is the C-D stretching mode vCD of bidentate formate, to weaken and the vCD of monodentate formate at 2190 cm-1 to appear. The spectral changes recovered in 100 ps time scale but not fully above 400 K. The observations suggested that the formate in bidentate configuration transformed to monodentate formate prior to decomposition. Temperature- and time-dependent features indicated that the two types of formate were in equilibrium and the equilibrium shifted towards the monodentate form by a rapid laser-induced temperature jump of about 250-300 K. The experiment enabled us to identify short-lived species and to estimate kinetic parameters. The enthalpy difference between bidentate and monodentate formates and the activation energy for the decomposition of monodentate formate were 19 ± 5 and 30 ± 10 kJ/mol, respectively..
91. Kazunari Domen, Athula Bandara, Jun Kubota, Ken Onda, Akihide Wada, Satoru S. Kano, Chiaki Hirose, SFG study of unstable surface species by picosecond pump-probe method, Surface Science, https://doi.org/10.1016/S0039-6028(99)00301-5, 427-428, 349-357, 1999.06, We succeeded in identifying the intermediates in thermal decomposition reactions by utilizing the combination of an instantaneous temperature jump induced by the irradiation of picosecond laser pulses and the subsequent observation by time-resolved sum-frequency generation (TR-SFG) spectroscopy. The short-lived reactive intermediates in the decomposition of formate on NiO(111) and Ni(111) surfaces were identified. The irradiation of 1064 nm laser pulses caused the vibrational peak of the C-D stretching mode vCD of bidentate formate on NiO(111) to weaken and the vCD band of monodentate formate to appear. The result on Ni(111) showed the weakening of the vCD band of bridging formate and the appearance of the C double bond O stretching mode vC double bond O of monodentate formate. The spectral changes recovered on a 100 ps time scale but not fully above 400 K for the NiO(111) system and 320 K for the Ni(111) system, indicating the onset of thermal decomposition in the high-temperature period. The observations suggested that the formate in the stable bidentate/bridging configurations transformed to unstable monodentate formate prior to decomposition. Temperature- and time-dependent features indicated that the two types of formate were in equilibrium and the equilibrium shifted towards the monodentate form by the rapid laser-induced temperature jump of about 250-300 K..
92. S. Katano, A. Bandara, J. Kubota, Ken Onda, A. Wada, K. Domen, C. Hirose, Screening of SFG signals from bridged CO on Ni(111) by the coexistence of linear CO, Surface Science, https://doi.org/10.1016/S0039-6028(99)00299-X, 427-428, 337-342, 1999.06, A spectroscopic study of adsorbed CO on Ni(111) was carried out using infrared-visible sum-frequency generation (SFG) and infrared reflection absorption spectroscopy (IRAS). An anomalous coverage-dependence of the SFG signal intensities of bridged and linear CO was found. While the SFG signal of bridged CO at 1920 cm-1 gained intensity with increasing CO coverage in the region between 0 and 0.4 ML (1 monolayer, ML: saturation coverage at 130 K), it weakened and disappeared when linear CO (2074 cm-1) was present on the surface above 0.47 ML. The IRA peak of bridged CO, however, did not disappear at the increased coverage suggesting that the weakening of the SFG signal of bridged CO is not caused by the decrease in the coverage but by the suppression of the SFG process of bridged CO by the coexistence of linear CO. The spectra of isotope mixtures of C16O and C18O clearly indicated that the weakening of SFG signals is not caused by the dipole-dipole coupling between the vibrational modes of bridged and linear CO. The anomalous coverage-dependence of the SFG signal was ascribed to the change of the Raman tensor by the coadsorption of linear CO..
93. Athula Bandara, Jun Kubota, Ken Onda, Akihide Wada, Satoru S. Kano, Kanzunari Domen, Chiaki Hirose, Time-resolved SFG study of the vibrational excitation of adsorbed CO on Ni(111) and NiO(111) surfaces under the irradiation of UV and visible photons, Surface Science, https://doi.org/10.1016/S0039-6028(99)00298-8, 427-428, 331-336, 1999.06, The effects of irradiation by picosecond ultraviolet (266 nm) and visible (532 nm) pulses of CO-covered Ni(111) and NiO(111)/Ni(111) surfaces were investigated by infrared-visible sum-frequency generation (SFG) spectroscopy. As a result of the irradiation of UV pulses, the SFG signal by the v = 1 ← 0 bands of the CO stretching mode weakened and transient peaks tentatively assigned to the v = 2 ← 1 hot band transition appeared with a downshift by 18 and 14 cm-1 from v = 1 ← 0 bands on the metal and oxide-layered surfaces, respectively. The time profile of the weakening and that of the transient signal were coincident with the same rise and fall times of less than approx. 10 ps. The irradiation of visible pulses also induced a weakening of the v = 1 ← 0 bands and transient peaks, but the decay time on the metal surface was longer than 100 ps. The temporal and spectral features imply that the persistent v = 1 ← 0 band and the transient band originated from the same CO molecule. Short decay times observed on irradiating UV pulses on to the Ni(111) and NiO(111) surfaces and that of the visible pulses on to the NiO(111) surface suggest that the photo-generated hot electrons induced the observed spectral changes. The long decay time observed on irradiating visible pulses on to the Ni(111) surface suggests the occurrence of a thermally driven process. Participation of the negative ion state was considered to be one of the possible mechanisms for the rapidly decaying features..
94. Ken Onda, Michio Yaginuma, Akihide Wada, Kazunari Domen, Chiaki Hirose, Vibrational relaxation process of polyatomic molecules adsorbed in zeolites, Laser Chemistry, 19, 1-4, 321-324, 1999, The vibrational relaxation lifetimes of the CO stretching mode of Cr(CO)6 and Mo(CO)6 adsorbed in the cage of the HY, DY and NaY-type zeolites were measured at various temperature by pump-probe method using picosecond infrared laser pulses. It was shown by comparing the lifetimes that the accepting modes include both the vibrational modes of the metal carbonyl and those associated with the cations on the zeolite surface. The analysis of the temperature dependence of the lifetimes revealed that the number of the excited accepting modes are four with their energy lying around 500 cm-1..
95. T. Fujino, M. Kashitani, Ken Onda, A. Wada, K. Domen, C. Hirose, M. Ishida, F. Goto, S. S. Kano, Picosecond infrared pump-probe spectrum of D2O adsorbed at acid OD group of zeolite, Journal of Chemical Physics, https://doi.org/10.1063/1.476816, 109, 6, 2460-2466, 1998.12, Spectroscopic and dynamic features of the vibrationally excited D2O/zeolite system have been investigated by two-color infrared-infrared pump-probe experiment. The frequency- and delay-scanned probe intensities were measured by tuning the pump laser to the OD stretching bands of the D2O molecule hydrogen bonded to the acidic OD group of mordenite zeolite. Two types of pump-induced signals were observed: the ones which have the population lifetime of 43±5 ps and display frequency shift by the pumping frequency, and the others, which have the lifetime of about 15 ps and exhibit no such frequency shift. Possible origins of the signals are discussed..
96. Tetsuo Yuzawa, Tatsutoshi Shioda, Jun Kubota, Ken Onda, Akihide Wada, Kazunari Domen, Chiaki Hirose, Polarization characteristics from SFG spectra of clean and regulatively oxidized Ni(100) surfaces adsorbed by propionate and formate, Surface Science, 416, 1-2, 1998.10, Experiments have been carried out on propionate and formate adsorbed on clean and regulatively oxidized Ni(100) surfaces to scrutinize the notion that the polarization characteristics of infrared-visible sum frequency generation (SFG) signals of molecules on metallic surfaces are different from those due to molecules on insulating surfaces. Both propionate and formate give signals only for p-polarized visible pulses on clean metal and oxygen-saturated surfaces, but signals due to s-polarized visible pulses appeared from the propionate-covered surface as soon as the surface was covered with a monolayer of NiO(100). Vibrational peaks of propionate were located at 2887, 2945, and 2988 cm-1 and were assigned to the symmetric stretching mode of CH2 group, the symmetric stretching mode of CH3 group, and the degenerate stretching mode of CH3 group, respectively. The peak of the surface formate was located at 2948 cm-1 on the clean metal surface, but another band appeared at 2860 cm-1 on oxide-layered surfaces. The causes of the appearance of the (sp) polarization signals by the presence of the NiO layer are discussed..
97. Athula Bandara, Jun Kubota, Ken Onda, Akihide Wada, S. S. Kano, Kazunari Domen, Chiaki Hirose, Short-lived reactive intermediate in the decomposition of formate on NiO(111) surface observed by picosecond temperature jump, Journal of Physical Chemistry B, 102, 31, 5951-5954, 1998.07, Unstable intermediates are indispensable ingredients of chemical reactions, but their identification at solid surfaces has been hampered by the lack of techniques to detect short-lived and fractionally generated species. We succeeded in identifying the intermediate in a thermal decomposition reaction by utilizing the picosecond temperature jump induced by the irradiation of laser pulses and the observation by sum-frequency generation spectroscopy. We verified that the decomposition of formate on the NiO(111) surface is preceded by the transformation of stable bidentate formate to unstable monodentate formate, which is the reactive intermediate, and two species are found to be in equilibrium before decomposition..
98. Athula Bandara, Satoshi Katano, Jun Kubota, Ken Onda, Akihide Wada, Kazunari Domen, Chiaki Hirose, The effect of co-adsorption of on-top CO on the sum-frequency generation signal of bridge CO on the Ni(111) surface, Chemical Physics Letters, 290, 1-3, 261-267, 1998.06, Anomalous coverage dependence was observed on the sum-frequency generation signal of bridge CO on the Ni(111) surface when co-adsorbed by on-top CO. The bridge CO peak was distinct and normal from 0.04 to ~0.40 monolayer (ML) but weakened after the appearance of the on-top CO peak at 0.47 ML and disappeared at 0.60 ML, although both peaks were observed in the infrared reflection absorption spectra up to 1.0 ML. The spectra of mixtures of C16O and C18O eliminate the possibility that the weakening of the bridge CO band is caused by its coupling with the on-top band..
99. Ken Onda, Michio Yaginuma, Toshiya Yokota, Akihide Wada, Kazunari Domen, Chiaki Hirose, Satoru S. Kano, Relaxation process of CO stretching mode of metal carbonyls adsorbed on Y zeolites studied by pump-probe experiment, Journal of Chemical Physics, https://doi.org/10.1063/1.476004, 108, 14, 5935-5941, 1998.04, The vibrational relaxation lifetimes of Mo(CO)6 and Cr(CO)6, adsorbed in a cage of HY, DY, and NaY zeolites, were measured at various temperatures by the pump-probe method using picosecond infrared laser pulses. The lifetimes at 110 K of the υ = 1 level of the CO stretching mode were 70±4, 53±5, and 150±9 ps for Mo(CO)6 on HY, DY, and NaY zeolite, respectively, and 87 ±6, 74±7, and 140±12 ps for Cr(CO)6, respectively. It was shown by comparison of the lifetimes at 110 K, as well as the temperature dependence, that the rate of vibrational relaxation is determined by the coupling of the CO stretching mode with four quanta of the accepting modes comprised of: (i) modes with displacement of central metals Cr or Mo in all of the three zeolite systems, (ii) modes associated with the OH and OD groups on zeolite surface in the HY and DY zeolite systems, and probably (iii) the frame vibrational modes of zeolite with energies at 300-800 cm-1..
100. Athula Bandara, Shinsaku Dobashi, Jun Kubota, Ken Onda, Akihide Wada, Kazunari Domen, Chiaki Hirose, Satoru S. Kano, Adsorption of CO and NO on NiO (111)/Ni(111) surface studied by infrared-visible sum frequency generation spectroscopy, Surface Science, 387, 1-3, 312-319, 1997.10, Adsorption of CO and NO on NiO(111) thin films epitaxially grown on Ni(111) substrate has been studied by infrared-visible sum frequency generation (SFG) and the results were compared with those of infrared reflection absorption spectroscopy. From SFG measurements, the C-O stretching band of adsorbed CO was observed at 2144 cm-1 for both p- and s-polarized visible (532 nm) light whereas the adsorbed NO gave the N-O stretching band at 1800 cm-1 only for the p-polarized visible light. These observations suggested that the CO molecule was inclined to the surface whereas the tilt angle of NO from the surface normal was smaller than that of CO. The adsorption sites of CO and NO molecules are located on the slopes of trigonal microfacets formed by the reconstruction of the NiO(111) surface. CO adsorbed on Ni(111) instead of NiO(111) was also examined: the SFG signal corresponding to the C-O stretching mode of linearly bonded CO was observed at 2076 cm-1 only for the p-polarized visible light, but that of the bridge-bonded one (at saturation coverage) was not detected by SFG..
101. Tetsuo Yuzawa, Jun Kubota, Ken Onda, Akihide Wada, Kazunari Domen, Chiaki Hirose, A TPD and SFG study of propionic acid adsorbed on Ni(110) surface, Journal of Molecular Structure, https://doi.org/10.1016/S0022-2860(97)00151-8, 413-414, 307-312, 1997.09, Propionic acid adsorbed on Ni(110) surface was studied by temperature programmed desorption (TPD) and infrared-visible sum frequency generation (SFG) spectroscopy. The TPD results indicated that propionic acid molecularly adsorbed on the Ni(110) surface at 113 K and that the multilayers of propionic acid started to desorb at 213 K. The molecularly adsorbed propionic acid on the first layer decomposed on further heating to form propionate species which decomposed at 390 K to H2(g) and CO2(g), leaving carbon on the surface. The SFG spectra of the propionic acid adsorbed in various thickness at 113 K gave vibrational peaks at 2885 cm-1, 2956 cm-1, and 2990 cm-1, assigned to the symmetric CH stretching mode of the CH2 group, symmetric CH stretching mode of the CH3 group, and antisymmetric CH stretching mode of the CH2 group, respectively, of propionic acid. The multilayered films gave peaks under both the (pp) and (sp) polarization combinations of visible and infrared pulses but the signals disappeared for the (sp) polarization combination when the coverage was lower than one monolayer (ML). The absence of the SFG peaks for the (sp) polarization combination on the low coverage surface was ascribed to the difference in molecular orientation and molecular SFG tensor. The SFG spectrum of the surface which was dosed at 300 K gave vibrational peaks at 2885 cm-1, 2948 cm-1 and 2990 cm-1 only for the (pp) polarization combination. These peaks were assigned to the symmetric CH stretching mode of the CH2 group, symmetric CH stretching mode of the CH3 group, and the antisymmetric CH stretching mode of the CH2 group of propionate, the product of dissociative adsorption..
102. Hideki Katagiri, Tokuei Sako, Akiyoshi Hishikawa, Takeki Yazaki, Ken Onda, Kaoru Yamanouchi, Kouichi Yoshino, Experimental and theoretical exploration of photodissociation of SO2 via the C1B2 state
Identification of the dissociation pathway, Journal of Molecular Structure, https://doi.org/10.1016/S0022-2860(97)00199-3, 413-414, 589-614, 1997.09, The photodissociation reaction of SO2 via the C1B2 state, SO2 (C1B2) → SO(3Σ(~)) + O(3P) was investigated by experimental and theoretical approaches cooperatively to clarify its dissociation mechanism. We measured the laser induced fluorescence (LIF) spectrum of the C-X band in the short UV wavelength region (210-200 nm) under jet-cooled conditions. The fluorescence quantum yields and the dissociation rates of individual vibronic levels were determined in the 220-200-nm region using (i) the LIF spectrum measured in the present study, (ii) that measured previously by Yamanouchi et al. (J. Mol. Struct. 352/353 (1995) 541) in the longer wavelength region above 210 nm, and (iii) the high-resolution absorption spectrum measured by Freeman et al. (Planet. Space. Sci. 32 (1984) 1125). The dissociation rates were also derived in the 210-200-nm region from the broadening of the rotational lines of the C-X vibronic transitions. It was found that the dissociation rates determined through two different procedures were consistent with each other, and that the rate increases almost exponentially as an excess energy above the dissociation threshold increases though there is a certain fluctuation of the dissociation rates reflecting a mode specificity. We also performed theoretical ab initio calculations to derive potential energy surfaces (PESs) of the electronic ground states and low-lying electronically excited states of SO2 within the MCSCF and MRCI levels. The theoretical calculations showed that (i) the PES of the 21A' state (the C1B2 state in C(2v) symmetry), correlating with the SO(1Δ) + O(1D) asymptote, crosses with the repulsive singlet (31A') state, correlating with the SO(3Σ-) + O(3P) asymptote, to form a pseudo-seam, (ii) the crossing pseudo-seam of these two PESs is located near the equilibrium bent angles for the X and C states along the energy contour of ~9700 cm-1 measured from the SO(3Σ-) + O(3P) dissociation limit, and (iii) the crossing seam between the 21A' (C1B2) and repulsive 23A' states is located in a lower energy region than the singlet seam; at ~6700 cm-1 measured from the SO(3Σ-) + O(3P) dissociation limit. On the basis of the above experimental and theoretical results together with the previous experimental evidence, we propose that (i) the photodissociation reaction via the C state proceeds mainly through the vibronic mixing between the C state vibronic levels with the quasi-bound dissociation continuum of the electronic ground X1A1 state, and (ii) the additional dissociation channels may be open through the crossing seam with the repulsive singlet (31A') state and that with the repulsive triplet (23A') state in their narrow crossing energy regions..
103. Tomoki Tasaka, Ken Onda, Akiyoshi Hishikawa, Kaoru Yamanouchi, PHOFEX spectroscopy of HgNe and HgAr
Determination of the dissociation energies of the X1σ+, A3Π 0+, and B3Π1 states, Bulletin of the Chemical Society of Japan, https://doi.org/10.1246/bcsj.70.1039, 70, 5, 1039-1045, 1997, Photofragment excitation (PHOFEX) spectroscopy was applied to determine the dissociation energies of the B3Π1 states of HgAr and HgNe. The PHOFEX spectra were measured in the wavelength region where the laser-induced fluorescence spectrum of the B3Π1-X 1Σ+ transition exhibits a continuum structure by probing the photolysis product of Hg(63P1) through the Hg(83S1-63P1) transition. By the spectral simulation of the threshold behavior of the highresolution (Δν≈0.08 cm-1) PHOFEX spectrum, the thresholds for the photodissociation reaction, HgRg→Hg(63P1)+Rg, were determined to be 39447.9(3) and 39536.0(5) cm-1 for Rg = Ne and Ar, respectively. From these thresholds, the dissociation energies, Do's, of the B3Π1 states of HgNe and HgAr were determined to be Do(B3Π1; HgNe) = 9.8(3) and D o(B3π1; HgAr) = 61.8(5) cm-1, respectively. This direct determination of the dissociation energies of the B3Π1states led to a determination of the D o's for the X1Σ+ and A 3Π0+ states; D0(X1Σ +; HgNe) = 35.6(3), D0(X1Σ+; HgAr) = 123.7(5), D0(A3Π0+; HgNe) = 68.7(3), and D0(A3Π0+; HgAr) = 348.8(6) cm -1. In addition, the (ν′,0) vibronic bands of the B 3Π1-X1Σ+ transition of HgAr were re-measured with high resolution for ν′ = 0-8. From the transition wavenumbers of these vibronic bands, the Morse potential parameters were determined with high precision as ωe = 11.94(3) cm -1 and ωcxe = 0.594(3) cm-1..
104. Ken Onda, Kaoru Yamanouchi, Electronic structure of metal-rare gas dimers with sp configuration
Application to strong spin-orbit interaction in HgAr, Journal of Chemical Physics, https://doi.org/10.1063/1.471683, 104, 23, 9376-9387, 1996.06, In order to describe the electronic states of metal (M)-rare gas (Rg) van der Waals dimers having an sp configuration with a strong spin-orbit interaction, we derived an elf parity adapted molecular Hamiltonian matrix by adopting a symmetry-adapted atomic orbital approach. The molecular Hamiltonian was constructed by introducing (i) the interaction between the p electron and the attached rare gas atom, VRg, (ii) the exchange interaction between the s and p orbitals, e2/rsp, and (iii) the spin-orbit interaction for the p electron. As a basis set, twelve molecular electronic wave functions were derived by taking into account their elf parities. We applied the derived molecular Hamiltonian matrix to the first excited 6s6p configuration of HgAr by performing a least-squares fit to the spectroscopically determined term values for the v=0 levels of the a 3Π0-, A 3Π0+, B 3Π1, b 3Π2, and C 1Π1 states. From the results of the least-squares fit, we clarified how the above interactions (i)-(iii) split twelve degenerate molecular wave functions into the eight electronic eigenstates; i.e., a 3Π0-, A 3Π0+, B 3Π1, b 3Π2, c 3Σ1 +, d 3Σ0- +, C 1Π1, and D 1Σ0 +. On the basis of (i) a critical comparison between the atomic Hamiltonian matrix for Hg and the determined molecular Hamiltonian matrix and (ii) an examination of the mixing among the symmetry-adapted molecular wave functions, characteristic features of the electronic structure arising from the formation of a van der Waals bond, were extracted..
105. T. Fujino, M. Furuki, M. Kashitani, Ken Onda, J. Kubota, J. N. Kondo, A. Wada, K. Domen, C. Hirose, F. Wakabayashi, M. Ishida, F. Goto, S. S. Kano, The effect of adsorbed noble gas atoms on vibrational relaxation of hydroxyl group in zeolite, Journal of Chemical Physics, https://doi.org/10.1063/1.471873, 105, 1, 279-288, 1996.01, By means of tunable infrared pump-probe experiment, vibrational relaxation rates (v = 1→0) of OH stretching mode of Brønsted acidic hydroxyl groups in a mordenite zeolite, which were interacting with noble gases (He, Ar, Kr, or Xe), were measured. The population lifetime, T1, decreased with the increase of the interaction with noble gases, e.g., 170 ps and 58 ps for isolated hydroxyl groups and ones interacting with Xe, respectively. Using a simple linear chain model, the mechanism for the enhancement of the relaxation rate was studied..
106. L. Krim, C. Jouvet, B. Soep, Ken Onda, K. Yamanouchi, J. P. Visticot, Induction of optical transitions through complexation within Hg-rare gas van der Waals systems, Journal of Chemical Physics, https://doi.org/10.1063/1.470475, 103, 14, 5956-5963, 1995, The high repulsive states of HgAr and HgNe van der Waals complexes, correlating with Hg 6s6d atomic states have been investigated by double resonance spectroscopy, through the first excited state A 30 + and B 31 of the complexes. The repulsive potentials have been fitted through numerical Franck-Condon simulations. They have been characterized by perturbative calculation as quasi-pure 6dΣ potentials in Hund's case a. The strong Hg-rare gas electrostatic interaction potential overruns the spin-orbit interaction at distances shorter than 7 Å. These observed repulsive states are mostly of Ω=1 character correlating with 3D3 at infinite distances. The contribution from the potential of Ω=0- symmetry correlating with 1D 2 is of minor importance. Therefore, the absorption in the repulsive states of the complex arises mostly from proximity induced absorption in an optically forbidden transition 3P13D 3. A perturbative model accounts well for the bound free absorption intensities experimentally observed..
107. Ken Onda, Yasuhiro Iwasawa, Akihide Wada, Vibrational relaxation of adsorbate and adsorbent in the CO-adsorbed DM-20 zeolite system, Chemical Physics Letters, https://doi.org/10.1016/S0009-2614(03)00071-X, 370, 3-4, 437-442, 2003.03, We made the first observation of transient vibrational spectra of both adsorbate and adsorbent at molecule-adsorbed surface using a two-color infrared picosecond laser system. The transient measurements were carried out on the CO-adsorbed surface hydroxyl group (OD) of DM-20 zeolite by pumping the CO stretching mode (2175 cm-1) or OD stretching mode (2470 cm-1) and probing over the CO and OD stretching region (2000-2700 cm-1). The T1 lifetime for OD was 5.8 ps and that for CO was 540 ps. By comparison with other CO-adsorbed systems, vibrational relaxation mechanisms of CO on various surfaces are discussed. We also found evidence of energy flows from the vibrational mode of the adsorbent (OD stretching) to that of the adsorbate (CO stretching) via van der Waals interaction..
108. H. Ishida, R. Mizoguchi, Ken Onda, C. Hirose, S. S. Kano, A. Wada, Second harmonic observation of Cu(1 1 1) surface
In situ measurements during molecular adsorption, Surface Science, https://doi.org/10.1016/S0039-6028(02)02651-1, 526, 1-2, 201-207, 2003.02, Second harmonic generation (SHG) spectroscopy using a tunable femtosecond laser has been demonstrated as an effective and a practical in situ monitor of surface electronic states during adsorption processes. We have successfully shown this technique to be suitable for the study of surface electronic states, not only those induced by adsorbed molecules but also those associated with clean surfaces. By observing the change in the SH signals from a Cu(1 1 1) surface during exposure to benzene or CO molecules, we discovered new resonances of the clean Cu surface that could not be ascribed to the well-known surface state (SS)-image state (IS) transition. One of these was from a surface site that is less likely to adsorb benzene, where the SH signal intensity was kept constant until the Cu surface was covered by the sub-monolayer..
109. Athula Bandara, Satoru S. Kano, Ken Onda, Satoshi Katano, Jun Kubota, Kazunari Domen, Chiaki Hirose, Akihide Wada, SFG spectroscopy of CO/Ni(111)
UV pumping and the transient hot band transition of adsorbed CO, Bulletin of the Chemical Society of Japan, https://doi.org/10.1246/bcsj.75.1125, 75, 5, 1125-1132, 2002.05, A UV excitation by a picosecond pulse at 266 nm induced an unusual shoulder on the v CO = 1 ← 0 resonance peak of CO/Ni(111) monitored by sum-frequency generation (SFG) of visible and IR pulses. The observed line shape was reproduced by the use of a dipole-dipole interaction model with the coherent potential approximation (CPA) where the hot band transition with a population ratio of 0.3 to 0.7 (v = 1 to v = 0) was assumed. Neither the transition to the two-phonon bound state nor the coupling with the low-frequency phonon modes explained the observed changes. The shoulder appeared only during the UV excitation, which indicated that the electronically driven excitation, presumably by the hot electrons generated by the irradiation, dominated the process. As possible mechanisms, the involvement of an intermediate negative ion resonance state and/or the non-adiabatic coupling of electronic states with C-O stretching mode were considered..
110. Ken Onda, Masaru Nakagawa, Toshiaki Asakai, Reiko Watase, Akihide Wada, Kunihiro Ichimura, Chiaki Hirose, Controlling packing structure of hydrophobic alkyl tails of monolayered films of ion-paired macrocyclic amphiphiles as studied by sum-frequency generation spectroscopy, Journal of Physical Chemistry B, https://doi.org/10.1021/jp013939v, 106, 15, 3855-3859, 2002.04, Two kinds of monolayered films of ion-paired macrocyclic amphiphiles formed from anionic surfactant 1,2-bis[(dodecyloxy)carbonyl]ethane-1-sulfonate and tetracationic macrocycles with different molecular sizes were prepared on hydrophilic silica plates by the Langmuir-Blodgett technique. Vibrational sum-frequency generation (SFG) spectroscopy was applied to investigate the packing structure of the alkyl chains of the films. The SF intensity ratio of the CH symmetric stretching mode of the methylene group to that of methyl group in the alkyl chains increased when the occupied area of the macrocycle was larger than the sum of the cross-sectional areas of the hydrophobic alkyl tails. The appearance of a methylene CH band in the SFG spectra implies the presence of gauche conformation in the alkyl chains. This indicates that the molecular size of the cyclic tetracation hydrophilic head exclusively governs the packing structure of the alkyl group hydrophobic tails in the monolayer state. The results show that monolayer fabrication using ion-paired macrocyclic amphiphiles has advantages in allowing control over the packing structure of the outermost alkyl chains of solid surfaces through the choice of the size of the tetracation..
111. Ken Onda, Akihide Wada, Kazunari Domen, Chiaki Hirose, Realtime observation of the desorption process of isobutene from zeolite using picosecond infrared lasers, Surface Science, https://doi.org/10.1016/S0039-6028(01)01970-7, 502-503, 319-324, 2002.04, The transient absorption spectra of OD stretching mode of isobutene-adsorbed hydroxyl groups of DM20 zeolite were measured by picosecond infrared-infrared pump-probe experiment. The vibrational relaxation time (T1) of the ν = 1 state was estimated to be 12±2 ps from the temporal profile of the bleaching and hot bands of the OD stretching mode. We found a band indicating the production of a short-lived species at 40 ps after excitation. Moreover we observed the course of isobutene-desorption in the time region from 40 to 100 ps. This result implies that this desorption does not occur immediately but occurs after local vibrational relaxation..
112. K. Kusafuka, H. Noguchi, Ken Onda, J. Kubota, K. Domen, C. Hirose, A. Wada, Time-resolved study of formate on Ni(1 1 1) by picosecond SFG spectroscopy, Surface Science, https://doi.org/10.1016/S0039-6028(01)01969-0, 502-503, 313-318, 2002.04, Time-resolved vibrational measurements were carried out on formate (HCOO) adsorbed on Ni(1 1 1) surface by combining the sum-frequency generation method and picosecond laser system (time resolution of 6 ps). Rapid intensity decrease (within the time resolution) followed by intensity recovery (time constant of several tens of ps) of CH stretching signal was observed when picosecond 800 nm pulse was irradiated on the sample surface. From the results of temperature and pump fluence dependences of temporal behaviour of signal intensity, we concluded that the observed intensity change was induced by non-thermal process. Mechanism of the temporal intensity change was discussed..
113. Ken Onda, Kinuka Tanabe, Hidenori Noguchi, Akihide Wada, Takafumi Shido, Aritomo Yamaguchi, Yasuhiro Iwasawa, Studies of the surface deuterioxyl group and adsorbed D2O on γ-Al2O3 using picosecond infrared pump probe spectroscopy, Journal of Physical Chemistry B, https://doi.org/10.1021/jp0118625, 105, 46, 11456-11461, 2001.11, Picosecond infrared-infrared pump-probe experiments in the OD stretching region were carried out on dehydrated and multilayer water-adsorbed γ-Al2O3. For the dehydrated γ-Al2O3, transient bands assigned to the bleaching and hot bands of the OD stretching mode of isolated surface hydroxyl groups were observed, and the population lifetime (T1 of the vibrational excited state (v = 1) of the mode was 200 ± 10 ps at 293 K. The characteristic temperature dependence of T1 is indicative of a seven-phonon process. For the water-adsorbed γ-Al2O3, the transient bleaching and transient hot bands with a lifetime of 10-12 ps were observed by excitation of the absorption peak at 2630 cm-1. In addition to these bands, weak transient bands were observed in the low-frequency region. These results are considered evidence of the existence of isolated water molecules on the γ-Al2O3 surface. Pure dephasing is also discussed based on the bandwidth of the observed transient bands..
114. K. Domen, K. Kusafuka, A. Bandara, M. Hara, J. N. Kondo, J. Kubota, Ken Onda, A. Wada, C. Hirose, Observation of unstable reaction intermediate by picosecond tunable infrared laser pulses, Studies in Surface Science and Catalysis, 130 A, 365-370, 2000.12, The picosecond tunable infrared laser pulses enabled us to carry out the vibrational spectroscopic measurements of the species adsorbed on surfaces with picosecond time-resolution. The irradiation of near-infrared pump pulses causes thermal excitation of the surface, and the change of adsorbates was probed by sum-frequency generation (SFG) spectroscopy. We applied this method to the direct observation of unstable species of formate on NiO(111) surface. The stable bidentate formate on NiO(111) was transformed to monodentate by the irradiation of pump pulses, and the dynamic behavior of the newly observed monodentate formate was followed as a precursor for the decomposition of formate. The application of the present method has extended to the study of methoxy species on Ni(111)..
115. Athula Bandara, Jim Kubota, Ken Onda, Akihide Wada, Kazunari Domen, Chiaki Hirose, Short-lived reactive formate species on NiO(111) observed by picosecond temperature jump, Surface Science, https://doi.org/10.1016/S0039-6028(99)00076-X, 433, 83-87, 1999.08, The combination of an instantaneous temperature jump induced by the irradiation of picosecond laser pulses and the subsequent observation by sum-frequency generation spectroscopy enabled the identification of a short-lived reactive intermediate in the decomposition of formate on NiO(111) surface. The irradiation of 1064 nm laser pulses caused the vibrational peak at 2160 cm-1, which is the C-D stretching mode vCD of bidentate formate, to weaken and the vCD of monodentate formate at 2190 cm-1 to appear. The spectral changes recovered in 100 ps time scale but not fully above 400 K. The observations suggested that the formate in bidentate configuration transformed to monodentate formate prior to decomposition. Temperature- and time-dependent features indicated that the two types of formate were in equilibrium and the equilibrium shifted towards the monodentate form by a rapid laser-induced temperature jump of about 250-300 K. The experiment enabled us to identify short-lived species and to estimate kinetic parameters. The enthalpy difference between bidentate and monodentate formates and the activation energy for the decomposition of monodentate formate were 19 ± 5 and 30 ± 10 kJ/mol, respectively..
116. Kazunari Domen, Athula Bandara, Jun Kubota, Ken Onda, Akihide Wada, Satoru S. Kano, Chiaki Hirose, SFG study of unstable surface species by picosecond pump-probe method, Surface Science, https://doi.org/10.1016/S0039-6028(99)00301-5, 427-428, 349-357, 1999.06, We succeeded in identifying the intermediates in thermal decomposition reactions by utilizing the combination of an instantaneous temperature jump induced by the irradiation of picosecond laser pulses and the subsequent observation by time-resolved sum-frequency generation (TR-SFG) spectroscopy. The short-lived reactive intermediates in the decomposition of formate on NiO(111) and Ni(111) surfaces were identified. The irradiation of 1064 nm laser pulses caused the vibrational peak of the C-D stretching mode vCD of bidentate formate on NiO(111) to weaken and the vCD band of monodentate formate to appear. The result on Ni(111) showed the weakening of the vCD band of bridging formate and the appearance of the C double bond O stretching mode vC double bond O of monodentate formate. The spectral changes recovered on a 100 ps time scale but not fully above 400 K for the NiO(111) system and 320 K for the Ni(111) system, indicating the onset of thermal decomposition in the high-temperature period. The observations suggested that the formate in the stable bidentate/bridging configurations transformed to unstable monodentate formate prior to decomposition. Temperature- and time-dependent features indicated that the two types of formate were in equilibrium and the equilibrium shifted towards the monodentate form by the rapid laser-induced temperature jump of about 250-300 K..
117. S. Katano, A. Bandara, J. Kubota, Ken Onda, A. Wada, K. Domen, C. Hirose, Screening of SFG signals from bridged CO on Ni(111) by the coexistence of linear CO, Surface Science, https://doi.org/10.1016/S0039-6028(99)00299-X, 427-428, 337-342, 1999.06, A spectroscopic study of adsorbed CO on Ni(111) was carried out using infrared-visible sum-frequency generation (SFG) and infrared reflection absorption spectroscopy (IRAS). An anomalous coverage-dependence of the SFG signal intensities of bridged and linear CO was found. While the SFG signal of bridged CO at 1920 cm-1 gained intensity with increasing CO coverage in the region between 0 and 0.4 ML (1 monolayer, ML: saturation coverage at 130 K), it weakened and disappeared when linear CO (2074 cm-1) was present on the surface above 0.47 ML. The IRA peak of bridged CO, however, did not disappear at the increased coverage suggesting that the weakening of the SFG signal of bridged CO is not caused by the decrease in the coverage but by the suppression of the SFG process of bridged CO by the coexistence of linear CO. The spectra of isotope mixtures of C16O and C18O clearly indicated that the weakening of SFG signals is not caused by the dipole-dipole coupling between the vibrational modes of bridged and linear CO. The anomalous coverage-dependence of the SFG signal was ascribed to the change of the Raman tensor by the coadsorption of linear CO..
118. Athula Bandara, Jun Kubota, Ken Onda, Akihide Wada, Satoru S. Kano, Kanzunari Domen, Chiaki Hirose, Time-resolved SFG study of the vibrational excitation of adsorbed CO on Ni(111) and NiO(111) surfaces under the irradiation of UV and visible photons, Surface Science, https://doi.org/10.1016/S0039-6028(99)00298-8, 427-428, 331-336, 1999.06, The effects of irradiation by picosecond ultraviolet (266 nm) and visible (532 nm) pulses of CO-covered Ni(111) and NiO(111)/Ni(111) surfaces were investigated by infrared-visible sum-frequency generation (SFG) spectroscopy. As a result of the irradiation of UV pulses, the SFG signal by the v = 1 ← 0 bands of the CO stretching mode weakened and transient peaks tentatively assigned to the v = 2 ← 1 hot band transition appeared with a downshift by 18 and 14 cm-1 from v = 1 ← 0 bands on the metal and oxide-layered surfaces, respectively. The time profile of the weakening and that of the transient signal were coincident with the same rise and fall times of less than approx. 10 ps. The irradiation of visible pulses also induced a weakening of the v = 1 ← 0 bands and transient peaks, but the decay time on the metal surface was longer than 100 ps. The temporal and spectral features imply that the persistent v = 1 ← 0 band and the transient band originated from the same CO molecule. Short decay times observed on irradiating UV pulses on to the Ni(111) and NiO(111) surfaces and that of the visible pulses on to the NiO(111) surface suggest that the photo-generated hot electrons induced the observed spectral changes. The long decay time observed on irradiating visible pulses on to the Ni(111) surface suggests the occurrence of a thermally driven process. Participation of the negative ion state was considered to be one of the possible mechanisms for the rapidly decaying features..
119. Ken Onda, Michio Yaginuma, Akihide Wada, Kazunari Domen, Chiaki Hirose, Vibrational relaxation process of polyatomic molecules adsorbed in zeolites, Laser Chemistry, 19, 1-4, 321-324, 1999, The vibrational relaxation lifetimes of the CO stretching mode of Cr(CO)6 and Mo(CO)6 adsorbed in the cage of the HY, DY and NaY-type zeolites were measured at various temperature by pump-probe method using picosecond infrared laser pulses. It was shown by comparing the lifetimes that the accepting modes include both the vibrational modes of the metal carbonyl and those associated with the cations on the zeolite surface. The analysis of the temperature dependence of the lifetimes revealed that the number of the excited accepting modes are four with their energy lying around 500 cm-1..
120. T. Fujino, M. Kashitani, Ken Onda, A. Wada, K. Domen, C. Hirose, M. Ishida, F. Goto, S. S. Kano, Picosecond infrared pump-probe spectrum of D2O adsorbed at acid OD group of zeolite, Journal of Chemical Physics, https://doi.org/10.1063/1.476816, 109, 6, 2460-2466, 1998.12, Spectroscopic and dynamic features of the vibrationally excited D2O/zeolite system have been investigated by two-color infrared-infrared pump-probe experiment. The frequency- and delay-scanned probe intensities were measured by tuning the pump laser to the OD stretching bands of the D2O molecule hydrogen bonded to the acidic OD group of mordenite zeolite. Two types of pump-induced signals were observed: the ones which have the population lifetime of 43±5 ps and display frequency shift by the pumping frequency, and the others, which have the lifetime of about 15 ps and exhibit no such frequency shift. Possible origins of the signals are discussed..
121. Tetsuo Yuzawa, Tatsutoshi Shioda, Jun Kubota, Ken Onda, Akihide Wada, Kazunari Domen, Chiaki Hirose, Polarization characteristics from SFG spectra of clean and regulatively oxidized Ni(100) surfaces adsorbed by propionate and formate, Surface Science, 416, 1-2, 1998.10, Experiments have been carried out on propionate and formate adsorbed on clean and regulatively oxidized Ni(100) surfaces to scrutinize the notion that the polarization characteristics of infrared-visible sum frequency generation (SFG) signals of molecules on metallic surfaces are different from those due to molecules on insulating surfaces. Both propionate and formate give signals only for p-polarized visible pulses on clean metal and oxygen-saturated surfaces, but signals due to s-polarized visible pulses appeared from the propionate-covered surface as soon as the surface was covered with a monolayer of NiO(100). Vibrational peaks of propionate were located at 2887, 2945, and 2988 cm-1 and were assigned to the symmetric stretching mode of CH2 group, the symmetric stretching mode of CH3 group, and the degenerate stretching mode of CH3 group, respectively. The peak of the surface formate was located at 2948 cm-1 on the clean metal surface, but another band appeared at 2860 cm-1 on oxide-layered surfaces. The causes of the appearance of the (sp) polarization signals by the presence of the NiO layer are discussed..
122. Athula Bandara, Jun Kubota, Ken Onda, Akihide Wada, S. S. Kano, Kazunari Domen, Chiaki Hirose, Short-lived reactive intermediate in the decomposition of formate on NiO(111) surface observed by picosecond temperature jump, Journal of Physical Chemistry B, 102, 31, 5951-5954, 1998.07, Unstable intermediates are indispensable ingredients of chemical reactions, but their identification at solid surfaces has been hampered by the lack of techniques to detect short-lived and fractionally generated species. We succeeded in identifying the intermediate in a thermal decomposition reaction by utilizing the picosecond temperature jump induced by the irradiation of laser pulses and the observation by sum-frequency generation spectroscopy. We verified that the decomposition of formate on the NiO(111) surface is preceded by the transformation of stable bidentate formate to unstable monodentate formate, which is the reactive intermediate, and two species are found to be in equilibrium before decomposition..
123. Athula Bandara, Satoshi Katano, Jun Kubota, Ken Onda, Akihide Wada, Kazunari Domen, Chiaki Hirose, The effect of co-adsorption of on-top CO on the sum-frequency generation signal of bridge CO on the Ni(111) surface, Chemical Physics Letters, 290, 1-3, 261-267, 1998.06, Anomalous coverage dependence was observed on the sum-frequency generation signal of bridge CO on the Ni(111) surface when co-adsorbed by on-top CO. The bridge CO peak was distinct and normal from 0.04 to ~0.40 monolayer (ML) but weakened after the appearance of the on-top CO peak at 0.47 ML and disappeared at 0.60 ML, although both peaks were observed in the infrared reflection absorption spectra up to 1.0 ML. The spectra of mixtures of C16O and C18O eliminate the possibility that the weakening of the bridge CO band is caused by its coupling with the on-top band..
124. Ken Onda, Michio Yaginuma, Toshiya Yokota, Akihide Wada, Kazunari Domen, Chiaki Hirose, Satoru S. Kano, Relaxation process of CO stretching mode of metal carbonyls adsorbed on Y zeolites studied by pump-probe experiment, Journal of Chemical Physics, https://doi.org/10.1063/1.476004, 108, 14, 5935-5941, 1998.04, The vibrational relaxation lifetimes of Mo(CO)6 and Cr(CO)6, adsorbed in a cage of HY, DY, and NaY zeolites, were measured at various temperatures by the pump-probe method using picosecond infrared laser pulses. The lifetimes at 110 K of the υ = 1 level of the CO stretching mode were 70±4, 53±5, and 150±9 ps for Mo(CO)6 on HY, DY, and NaY zeolite, respectively, and 87 ±6, 74±7, and 140±12 ps for Cr(CO)6, respectively. It was shown by comparison of the lifetimes at 110 K, as well as the temperature dependence, that the rate of vibrational relaxation is determined by the coupling of the CO stretching mode with four quanta of the accepting modes comprised of: (i) modes with displacement of central metals Cr or Mo in all of the three zeolite systems, (ii) modes associated with the OH and OD groups on zeolite surface in the HY and DY zeolite systems, and probably (iii) the frame vibrational modes of zeolite with energies at 300-800 cm-1..
125. Athula Bandara, Shinsaku Dobashi, Jun Kubota, Ken Onda, Akihide Wada, Kazunari Domen, Chiaki Hirose, Satoru S. Kano, Adsorption of CO and NO on NiO (111)/Ni(111) surface studied by infrared-visible sum frequency generation spectroscopy, Surface Science, 387, 1-3, 312-319, 1997.10, Adsorption of CO and NO on NiO(111) thin films epitaxially grown on Ni(111) substrate has been studied by infrared-visible sum frequency generation (SFG) and the results were compared with those of infrared reflection absorption spectroscopy. From SFG measurements, the C-O stretching band of adsorbed CO was observed at 2144 cm-1 for both p- and s-polarized visible (532 nm) light whereas the adsorbed NO gave the N-O stretching band at 1800 cm-1 only for the p-polarized visible light. These observations suggested that the CO molecule was inclined to the surface whereas the tilt angle of NO from the surface normal was smaller than that of CO. The adsorption sites of CO and NO molecules are located on the slopes of trigonal microfacets formed by the reconstruction of the NiO(111) surface. CO adsorbed on Ni(111) instead of NiO(111) was also examined: the SFG signal corresponding to the C-O stretching mode of linearly bonded CO was observed at 2076 cm-1 only for the p-polarized visible light, but that of the bridge-bonded one (at saturation coverage) was not detected by SFG..
126. Tetsuo Yuzawa, Jun Kubota, Ken Onda, Akihide Wada, Kazunari Domen, Chiaki Hirose, A TPD and SFG study of propionic acid adsorbed on Ni(110) surface, Journal of Molecular Structure, https://doi.org/10.1016/S0022-2860(97)00151-8, 413-414, 307-312, 1997.09, Propionic acid adsorbed on Ni(110) surface was studied by temperature programmed desorption (TPD) and infrared-visible sum frequency generation (SFG) spectroscopy. The TPD results indicated that propionic acid molecularly adsorbed on the Ni(110) surface at 113 K and that the multilayers of propionic acid started to desorb at 213 K. The molecularly adsorbed propionic acid on the first layer decomposed on further heating to form propionate species which decomposed at 390 K to H2(g) and CO2(g), leaving carbon on the surface. The SFG spectra of the propionic acid adsorbed in various thickness at 113 K gave vibrational peaks at 2885 cm-1, 2956 cm-1, and 2990 cm-1, assigned to the symmetric CH stretching mode of the CH2 group, symmetric CH stretching mode of the CH3 group, and antisymmetric CH stretching mode of the CH2 group, respectively, of propionic acid. The multilayered films gave peaks under both the (pp) and (sp) polarization combinations of visible and infrared pulses but the signals disappeared for the (sp) polarization combination when the coverage was lower than one monolayer (ML). The absence of the SFG peaks for the (sp) polarization combination on the low coverage surface was ascribed to the difference in molecular orientation and molecular SFG tensor. The SFG spectrum of the surface which was dosed at 300 K gave vibrational peaks at 2885 cm-1, 2948 cm-1 and 2990 cm-1 only for the (pp) polarization combination. These peaks were assigned to the symmetric CH stretching mode of the CH2 group, symmetric CH stretching mode of the CH3 group, and the antisymmetric CH stretching mode of the CH2 group of propionate, the product of dissociative adsorption..
127. Hideki Katagiri, Tokuei Sako, Akiyoshi Hishikawa, Takeki Yazaki, Ken Onda, Kaoru Yamanouchi, Kouichi Yoshino, Experimental and theoretical exploration of photodissociation of SO2 via the C1B2 state
Identification of the dissociation pathway, Journal of Molecular Structure, https://doi.org/10.1016/S0022-2860(97)00199-3, 413-414, 589-614, 1997.09, The photodissociation reaction of SO2 via the C1B2 state, SO2 (C1B2) → SO(3Σ(~)) + O(3P) was investigated by experimental and theoretical approaches cooperatively to clarify its dissociation mechanism. We measured the laser induced fluorescence (LIF) spectrum of the C-X band in the short UV wavelength region (210-200 nm) under jet-cooled conditions. The fluorescence quantum yields and the dissociation rates of individual vibronic levels were determined in the 220-200-nm region using (i) the LIF spectrum measured in the present study, (ii) that measured previously by Yamanouchi et al. (J. Mol. Struct. 352/353 (1995) 541) in the longer wavelength region above 210 nm, and (iii) the high-resolution absorption spectrum measured by Freeman et al. (Planet. Space. Sci. 32 (1984) 1125). The dissociation rates were also derived in the 210-200-nm region from the broadening of the rotational lines of the C-X vibronic transitions. It was found that the dissociation rates determined through two different procedures were consistent with each other, and that the rate increases almost exponentially as an excess energy above the dissociation threshold increases though there is a certain fluctuation of the dissociation rates reflecting a mode specificity. We also performed theoretical ab initio calculations to derive potential energy surfaces (PESs) of the electronic ground states and low-lying electronically excited states of SO2 within the MCSCF and MRCI levels. The theoretical calculations showed that (i) the PES of the 21A' state (the C1B2 state in C(2v) symmetry), correlating with the SO(1Δ) + O(1D) asymptote, crosses with the repulsive singlet (31A') state, correlating with the SO(3Σ-) + O(3P) asymptote, to form a pseudo-seam, (ii) the crossing pseudo-seam of these two PESs is located near the equilibrium bent angles for the X and C states along the energy contour of ~9700 cm-1 measured from the SO(3Σ-) + O(3P) dissociation limit, and (iii) the crossing seam between the 21A' (C1B2) and repulsive 23A' states is located in a lower energy region than the singlet seam; at ~6700 cm-1 measured from the SO(3Σ-) + O(3P) dissociation limit. On the basis of the above experimental and theoretical results together with the previous experimental evidence, we propose that (i) the photodissociation reaction via the C state proceeds mainly through the vibronic mixing between the C state vibronic levels with the quasi-bound dissociation continuum of the electronic ground X1A1 state, and (ii) the additional dissociation channels may be open through the crossing seam with the repulsive singlet (31A') state and that with the repulsive triplet (23A') state in their narrow crossing energy regions..
128. Tomoki Tasaka, Ken Onda, Akiyoshi Hishikawa, Kaoru Yamanouchi, PHOFEX spectroscopy of HgNe and HgAr
Determination of the dissociation energies of the X1σ+, A3Π 0+, and B3Π1 states, Bulletin of the Chemical Society of Japan, https://doi.org/10.1246/bcsj.70.1039, 70, 5, 1039-1045, 1997, Photofragment excitation (PHOFEX) spectroscopy was applied to determine the dissociation energies of the B3Π1 states of HgAr and HgNe. The PHOFEX spectra were measured in the wavelength region where the laser-induced fluorescence spectrum of the B3Π1-X 1Σ+ transition exhibits a continuum structure by probing the photolysis product of Hg(63P1) through the Hg(83S1-63P1) transition. By the spectral simulation of the threshold behavior of the highresolution (Δν≈0.08 cm-1) PHOFEX spectrum, the thresholds for the photodissociation reaction, HgRg→Hg(63P1)+Rg, were determined to be 39447.9(3) and 39536.0(5) cm-1 for Rg = Ne and Ar, respectively. From these thresholds, the dissociation energies, Do's, of the B3Π1 states of HgNe and HgAr were determined to be Do(B3Π1; HgNe) = 9.8(3) and D o(B3π1; HgAr) = 61.8(5) cm-1, respectively. This direct determination of the dissociation energies of the B3Π1states led to a determination of the D o's for the X1Σ+ and A 3Π0+ states; D0(X1Σ +; HgNe) = 35.6(3), D0(X1Σ+; HgAr) = 123.7(5), D0(A3Π0+; HgNe) = 68.7(3), and D0(A3Π0+; HgAr) = 348.8(6) cm -1. In addition, the (ν′,0) vibronic bands of the B 3Π1-X1Σ+ transition of HgAr were re-measured with high resolution for ν′ = 0-8. From the transition wavenumbers of these vibronic bands, the Morse potential parameters were determined with high precision as ωe = 11.94(3) cm -1 and ωcxe = 0.594(3) cm-1..
129. Ken Onda, Kaoru Yamanouchi, Electronic structure of metal-rare gas dimers with sp configuration
Application to strong spin-orbit interaction in HgAr, Journal of Chemical Physics, https://doi.org/10.1063/1.471683, 104, 23, 9376-9387, 1996.06, In order to describe the electronic states of metal (M)-rare gas (Rg) van der Waals dimers having an sp configuration with a strong spin-orbit interaction, we derived an elf parity adapted molecular Hamiltonian matrix by adopting a symmetry-adapted atomic orbital approach. The molecular Hamiltonian was constructed by introducing (i) the interaction between the p electron and the attached rare gas atom, VRg, (ii) the exchange interaction between the s and p orbitals, e2/rsp, and (iii) the spin-orbit interaction for the p electron. As a basis set, twelve molecular electronic wave functions were derived by taking into account their elf parities. We applied the derived molecular Hamiltonian matrix to the first excited 6s6p configuration of HgAr by performing a least-squares fit to the spectroscopically determined term values for the v=0 levels of the a 3Π0-, A 3Π0+, B 3Π1, b 3Π2, and C 1Π1 states. From the results of the least-squares fit, we clarified how the above interactions (i)-(iii) split twelve degenerate molecular wave functions into the eight electronic eigenstates; i.e., a 3Π0-, A 3Π0+, B 3Π1, b 3Π2, c 3Σ1 +, d 3Σ0- +, C 1Π1, and D 1Σ0 +. On the basis of (i) a critical comparison between the atomic Hamiltonian matrix for Hg and the determined molecular Hamiltonian matrix and (ii) an examination of the mixing among the symmetry-adapted molecular wave functions, characteristic features of the electronic structure arising from the formation of a van der Waals bond, were extracted..
130. Ken Onda, Kaoru Yamanouchi, Interatomic potentials of singlet s-Rydberg series of a HgNe van der Waals dimer
Evidence for stabilization by superexchange interaction, Journal of Chemical Physics, https://doi.org/10.1063/1.469171, 102, 3, 1129-1140, 1995, The interatomic potentials of the singlet Rydberg series of the HgNe dimer 1Σ+ Hg(n1S0)Ne (n=7-9) were determined over a wide range of interatomic distance by the analysis of the optical-optical double resonance (OODR) spectra measured in the present study via the A 3Π0 + and B 3Π1 states. The interatomic potential for n=7 consists of one bound vibrational level (v=0), three quasibound levels (v=1-3) trapped inside a potential barrier, and one weakly trapped quasibound level just above the potential barrier. The dissociation energy (De) of this potential is 50(3) cm-1 and the large potential barrier with a height of 133(2) cm-1 is located at 4.20(3) Å, while the interatomic potentials for n=8 and n=9 consist of a deep bound well, whose dissociation energies (De) are 240(3) and 297(3) cm-1, respectively. From these potentials, it was shown that the principal quantum number dependence (n) of the interatomic potential originates mainly from that of the exchange repulsion between the Rydberg electron of Hg and the attached Ne atom in a similar manner as the triplet Rydberg series of HgNe, 3Σ+ Hg(n 3S1)Ne (n=7-10) [K. Onda et al. J. Chem. Phys. 101, 7290 (1994)]. By comparing the potentials of the singlet with the triplet Rydberg series, it was found that an interatomic potential of the singlet state is always deeper than that of the triplet state for the same n. This difference between singlet and triplet was interpreted by a superexchange interaction model [P N. Anderson, Phys. Rev. 79, 350 (1950)], in which a singlet Rydberg state becomes more stable due to small spin density on the Ne atom induced by the kinetic exchange interaction between the Hg+ ion and the Ne atom..
131. Ken Onda, Kaoru Yamanouchi, Misaki Okunishi, Soji Tsuchiya, Interatomic potentials of triplet s-Rydberg series of HgNe and HgAr van der Waals dimers, Journal of Chemical Physics, https://doi.org/10.1063/1.468286, 101, 9, 7290-7299, 1994, The optical-optical double resonance (OODR) spectra of Rydberg 3Σ+ states of Hg(n3S1)Ne (n=8-10) and Hg(83S1)Ar were measured by using A and B states as intermediate states in the OODR process. The interatomic potentials of three states of HgNe and one state of HgAr were determined over a wide range of interatomic distance, R=3-7 Å, by the analysis of the vibrational structure of their OODR spectra. It was found that the potential shape varies sensitively with n and converges to that of the ion core, HgNe+. Dissociation energies (De) of the Rydberg states for the n=8, 9, and 10 were derived to be 209(2), 284(2), and 309(2) cm-1, respectively. Using the quantum defect orbital [G. Simons, J. Chem. Phys. 60, 645 (1974)], which represents a hydrogenic radial wave function for a Rydberg state with a given quantum defect, was introduced to interpret the characteristic n dependence of the interatomic potential. It was shown that the interatomic potential for the Rydberg states can be expressed by the sum of the ion core potential, Vion(R), and the repulsive potential, Vex(R), which originates mainly from the exchange repulsion between the Rydberg electron and the attached rare gas atom. The interatomic potential for Hg(8 3S1)Ar, whose dissociation energy [De=1602(4) cm-1] is much deeper than that of Hg(83S1)Ne, was also interpreted consistently by expressing the potential as V ion(R) + Vex(R)..
132. Misaki Okunishi, Kaoru Yamanouchi, Ken Onda, Soji Tsuchiya, Interatomic potential of the HgNe van der Waals complex in the E( 3Σ+) Rydberg state, Journal of Chemical Physics, 98, 4, 2675-2681, 1993, The lowest Rydberg state E(3Σ+) of the HgNe van der Waals complex has been investigated by optical-optical double resonance (OODR) spectroscopy using two intermediate electronic states of A 3O+ and B31. The E-B band exhibits an oscillatory free-bound continuum, which reflects a Franck-Condon projection of the wave function of the B state onto the repulsive part of the E state potential. In the E-A band, two relatively sharp peaks are observed together with a free-bound continuum showing an asymmetric interference structure. The observed intensity patterns of the E-B and E-A bands are interpreted by a potential barrier characteristic of the E state potential, which traps two quasibound vibrational states in the inner well. Based on (i) the observed Franck-Condon pattern of the free-bound transitions; (ii) the transition's wave number of the bound-bound transitions in the E-A band; and (iii) the rotational constants of the quasibound (v=0 and v=1) levels in the E state, the interatomic potential of the E state is derived by a numerical simulation. The height of the potential barrier measured from the dissociation limit of Hg (7 3S1)+Ne is determined to be 153 cm-1 and the interatomic distance at the top of the barrier to be located at 3.9(1) Å..
133. T. Fujino, M. Furuki, M. Kashitani, Ken Onda, J. Kubota, J. N. Kondo, A. Wada, K. Domen, C. Hirose, F. Wakabayashi, M. Ishida, F. Goto, S. S. Kano, The effect of adsorbed noble gas atoms on vibrational relaxation of hydroxyl group in zeolite, Journal of Chemical Physics, https://doi.org/10.1063/1.471873, 105, 1, 279-288, 1996.01, By means of tunable infrared pump-probe experiment, vibrational relaxation rates (v = 1→0) of OH stretching mode of Brønsted acidic hydroxyl groups in a mordenite zeolite, which were interacting with noble gases (He, Ar, Kr, or Xe), were measured. The population lifetime, T1, decreased with the increase of the interaction with noble gases, e.g., 170 ps and 58 ps for isolated hydroxyl groups and ones interacting with Xe, respectively. Using a simple linear chain model, the mechanism for the enhancement of the relaxation rate was studied..
134. L. Krim, C. Jouvet, B. Soep, Ken Onda, K. Yamanouchi, J. P. Visticot, Induction of optical transitions through complexation within Hg-rare gas van der Waals systems, Journal of Chemical Physics, https://doi.org/10.1063/1.470475, 103, 14, 5956-5963, 1995, The high repulsive states of HgAr and HgNe van der Waals complexes, correlating with Hg 6s6d atomic states have been investigated by double resonance spectroscopy, through the first excited state A 30 + and B 31 of the complexes. The repulsive potentials have been fitted through numerical Franck-Condon simulations. They have been characterized by perturbative calculation as quasi-pure 6dΣ potentials in Hund's case a. The strong Hg-rare gas electrostatic interaction potential overruns the spin-orbit interaction at distances shorter than 7 Å. These observed repulsive states are mostly of Ω=1 character correlating with 3D3 at infinite distances. The contribution from the potential of Ω=0- symmetry correlating with 1D 2 is of minor importance. Therefore, the absorption in the repulsive states of the complex arises mostly from proximity induced absorption in an optically forbidden transition 3P13D 3. A perturbative model accounts well for the bound free absorption intensities experimentally observed..
135. Ken Onda, Kaoru Yamanouchi, Interatomic potentials of singlet s-Rydberg series of a HgNe van der Waals dimer
Evidence for stabilization by superexchange interaction, Journal of Chemical Physics, https://doi.org/10.1063/1.469171, 102, 3, 1129-1140, 1995, The interatomic potentials of the singlet Rydberg series of the HgNe dimer 1Σ+ Hg(n1S0)Ne (n=7-9) were determined over a wide range of interatomic distance by the analysis of the optical-optical double resonance (OODR) spectra measured in the present study via the A 3Π0 + and B 3Π1 states. The interatomic potential for n=7 consists of one bound vibrational level (v=0), three quasibound levels (v=1-3) trapped inside a potential barrier, and one weakly trapped quasibound level just above the potential barrier. The dissociation energy (De) of this potential is 50(3) cm-1 and the large potential barrier with a height of 133(2) cm-1 is located at 4.20(3) Å, while the interatomic potentials for n=8 and n=9 consist of a deep bound well, whose dissociation energies (De) are 240(3) and 297(3) cm-1, respectively. From these potentials, it was shown that the principal quantum number dependence (n) of the interatomic potential originates mainly from that of the exchange repulsion between the Rydberg electron of Hg and the attached Ne atom in a similar manner as the triplet Rydberg series of HgNe, 3Σ+ Hg(n 3S1)Ne (n=7-10) [K. Onda et al. J. Chem. Phys. 101, 7290 (1994)]. By comparing the potentials of the singlet with the triplet Rydberg series, it was found that an interatomic potential of the singlet state is always deeper than that of the triplet state for the same n. This difference between singlet and triplet was interpreted by a superexchange interaction model [P N. Anderson, Phys. Rev. 79, 350 (1950)], in which a singlet Rydberg state becomes more stable due to small spin density on the Ne atom induced by the kinetic exchange interaction between the Hg+ ion and the Ne atom..
136. Ken Onda, Kaoru Yamanouchi, Misaki Okunishi, Soji Tsuchiya, Interatomic potentials of triplet s-Rydberg series of HgNe and HgAr van der Waals dimers, Journal of Chemical Physics, https://doi.org/10.1063/1.468286, 101, 9, 7290-7299, 1994, The optical-optical double resonance (OODR) spectra of Rydberg 3Σ+ states of Hg(n3S1)Ne (n=8-10) and Hg(83S1)Ar were measured by using A and B states as intermediate states in the OODR process. The interatomic potentials of three states of HgNe and one state of HgAr were determined over a wide range of interatomic distance, R=3-7 Å, by the analysis of the vibrational structure of their OODR spectra. It was found that the potential shape varies sensitively with n and converges to that of the ion core, HgNe+. Dissociation energies (De) of the Rydberg states for the n=8, 9, and 10 were derived to be 209(2), 284(2), and 309(2) cm-1, respectively. Using the quantum defect orbital [G. Simons, J. Chem. Phys. 60, 645 (1974)], which represents a hydrogenic radial wave function for a Rydberg state with a given quantum defect, was introduced to interpret the characteristic n dependence of the interatomic potential. It was shown that the interatomic potential for the Rydberg states can be expressed by the sum of the ion core potential, Vion(R), and the repulsive potential, Vex(R), which originates mainly from the exchange repulsion between the Rydberg electron and the attached rare gas atom. The interatomic potential for Hg(8 3S1)Ar, whose dissociation energy [De=1602(4) cm-1] is much deeper than that of Hg(83S1)Ne, was also interpreted consistently by expressing the potential as V ion(R) + Vex(R)..
137. Misaki Okunishi, Kaoru Yamanouchi, Ken Onda, Soji Tsuchiya, Interatomic potential of the HgNe van der Waals complex in the E( 3Σ+) Rydberg state, Journal of Chemical Physics, 98, 4, 2675-2681, 1993, The lowest Rydberg state E(3Σ+) of the HgNe van der Waals complex has been investigated by optical-optical double resonance (OODR) spectroscopy using two intermediate electronic states of A 3O+ and B31. The E-B band exhibits an oscillatory free-bound continuum, which reflects a Franck-Condon projection of the wave function of the B state onto the repulsive part of the E state potential. In the E-A band, two relatively sharp peaks are observed together with a free-bound continuum showing an asymmetric interference structure. The observed intensity patterns of the E-B and E-A bands are interpreted by a potential barrier characteristic of the E state potential, which traps two quasibound vibrational states in the inner well. Based on (i) the observed Franck-Condon pattern of the free-bound transitions; (ii) the transition's wave number of the bound-bound transitions in the E-A band; and (iii) the rotational constants of the quasibound (v=0 and v=1) levels in the E state, the interatomic potential of the E state is derived by a numerical simulation. The height of the potential barrier measured from the dissociation limit of Hg (7 3S1)+Ne is determined to be 153 cm-1 and the interatomic distance at the top of the barrier to be located at 3.9(1) Å..
138. K. Onda, M. Takahashi, Y. Ishikawa, K. Sugita, K. Tanaka, S. Arai, D. M. Rayner, P. A. Hackett, MECHANISM OF HYDROGENATION OF ETHYLENE VIA PHOTOPRODUCED UNSATURATED IRON CARBONYL IN THE GAS-PHASE, JOURNAL OF PHYSICAL CHEMISTRY, 95, 2, 758-761, 1991.01.