Taka-aki Ishibashi

Time-resolved infrared absorption spectroscopy of photogenerated electrons in platinized TiO2 particles

Abstract Kinetics of photogenerated electrons in platinized TiO2 (P-25) particles were observed by time-resolved infrared (IR) absorption spectroscopy with AC-coupled detection of cw probe light. The sample irradiated by UV pulse (355 nm and 10 ns) gave transient absorption which monotonously increased in intensity with decreasing wavenumber from 3000 to 900 cm −1 . Photogenerated electrons trapped in shallow mid-gap states were proposed to originate the transient absorption. The decay kinetics of the trapped electrons were fitted to a multi-exponential formula with six lifetimes ranging from 1×10−7 to 3×10−1 s.

Partially coherent anti‐Stokes Raman scattering (PCARS): A comparative band‐shape analysis of the spontaneous Raman and PCARS spectra of toluene

Partially coherent anti‐Stokes Raman scattering (PCARS) spectrum of liquid toluene has been measured. The observed shape of the 1004 cm−1 band is asymmetric unlike that of the spontaneous Raman spectrum. A least‐squares fitting shows that this asymmetric PCARS band‐shape is fitted well to ‖χ(3)‖2 but not to the imaginary part of χ(3). The origin of the observed band asymmetry is the nonresonant term of χ(3), which is real and does not affect the spontaneous Raman band shapes. These results support our previous interpretation that the PCARS intensity originates from the relaxation of the CARS phase matching condition due to the optical inhomogeneity of the sample.

A multiplex infrared-visible sum-frequency spectrometer with wavelength tunability of the visible probe

We present a multiplex infrared-visible sum-frequency (SF) spectrometer with wavelength tunability of the visible probe. The spectrometer is based on a two-channel optical parametric amplifier producing a broad-bandwidth femtosecond infrared pulse and a narrow-bandwidth picosecond visible pulse (470–800 and 401 nm). A SF spectrum over a 400 cm−1 spectral region was obtainable by a single measurement. SF spectra of an octadecanethiol monolayer on a gold substrate were measured with visible probes of different wavelengths from 401 to 633 nm. The spectral shape showed remarkable dependence on the visible probe wavelength. The dependence was ascribed to resonance to a surface electronic state of the substrate existing in the visible region.

Microsecond kinetics of photocatalytic oxidation on Pt/TiO2 traced by vibrational spectroscopy

Abstract 2-Propanol oxidation to acetone was examined by time-resolved infrared spectroscopy on a Pt/TiO 2 photocatalyst in an aqueous solution. Holes generated by the band-gap excitation were found to attach to the adsorbed reactant within the first 0.5 μs. Subsequent rearrangement of atoms in the hole-attached reactant was observed on a series of time-resolved vibrational spectra. The CO stretching band of a reaction intermediate to be converted to acetone appeared at time delays of 0–20 μs. The observed wavenumber of the intermediate, 1640 cm −1 , was compared with theoretically predicted CO stretching frequencies of possible species. An acceptable agreement was obtained with the anion radical of acetone adsorbed on the catalyst.

Polarization-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy: a new probe of molecular symmetry through accurate determination of the Raman depolarization ratio

A broad-band multiplex CARS spectroscopic system was constructed for precise polarization-controlled measurements. The directions of polarization of the two incident laser beams and that of the CARS signal beam are controlled precisely by two polarizers and an analyzer set on high-precision prism mounts. Polarized CARS spectra were recorded with high signal-to-noise ratios for a number of different analyzer settings, typically at intervals of 1° of the analyzer axis. The observed changes in band shapes are interpreted by a global simulation analysis in which all the band shapes at different analyzer settings are reproduced using a small number of intrinsic bandshape parameters. The analysis leads to very accurate determination of depolarization ratios, accurate within an error of ±0.002 in the most favorable case. The method may well be called polarization-resolved CARS spectroscopy. Depolarization ratios of two typical organic liquids, cyclohexane and 1,2-dichloroethane, were determined. The two non-totally symmetric bands of cyclohexane gave a value of 0.75 within an experimental uncertainty of 0.002. The two totally symmetric bands of 1,2-dichloroethane, which have been reported to be depolarized despite their symmetry, showed depolarization ratios very close to but meaningfully smaller than 0.75. Copyright

Chiral sum frequency spectroscopy of thin films of porphyrin J-aggregates.

Thin films of chiral porphyrin J-aggregates have been studied by vibrationally and electronically doubly resonant sum frequency generation (SFG) spectroscopy. It was revealed that the chiral supramolecular structures of porphyrin aggregates in solutions were retained in the thin film samples, and their chirality was determined by using chiral vibrational SFG spectroscopy. Electronic resonance profiles of some vibrational bands in achiral and chiral SFG were different from each other, and both were distinct from electronic absorption spectra. To account for these peculiar profiles, we have proposed interference effects of Raman tensor components in achiral and chiral SFG susceptibilities, which is analogous to that of resonance Raman scattering.

Chirality Discriminated by Heterodyne-Detected Vibrational Sum Frequency Generation

We first demonstrated chiral vibrational sum frequency generation (VSFG) in the heterodyne detection, which enables us to uniquely determine chiral second-order nonlinear susceptibility consisting of phase and amplitude and distinguish molecular chirality with high sensitivity. Liquid limonene was measured to evaluate the heterodyne-detected chiral VSFG developed in this study. R-(+)- and S-(-)-limonene showed clearly opposite signs in the complex spectra of the second-order nonlinear susceptibility in the CH stretching region. This is the first report of the chiral distinction by VSFG without any a priori knowledge about chiral and achiral spectral response. Furthermore, from the phase of the chiral VSFG field measured in the heterodyne detection, the origin of the chiral signal was ascribed to the bulk limonene. The heterodyne detection also improves detection limits significantly, allowing us to observe weak chiral signals in reflection. The heterodyne-detected chiral VSFG can provide information on absolute molecular configuration.

Pressure dependence of electron- and hole-consuming reactions in photocatalytic water splitting on Pt/TiO2 studied by time-resolved IR absorption spectroscopy

The decay kinetics of photogenerated electrons in the water splitting reaction on a Pt/TiO2 photocatalyst was studied by time-resolved IR absorption spectroscopy. The decay of the photogenerated electrons within 2 µs was decelerated when the catalyst was exposed to water vapor. The holes were con- sumed by the reaction with water instead of by the recombination with the electrons. On the other hand, the decay at 10-900 µs was accelerated by the exposure. The electrons were consumed by the reaction with water. The rate of the hole-consuming reaction was independent of the pressure of water vapor, whereas that of the electron-consuming reaction increased with the pressure from 1 to 10 Torr. The different pressure dependences indicate different reactants involved in the oxidative and reductive reactants.

Multiplex Infrared-Visible Sum-Frequency Spectrometer with a Phase-Conjugated Pulse Mixing Device for Narrow-Bandwidth Visible Probe Generation

We have constructed a multiplex infrared-visible sum-frequency (SF) spectrometer. Picosecond narrow-bandwidth visible laser light (full width at half-maximum (FWHM) ∼8 cm−1, 400 nm) was obtained by sum-frequency mixing of a pair of phase-conjugated broad-bandwidth pulses generated from femtosecond laser light (∼110 fs, 800 nm). Femtosecond broad-band infrared laser light (FWHM ∼280 cm−1, 3.4 μm) was obtained as a difference frequency mixing output of the signal and idler lights of an optical parametric amplifier pumped by the femtosecond laser light. Broad-bandwidth sum-frequency signal light (FWHM ∼280 cm−1) was generated by focusing the broad-bandwidth femtosecond infrared laser light and the narrow-bandwidth picosecond visible laser light onto a sample surface. The generated light was dispersed by a spectrograph and analyzed with a multichannel detector. An SF spectrum with a high signal-to-noise ratio over a 400 cm−1 spectral region with an ∼8 cm−1 resolution was obtained from an octadecanethiol self-assembled monolayer with a 60 s exposure time.

Effects of accumulated electrons on the decay kinetics of photogenerated electrons in Pt/TiO2 photocatalyst studied by time-resolved infrared absorption spectroscopy

Abstract The effects of accumulated electrons on the decay kinetics of photogenerated electrons in Pt/TiO 2 photocatalyst were examined using time-resolved infrared (TR-IR) absorption spectroscopy. When the catalyst was irradiated by a UV pulse in the presence of methanol gas, the lifetime of the electrons was drastically elongated through the effective hole-consuming reaction by the adsorbed methoxy groups. When the catalyst was further irradiated by a second UV pulse in the presence of these long-lived electrons, the number of electrons generated by the second pulse was smaller than that generated by the first pulse though these pulse energies were the same. By increasing the interval between these two pulses, the number of electrons generated by the second pulse was gradually restored to equal that generated by the first pulse. These changes were ascribed to the enhancement of the rate of electron–hole recombination due to the long-lived electrons.