Naotoshi Akamatsu

Formulas for the analysis of surface sum‐frequency generation spectrum by CH stretching modes of methyl and methylene groups

Derivation has been made of the expressions of surface vibrational sum‐frequency generation(SVSFG). The results include (a) the vibrational selection rule, (b) the expressions of the susceptibility tensors at the surfaces composed of functional groups with C3v and C2v local symmetry, (c) the formulas of the SFG tensor components which apply specifically to the vibrational resonance with the CH stretching modes of methyl and methylene groups, and (d) the interference effect by the nearby Fermi resonance band. Changes introduced by the slight distortion of C3v symmetry into Cs symmetry have also been discussed. Sample calculations have been made of the SVSFG of the CH stretch modes of oriented CH3 groups.

Formulas for the Analysis of the Surface SFG Spectrum and Transformation Coefficients of Cartesian SFG Tensor Components

Comprehensive expressions have been presented to facilitate the analysis of the surface sum-frequency generation (SFG) spectrum. The electric field components of the SFG beam for a given experimental setup have been related via appropriately defined Fresnel coefficients to the nonlinear source polarization, which in turn has been related to the electric fields of exciting visible and infrared beams through the macroscopic SFG susceptibility tensor. The coefficients of transformation have been given to relate the laboratory-fixed Cartesian components of the SFG tensor to the components described in a surface-fixed axis system. The tensor components have been further related to the components of the microscopic hyperpolarizability tensor of surface species, and the explicit expressions (in terms of the Euler angles defining molecular orientation) of the transformation coefficients are presented to describe the Cartesian tensor components described in a surface-fixed axis system by the molecule-fixed components.

Infrared-visible sum frequency generation study of HCOOH on an MgO(001) surface

Abstract Adsorption and decomposition of HCOOH on an MgO(001) single crystal surface were studied by means of infrared-visible sum frequency generation (SFG) spectroscopy and temperature programmed desorption (TPD) in UHV. In the SFG experiment, tunable ultrashort infrared pulses were produced by parametric oscillation/amplification in two pieces of LiNbO 3 single crystals and the tunable range was 2600–4000 cm −1 . HCOOH was adsorbed on MgO(001) as formate (HCOO) species at room temperature. The TPD study revealed that HCOOH mainly decomposed into CO and H 2 O at ∼ 510 K. By SFG, the C-H stretching band was observed at 2870 cm −1 with a shoulder at ∼ 2850 cm −1 . The in situ measurement of the change of the SFG signal intensity (2870 cm −1 ) was carried out during the dosage of HCOOH.

SFG study of rotational anisotropy of cadmium archidate Langmuir—Blodgett films

Abstract Anisotropic orientation of Langmuir—Blodgett (LB) films of cadmium arachidate on fused silica is studied by infrared—visible sum-frequency generation (SFG). The results of rotational anisotropy in the SFG signal show that the terminal methyl groups are tilted along the withdrawal direction for the 9- and 3-monolayer films but not for the 1-monolayer film.

Measurement of vibrational energy relaxation of a surface hydroxyl group by ultrashort tunable infrared pulses

Measurements were made in real time of the vibrational energy relaxation time, Ti, of the O-H stretching mode of surface hydroxyl groups at the so-called Bronsted acid sites of zeolites, by using tunable ultrashort infrared pulses (30 ps) generated by parametric amplification in two successive LiNbO 3 crystals. T 1 (υ=1→υ=0)of the O-H stretching mode was 161±8 ps for mordenite zeolite and 200±17 ps for Y-type zeolite both at 300 K. The dependence of Ti on the sample temperature was measured between 145 and 700 K, and it was suggested that the multiphonon (four phonons) decay of the vibrational energy to substrate phonons was operative.

Sn ←S1 and S1→S0 resonance CARS spectra of perylene in the S1 state

Resonance CARS spectrum of perylene in the first excited singlet (S1 ) state was observed in solution at room temperature. The observed shifts of Raman frequencies from the ground (S0 ) state values were related to the results of the PPP‐SCF‐CI MO calculation and the normal coordinate calculation. The experiment, intended to observe the resonance CARS signal of the S0 state, was found to give the signal predominated by the lines of the S1 state. It was also concluded that the peculiar feature originating from the simultaneous role of the pump beam depleted the molecules in the S0 state, populated the S1 state, and caused the resonance effect on CARS through the intense S1 →S0 transition.

The orientation analysis from polarization characteristics of the surface vibrational SFG spectrum of HCO2 on MgO(001) surface

Abstract Application of vibrational SFG (sum frequency generation) spectroscopy has been made to the study of the adsorption product of formic acid(HCOOH) on clean MgO(001) surface. The observed (CH) frequency of 2870 cm −1 revealed that the adsorbed species are formate (HCOO − ), and the analysis made on the polarization characteristics of signal intensity led to the conclusion that CH bonds stand perpendicular to the surface with rather broad distribution width of tilt angle: φ=0±30°. Formulas for the analysis have been presented.

Resonance polarization CARS of pyrene in the lowest excited singlet state

Polarization CARS of pyrene in the S1 state has been observed in solution at room temperature under the resonance condition with the Sn←S1 transient absorptions. The polarization behaviors of totally symmetric ag and nontotally symmetric b1g modes have been interpreted on the basis of an irreducible tensor formulation of the polarization CARS susceptibility which is applicable rigorously in resonance conditions, and anomalous depolarization ratio for the b1g mode corresponding to the case of ρl >3/4 in spontaneous Raman scattering has been deduced.

Resonance CARS study of long-lived species of diphenylhexatriene generated by monophotonic excitation in polar solvent

Abstract The transient absorption and resonance CARS (coherent anti-Stokes Raman scattering) spectra of photo-excited all-trans-1,6-diphenyl-1,3, 5-hexatriene (DPH) were studied and a hitherto unknown species was detected from the spectrum observed for an acetone solution. The excited species was generated by monophotonic excitation at 337 nm (ascertained from the dependence of the CARS intensity and that of the transient absorption on the UV power). A lifetime of ≈ 2 μs was obtained by time-resolved CARS. The new species was tentatively ascribed to a radical cation because of its anomalously long lifetime, agreement of the absorption maximum, and disagreement of CARS signals from those of DPH in the S 1 and T 1 states and from those of the radical anion.

Infrared studies of ethene hydrogenation over ZrO2

Hydrogenation of adsorbed ethene over ZrO2 at low temperature was observed by in situ infrared transmission spectroscopy. It was found that di-hydrogen was directly activated on the site where ethene was already adsorbed, which was confirmed by the comparison of adsorbed species produced during the reaction and adsorbed ethane species.