Hiromi Okamoto

Infrared and Raman spectra of 4-(dimethylamino)benzonitrile and isotopomers in the ground state and vibrational analysis

Abstract Vibrational analysis of 4-(dimethylamino)benzonitrile (DMABN) has been performed. Infrared and Raman spectra of DMABN and its four different isotope-labeled species have been recorded. Assignments of the vibrational bands are given on the basis of normal coordinate calculation on an empirical force field and of the Hartree–Fock and the density functional calculations. Most of the vibrational bands observed have been successfully assigned, and the isotope shifts are reasonably explained. It is clearly demonstrated from the measurements on the isotopomers that assignments of some bands in the literature are not correct. The characteristics of the empirical and the quantum-chemical force fields are compared. It has been found that the strong interaction between the dimethylamino group and the benzene ring is directly reflected in the force field.

Transient Raman spectra of the all-trans and 7-, 9-, 11- and 13-mono-cis isomers of retinal and the mechanism of the cis-trans isomerization in the lowest excited triplet state

Transient Raman studies of isomeric retinals reveal that the photoexcitation and subsequent intersystem crossing results (except for 13-cis) in an identical relaxed triplet species which gives exclusively the all-trans conformer in the ground state. The 13-cis isomer yields another relaxed triplet species. A scheme for the mechanism of the isomerization in T 1 is proposed in which the cis-trans partitioning takes place in the vibrationally, excited levels of the cis T 1 manifold and in which an efficient intersystem crossing occurring in these levels plays an essential role.

Resonance Raman excitation profiles of trans-azobenzene: contribution of a symmetry-forbidden electronic transition to Raman intensities

Abstract Resonance Raman excitation profiles of trans-azobenzene have been measured in the low-energy side of the symmetry-forbidden π∗ ← n absorption maximum at ≈ 450 nm. It is found that this absorption makes a significant contribution to the observed Raman intensities.

Solvation dynamics and vibrational relaxation in resonance Raman and fluorescence lineshapes of tetradesmethyl-β -carotene

Abstract We report the measurements and microscopic theoretical analysis of resonance fluorescence and Raman lineshapes of tetradesmethyl-β-carotene in isopentane at 190 and 230 K. We find the solvent correlation time to be 125 fs at 190 K, 111 fs at 230 K and the vibrational relaxation time to be 253 fs at both temperatures. The dependence of the Raman yield on the solvation dynamics and the detuning is predicted.

Picosecond transient infrared spectroscopy of electronically excited 4-dimethylamino-4′-nitrostilbene in the fingerprint region (1640-940 cm−1)

Abstract A picosecond infrared spectrometer covering nearly the whole fingerprint region has been constructed. The infrared absorption spectrum of electronically excited (S 1 state) 4-dimethylamino-4′-nitrostilbene, which is considered to have an intramolecular charge-transfer character, has been recorded. The molar absorption coefficients of observed prominent absorption bands have been estimated to be of the order of 10 3 mol −1 dm 3 cm −1 . Measurements of polarization dependence (pump polarization parallel/perpendicular to the probe polarization) have been performed for a few prominent bands. Assignments and polarization characteristics of the observed bands are discussed.

Femtosecond time-resolved polarized coherent anti-Stokes Raman studies on reorientational relaxation in benzonitrile

Abstract Femtosecond time-resolved polarized coherent anti-Stokes Raman scattering has been observed for the CN stretching vibration of benzonitrile in the neat liquid and in mixtures of benzonitrile and other liquid compounds. From isotropy and anisotropy measurements, rotational correlation functions have been determined directly in the time domain. In addition to a picosecond decay component, a faster decay component has been found in the case of neat benzonitrile. The Stokes—Einstein relation applies to low-viscosity mixtures of benzonitrile and other liquid compounds, but not to high-viscosity ones.

Femtosecond time-resolved coherent anti-Stokes Raman scattering from acetonitrile: solvent effects on the vibrational dephasing of the CN stretching band

Abstract Femtosecond time-resolved coherent anti-Stokes Raman scattering (CARS) has been observed for the fundamental band of the CN stretching mode (2252 cm −1 of acetonitrile. The CARS signal for the CN stretching shows a non-exponential decay. This signal behavior is attributed to interference between fundamental and hot-band transitions. It is shown that the dephasing of the CN stretching becomes faster with increasing hydrogen-bonding ability of the solvent (proton donor).

Picosecond transient infrared spectra and structure of S1 diphenylacetylene in solution

Abstract Picosecond transient infrared spectra of diphenylacetylene (DPA) were obtained in cyclohexane and acetonitrile solutions for the 1640–940 cm −1 wavenumber region, using a method based on optically heterodyned detection of absorption anisotropy. The observed transient species was assigned to the S 1 state of DPA on the basis of its temporal behavior. Comparison with the reported CARS spectra suggests that S 1 DPA has a planar (probably trans -bent) structure with a center of symmetry in both solutions.

Generation of ultrashort light pulses in the mid-infrared (3000−800 cm−1) by four-wave mixing

Abstract A convenient method of generating picosecond mid-infrared light pulses by four-wave mixing is reported. The generated radiation is widely tunable from near-infrared to 800 cm−1 (with barium fluoride as a mixing medium). Phase-matching conditions are discussed. Although the energy of a pulse generated by this method is rather low (of the order of 10−12 J), it is expected that the pulsed radiation is useful as a probe light in ultrafast time-resolved infrared spectroscopy.

Assignment and anharmonicity analysis of overtone and combination bands observed in the resonance Raman spectra of carotenoids

Abstract The resonance Raman spectra of all- trans carotenoids have been observed in the region of 5000-500 cm −1 for samples in glassy solution at 77 K and in the in vivo state at room temperature. Prominent bands in the wavenumber region higher than 2000 cm −1 are assigned to either overtones or combinations of three modes due to skeletal stretches and the CH 3 in-plane rock. From the wavenumbers of the observed Raman bands, anharmonicity constants for these three modes (including cross-term constants) are obtained. It is found that, for each carotenoid studied, the cross-term anharmonicity constant between the CC and CC stretches is significantly larger than the other anharmonicity constants.