P. Vohringer et al., SOLVENT AND FREQUENCY-DEPENDENCE OF VIBRATIONAL DEPHASING ON FEMTOSECOND TIME-SCALES, Journal of Raman spectroscopy, 26(7), 1995, pp. 535-551
The results of degenerate wavenumber third-order polarization (pump pr
obe) measurements of vibrational dephasing and relaxation in a variety
of solvents are reported, The 20 fs duration pulses are sufficiently
short to allow the preparation and detection of vibrational coherences
(i.e. wavepackets) to 800 cm(-1) oscillator wavenumber, The high qual
ity of the pump-probe data permits the unique determination of eight u
nderdamped modes for the cyanine dye under study, The relaxation of th
e underdamped modes does not show a significant solvent dependence, Th
e relaxation mechanisms that give rise to loss of vibrational coherenc
e and to the solvent-dependent relaxation of low-wavenumber (<100 cm(-
1)) modes are discussed, The wavenumber dependence of the vibrational
dephasing rate varies with a scaled exponential dependence exp(-omega/
omega(0)), or equally well with the inverse square of the vibrational
wavenumber, The exponential form would imply that vibrational dephasin
g occurs by population relaxation, The latter form, however, may imply
higher order coupling between the bath and solute through the solute
vibrational anharmonicity such that the off-diagonal contributions to
the prepared wavepacket relax by pure dephasing, Exponentially decayin
g contributions to the pump-probe signal are found to arise from overd
amped modes that are excited by the short laser pulses as opposed to a
solvation response, The solvent-dependent relaxation of these overdam
ped modes is determined to be similar to diffusive unimolecular barrie
r crossing dynamics observed in dense media.