SOLVENT AND FREQUENCY-DEPENDENCE OF VIBRATIONAL DEPHASING ON FEMTOSECOND TIME-SCALES

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.