Solvent and frequency dependence of vibrational dephasing on femtosecond time‐scales

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Abstract
The results of degenerate wavenumber third‐order polarization (pump‐probe) 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 quality of the pump‐probe data permits the unique determination of eight underdamped modes for the cyanine dye under study. The relaxation of the underdamped modes does not show a significant solvent dependence. The relaxation mechanisms that give rise to loss of vibrational coherence and to the solvent‐dependent relaxation of low‐wavenumber (−1) modes are discussed. The wavenumber dependence of the vibrational dephasing rate varies with a scaled exponential dependence exp( —ω/ω0), or equally well with the inverse square of the vibrational wavenumber. The exponential form would imply that vibrational dephasing 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 decaying contributions to the pump—probe signal are found to arise from overdamped modes that are excited by the short laser pulses as opposed to a solvation response. The solvent‐dependent relaxation of these overdamped modes is determined to be similar to diffusive unimolecular barrier crossing dynamics observed in dense media.