Vibrational energy redistribution in polyatomic liquids:: Ultrafast IR-Raman spectroscopy of nitromethane

Ultrafast anti-Stokes Raman spectroscopy of liquid nitromethane (NM) after mid-IR excitation in the C-H stretching region (similar to 3000 cm(-1)) is used to study vibrational energy redistribution with similar to 1 ps time resolution. Both vibrational energy relaxation (VER) and vibrational cooling (VC) are discussed. Raman probing of CCl4 mixed with the NM is used to monitor the buildup of excitation in the bath of collective lower frequency excitations (phonons). Combining the intramolecular and bath data, a new and intuitive way of visualizing VC in a polyatomic liquid is presented. In NM, VC occurs in three stages. First, energy deposited in the C-H stretch (and a small amount in first overtones of NO2 stretching and CH bending vibrations) is redistributed to every other vibration in a few picoseconds. Second, the higher energy daughter vibrations of the C-H stretch decay (similar to 1600-1400 cm(-1)) relax by populating the lower energy vibrations (similar to 1100-480 cm(-1)) in similar to 15 ps. Third, the lower energy vibrations excited in the first two stages relax by exciting the bath in 50-100 ps. Although the average vibrational energy decreases with time, this process differs from the usual vibrational cascade description of VC.