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 (∼3000 cm^-1) is used to study vibrational energy redistribution with ∼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 (∼1600-1400 cm^-1) relax by populating the lower energy vibrations (∼1100-480 cm^-1) in ∼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.