When vibrations interact: ultrafast energy relaxation of vibrational pairs in polyatomic liquids

The ultrafast IR–Raman technique is used to compare picosecond timescale vibrational energy relaxation (VR) of a polyatomic liquid, acetonitrile (CH3CN), following excitation of either a C–H stretching fundamental (∼3000 cm−1) or a pair of vibrations at about the same energy, the CN stretch and C–C stretch. When the vibrational pair is excited via combination band pumping, the CN stretch lifetime decreases by more than an order of magnitude, and an oscillation is observed in the C–C stretch population. These studies of a pair of interacting vibrations on the same molecule provide insight into the complex behavior expected from vibrationally excited nascent products of chemical reactions of condensed-phase polyatomic molecules.