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 (similar to 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. (C) 1998 Elsevier Science B.V. All rights reserved.