IR-UV double resonance studies of collision-induced vibrational energy transfer in acetylene

Abstract

Time resolved IR-UV double resonance spectroscopy enables state-selective kinetic studies of rotationally resolved vibrational energy transfer in gas phase acetylene-d2 (C2D2). As a prime example, C2D2/Ar collisions are found to equilibrate laser-induced populations in the 4151 vibrational level with the neighboring 42 and 52 levels, prior to significant transfer to the lower 41 and 51 levels. This vibrational bottleneck effect is attributable to the high efficiency of quasielastic intramolecular V−V energy transfer relative to V−T,R processes. In the contrasting case of C2D2 self-collisions, quasielastic intermolecular V−V energy transfer relieves the bottleneck. Relevant second-order rate constants have been measured, and selectivity with regard to angular momenta and state symmetries identified. Other related experiments aim to demonstrate ways in which intramolecular perturbations contribute to collision-induced rovibrational energy transfer, with potential for enhancement of laser-induced photochemistry. Novel experimental strategies, including Raman−UV double resonance1 and implementation of a narrowband optical parametric oscillator,2 are also being used in this context.