VIBRATIONAL-ENERGY TRANSFER FROM THE 6(1) LEVEL OF S-1 (B-1(2U)) BENZENE IN A SUPERSONIC EXPANSION .1. MONATOMIC COLLISION PARTNERS

Vibrational energy transfer has been monitored from the 61 level of 1B2u(S1) benzene seeded in rare gas supersonic free jet expansions at X/D=5, where the temperature is calculated to be ∼10 K. The monatomic collision partners helium, neon, argon, and krypton form the subject of this study. Consequently, transfer is limited to one mechanism, transfer of vibrational energy in benzene to translational energy of the collision pair. The vibrational energy transfer is followed using time resolved, dispersed fluorescence spectroscopy. While there are five possible destination levels, only three are found to be important. These are transfer to the 162 level and transfer to the spectrally unresolved 11 1 and 161 levels. Negligible transfer is observed to both of the remaining two accessible levels, 00 and 41. It is found that the branching ratio for the two destination channels is insensitive to the identity of the collision partner. The branching ratios are reproduced by calculations based on SSH-T theory. The calculations suggest that the insensitivity of the branching ratio to the collision partner is fortuitous: while the combined 111/161 channel retains approximately the same ratio to 162 for all collision partners, the relative importance of the 111 and 161 levels themselves is collision partner dependent. Evidence is presented suggesting that there is significant rotational excitation accompanying the vibrational energy transfer in the case of heavy collision partners. This study, by establishing the behavior of vibration to translation transfer, forms the basis for further studies of the role of vibration to rotation and, subsequently, vibration to vibration mechanisms in vibrational energy transfer in benzene at low temperatures. © 1995 American Institute of Physics.