Angular momentum influences on vibrational relaxation pathways from 6(1) benzene

摘要

Vibrational energy transfer from the 6(1) level of S-1(B-1(2u)) benzene has been studied at low collision energies in supersonic free jet expansions for the collision partners H-2, D-2, N-2, CH4, C2H2, and C-C3H6. Three of the four accessible vibrational relaxation channels in S-1 benzene are found to be significantly populated: the 16(2) level, the spectrally unresolved 11(1) and 16(1) levels, and the 0(0) level. A small amount of transfer to the 4(1) level was observed with Hz as a collision partner. It is found that: (i) transfer to 0(0) is generally efficient; and (ii) the state-to-state branching ratios change substantially with collision partner. This is quite different from the trends observed for monatomic collision partners, for which transfer to 0(0) is absent and the state-to-state branching ratios are largely independent of the collision partner's identity [E. R. Waclawik and W. D. Lawrance, J. Chem. Phys. 102, 2780 (1995)]. It is further observed that the rotational contours of collisionally populated levels change. For a particular collision partner the extent of rotational excitation in the destination level increases with increasing vibrational energy gap. For a particular destination level there is considerable variation in rotational excitation amongst collision partners. The state-to-state propensity differences between monatomic partners and diatomics and small polyatomics are suggested to arise because angular momentum constraints are influencing the vibrational state-to-state branching ratios. 6(1)-->0(0) transfer is most affected: it is observed only when the collision partner can accept energy as rotational motion, and its branching ratio is particularly sensitive to the collision partner identity. (C) 1988 American Institute of Physics. [References: 43]