Angular momentum influences on vibrational relaxation pathways from 6¹ benzene

Vibrational energy transfer from the 6¹ level of S₁(¹B_2u) benzene has been studied at low collision energies in supersonic free jet expansions for the collision partners H₂, D₂, N₂, CH₄, C₂H₂, and c-C₃H₆. Three of the four accessible vibrational relaxation channels in S₁ benzene are found to be significantly populated: the 16² level, the spectrally unresolved 11¹ and 16¹ levels, and the 0⁰ level. A small amount of transfer to the 4¹ level was observed with H₂ as a collision partner. It is found that: (i) transfer to 0⁰ 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 0⁰ 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-tostate 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. 6¹→0⁰ 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.