Vibrational energy transfer from four levels below 410 cm^-1 in S₁ p-difluorobenzene. : I. A strong collision partner dependence in state-to-state transfer by monatomics

Collision-induced vibrational energy transfer has been studied from four levels [30² (E_vib=240 cm^-1), 8² (E_vib=361 cm^-1), 27¹ (E_vib=403 cm^-1) and 6¹ (E_vib=410 cm^-1)] in S₁ p-difluorobenzene in supersonic free jet expansions of He, Ne, Ar, and Kr at ∼30-40 K. In broad terms the trends are similar to those observed previously in studies of aromatics: the transfer is highly selective, and one quantum changes in the low frequency modes are preferred. However, a significant collision partner dependence is observed, whereby changing from He through to Kr causes a substantial increase in multiple quanta (|Δv]>1) transfer. SSH-T calculations fail to capture this trend. The preference for |Δv|>1 transfer appears to be enhanced as the interaction time and attractive force on the collision partner increase. Consequently, it is predicted that (i) differences in the state-to-state branching ratios between collision partners will increase as the temperature is lowered; (ii) for a particular collision partner there will be an increase in |Δv|>1 transfer with decreasing temperature; and (iii) |Δv|>1 transfers will be most important for collision partners with small velocities (i.e., large masses), large intermolecular potential well depths (∈) and size (σ). The nearly isoenergetic 27¹ and 6¹ levels have virtually identical state-to-state branching ratios for Ar and small differences are observed for He. This suggests that the branching ratios are not particularly sensitive to the initial vibrational motion. Relaxation of 6¹ and 27¹ is inefficient compared with relaxation from 30² and 8².