State-to-state vibrational energy transfer from 6¹ benzene induced by low-energy collisions with nitrogen: Temperature-dependent propensities

State-to-state rate constants for vibrational energy transfer from the 6¹ level of S₁ benzene induced by collisions with N₂ have been measured as a function of temperature in the range 41-13 K in a supersonic free jet expansion. It is found that there is a change in the propensity for transfer into the destination channels as the temperature is decreased. The efficiency of transfer to the 16² level increases by a factor of ∼2 compared with transfer to the spectrally unresolved pair of levels 11¹/16¹ with the decrease in temperature over this range. Comparison with the room temperature results of Parmenter and Tang (Chem. Phys. 1978, 27, 127) shows that the branching ratio of 16² to 11¹/16¹ increases from <0.18 at room temperature to ∼1.0 at 13 K. The increase in propensity for 16² occurs despite the fact that transfer to this level involves a total change in vibrational quanta of 3, compared with 2 for 11¹ and 16¹. The trend in the state-to-state propensities for collision-induced vibrational energy transfer from 300 to 13 K suggests that they are approaching those seen in van der Waals predissociation as the temperature approaches 0 K.