State-to-state vibrational relaxation in 1B2u benzene-d6 induced by low energy collisions with He, H2, N2, and Ar

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Abstract
Time-resolved dispersed fluorescence spectroscopy has been used to study very low energy collision-induced vibrational relaxation in 1B2u benzene-d6 expanded in a supersonic free jet. Several expansion gases have been used to enable rate coefficients for vibrational relaxation to be obtained as a function of collision partner in the very low energy collision regime. Benzene-d6 is found to undergo vibrational deactivation with each of the chosen collision partners He, H2, N2, and Ar. This study focuses on measuring state-to-state relaxation rate coefficients and branching ratios for vibrational relaxation from the S1 vibrational level 61 (εvib =498 cm−1). Relaxation occurs to all the known S1 vibrational levels lying lower in energy than 61, namely 101 (εvib =454 cm−1), 162 (εvib =414 cm−1), 111 (εvib =382 cm−1), 41 (εvib =306 cm−1), 161 (εvib =207 cm−1), and 00 (εvib =0 cm−1). Two other unidentified channels also participate. Nitrogen and argon are found to be the most efficient collision partners in inducing relaxation (σobs/σhs ∼1.1, 1.7, respectively), whereas hydrogen and helium are found to be an order of magnitude less efficient (σobs/σhs ∼0.08, 0.02, respectively). The relaxation channels 61→101 and 61→111 are the dominant deactivation pathways for all the gases studied. The relative importance of relaxation pathways changes with collision partner. There appears to be a light or small collision partner effect, whereby the propensity for transfer through the channel 61→101 (Δεvib =−44 cm−1) seems to be redistributed among other channels, involving a greater energy or momentum gap, as the collision partner becomes lighter.