State-to-state vibrational energy transfer in S1 p-difluorobenzene at intermediate state densities: A change in propensity rules

Collision-induced vibrational energy transfer has been studied from two vibrational levels at intermediate state density in S1 p-difluorobenzene in a supersonic free jet expansion at ∼30–40 K. Transfer was studied from the 7181 (Evib=751 cm−1) and 84 (Evib=733 cm−1) states where ρvib is ∼0.4 states per cm−1. Data were obtained for He, Ne, H2, and D2 for both levels and also for Ar and Kr for 7181. There is some doubt concerning the influence of predissociation of van der Waals complexes on the spectra for these latter partners. The data analysis for 7181 is quantitative for all collision partners. For 84 the analysis is quantitative for H2 and D2 but qualitative for He and Ne because of poor signal levels. The state-to-state propensity rules in this region of the vibrational manifold are compared with those observed at lower state densities, particularly those from 82. The main feature to emerge is a lack of predictability of the major relaxation pathways. There is a clear increase in the importance of transfers involving multiple changes in vibrational quanta for all situations studied and at times such transfers totally dominate. This occurs in spite of the possibility for loss of one quantum of ν8, which is a very efficient channel in transfer from 82. Collision partners that show similar state-to-state branching ratios for 82 show quite different branching ratios for 7181 and for 84.