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 (E_vib=751 cm^-1) and 8⁴ (E_vib=733 cm^-1) states where ρ_vib 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.