State-to-state vibrational relaxation from levels at state densities up to 2.3 states per cm^-1 in p-difluorobenzene

Collision-induced vibrational energy transfer has been studied from three vibrational levels at intermediate state density in S₁ p-difluorobenzene in a supersonic free jet expansion. Transfer was studied from the 5¹ (E_vib=818cm^-1; ρ_vib=0.6percm^-1), 29² (E_vib=876cm^-1; ρ_vib=0.6percm^-1), and 5¹8² (E_vib=1179cm^-1; ρ_vib=2.3percm^-1) levels. The collision partners include a range of monatomics, diatomics, and polyatomics for 5¹ and 29². Hydrogen was the collision partner for 5¹8². For 29², transfers involving multiple changes in vibrational quanta are important, and generally such transfers dominate. This behavior is different from that observed at low state densities but is analogous to what has been observed previously at intermediate state densities in p-difluorobenzene [Mudjijono and W. D. Lawrance, J. Chem. Phys. 108, 4877 (1998)]. There is a suggestion in the data for c-propane and ethane that transfer to vibrational modes of these collision partners is occurring. 5¹ shows very inefficient relaxation. With the exception of N₂, there is no evidence in the spectra for significant transfer via channels involving multiple changes in vibrational quanta. The state-to-state branching ratios for transfer from 5¹8² were essentially in quantitative agreement with those expected based on transfer from 8². It appears that the in-plane mode ν₅, and combinations involving low frequency modes with ν₅, behave qualitatively differently to the lower frequency, out-of-plane modes. The lower frequency, out-of-plane modes change their state-to-state relaxation preferences with increasing vibrational state density, with multiple quantum changes becoming preferred, while the higher frequency in-plane ν₅ retains the state-to-state preferences seen at low state densities.