Do state-to-state propensity rules for vibrational energy transfer remain valid in the intermediate state density region?

Abstract

State-to-state branching ratios are reported for vibrational energy transfer (VET) from the 8 2 and 7 18 1 levels in S 1 ϱ-difluoro-benzene by He and H 2 in a supersonic free jet expansion. 8 2 is in the sparse region of the vibrational manifold ( E vib=350 cm −1; ϱ vib≈0.05 states / cm −1) while 7 18 1 is in the intermediate state density region ( E vib=751 cm −1; p vib≈0.4 states / cm −1). Two principal results emerge: (i) the propensity rules that apply for 8 2 (sparse region) are not followed for 7 18 1 (intermediate region) and (ii) He and H 2 show virtually identical state-to-state branching ratios for 8 2 but not for 7 18 1.