Is there a light collision partner effect in vibrational energy transfer? Relaxation of p-difluorobenzene by He and Ar

State-to-state branching ratios are reported for vibrational energy transfer (VET) from the 30² and 8² vibrational levels in S₁ (¹B_2u) p-difluorobenzene by the collision partners He and Ar in the collision region of a supersonic free-jet expansion. For VET from the 8² level, He removes more energy than does Ar. This is the expected light collision partner effect. For the 30² level, however, Ar unexpectedly removes more energy than He. The opposing behaviour observed for relaxation by He and Ar from two vibrational levels of the same molecule is inconsistent with a universal 'light collision partner effect'.