An active mode in both the collision-free and collision-induced vibrational dynamics of S₁ p-difluorobenzene

We present evidence that the vibrational mode v′₃₀ = 119.7 cm^-1, an out-of-plane fluorine bend, is specially active in collision-induced level-to-level vibrational energy transfer within S₁ p-difluorobenzene (pDFB). We summarize other evidence that shows v′₃₀ to be also an accelerating mode for collision-free intramolecular vibrational energy transfer in S₁ pDFB. These are the first findings of a common accelerating mode for both collision-free and collision-induced vibrational energy redistribution. The common role can be understood with the proposition that level mixing in the isolated molecule plays a dominant role in both processes. We have examined possible mechanisms for the enhanced coupling of v′₃₀: anharmonic mixing, Coriolis vibration-rotation mixing, and centrifugal vibration-rotation mixing. No single mechanism is identified as the source of the special activity of v′₃₀.