Evidence for mode-specific intramolecular vibrational redistribution in S₁ p-difluorobenzene

The extent of intramolecular vibrational redistribution (IVR) occurring within the fluorescence lifetime of S₁ p-difluorobenzene has been measured with single vibronic level fluorescence spectroscopy for the levels 0°, 6¹, 5¹, 5¹30¹, 5¹30², 3¹, 5², 5²30¹, 5²30², and 3¹5¹. We find that S₁ levels involving one or more quanta of excitation in v₃₀ undergo IVR to a greater extent than those with no excitation in v₃₀. We conclude that vibrational excitation in the out-of-plane mode v₃₀ enhances IVR and we postulate that this may be a consequence of a pseudo-double minimum potential associated with displacement in v₃₀.