Mode-dependent intramolecular vibrational redistribution in the S₁ state of jet-cooled p -difluorobenzene

Fluorescence spectra of p-difluorobenzene cooled in supersonic expansions have been measured following laser excitation of a variety of discrete absorption features in the S₁-S₀ transition. The spectra reveal that the onset for S₁ state mixing, as gauged from the fraction of unstructured emission in the dispersed fluorescence, occurs in the vicinity of ε_vib ∼ 1200 cm^-1. Estimates of the fraction of unstructured emission from emitting levels containing excitation in the out-of-plane mode v₃₀ indicate that state mixing is more advanced for these levels, as has been concluded previously from dispersed fluorescence studies of p-difluorobenzene at 300 K. Comparisons between the 300 K data and the results obtained for jet-cooled p-difluorobenzene support the view that rotation-vibration state mixing contributes to the intramolecular redistribution of vibrational energy in regions of low-to-medium vibrational state densities.