INTRAMOLECULAR VIBRATIONAL REDISTRIBUTION IN $S_{1}$ p-DIFLUOROBENZENE

1. R. A. Coveleskie and D. S. Parmenter, J. Mol. Spectrosc. (in press). 2. R. Zimmerman and T. M. Dunn, 35th Symposium on Molecular Spectroscopy, Columbus, Ohio (1980). 3. R. A. Coveleskie, D. A. Dolson, and C. S. Parmenter, J. Chem. Phys. 72, 5774 (1980).Author Institution:Steady-state collision free fluorescence spectra obtained by pumping prominent absorption maxima of p-difluorobenzene (pDFB) exhibit discrete vibrational $structure^{1}$ from $S_{1}$ levels with $E_{vib} \angle 2000$ $cm^{-1}$. The structure is overwhelmed by background congestion as higher $S_{1}$ levels are excited. The evolution from structured to background emission correlates well with the observed onset of absorption line broadening. Congested emission and level broadening are interpreted as spectroscopic views of intramolecular vibrational redistribution. Time-resolved fluorescence spectra have been obtained following 257.3 nm excitation of the $2189 cm^{-1}$ level in $S_{1} ~ pDFB.^{3}$ Electronic quenching of the $S_{1}$ state with up to 40 atm of $O_{2}$ establishers a reduced fluorescence lifetime for the emitting molecules thereby introducing time resolution into steady-state fluorescence observations. Structure fluorescence is enhanced relative to the background as the average $S_{1}$ lifetime is reduced from 5 ns to 10 ps