The Relative Influences of Ring Modes and Methyl Internal Rotation on the Cold Beam IVR of p ‐Fluorotoluene

Room temperature (300 K) experiments have earlier established that the replacement of a fluorine atom in p ‐difluorobenzene (pDFB) with a methyl group to make p ‐fluorotoluene (pFT) generates a qualitative difference in intramolecular vibrational redistribution (IVR) characteristics as seen in the S 1 states. Here we report S 1 –S 0 fluorescence excitation and dispersed single vibronic level (SVL) fluorescence spectra that have been obtained for IVR comparisons in the cold (5 K) environment of supersonic expansions. As in the 300 K experiments, the cold beam S 1 vibrational energy threshold for IVR is substantially lower in pFT. The vibrational congestion in dispersed fluorescence that reveals extensive S 1 level interactions first appears after pumping an S 1 pFT level near 800 cm −1 . In contrast, congestion in pDFB spectra is still absent from levels with twice this energy. Attention is directed to the relative S 1 ring level structures as a potential contributor to the distinctive IVR behaviors. Dispersed fluorescence from the S 1 zero point levels and fluorescence excitation spectra are combined with published information about S 1 fundamentals to show that the S 1 vibrational level structures of the two molecules are as closely related as those of an isotopomer pair. It is argued that the small differences in S 1 fundamentals cannot be a principal contribution to the qualitative IVR differences.