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

Abstract

AbstractRoom 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 S1 states. Here we report S1–S0 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 S1 vibrational energy threshold for IVR is substantially lower in pFT. The vibrational congestion in dispersed fluorescence that reveals extensive S1 level interactions first appears after pumping an S1 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 S1 ring level structures as a potential contributor to the distinctive IVR behaviors. Dispersed fluorescence from the S1 zero point levels and fluorescence excitation spectra are combined with published information about S1 fundamentals to show that the S1 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 S1 fundamentals cannot be a principal contribution to the qualitative IVR differences.