Intramolecular rate processes in highly vibrationally excited benzene

Abstract

Intracavity cw dye laser techniques incorporating photoacoustic detection have been used to measure visible absorption spectra of gas phase benzene (C6H6 and deuterated analogues C6H5D, p-C6H2D4, and C6HD5). The prominent spectral features are attributable to CH stretch overtones that correspond to highly localized (’’bond-selective’’) excitations of individual CH oscillators. These features have broad (∼100 cm−1 FWHM) Lorentzian line shapes that narrow as a function of increasing vibrational energy for most isotopic species. A simple model is presented to interpret the broad line shapes and their energy dependence as a manifestation of intramolecular V→V′ (vibration-to-vibration) energy redistribution and/or dephasing processes that occur rapidly (on 5×10−14 sec time scales), but nonstatistically, in isolated benzene and other polyatomic molecules.