Highly vibrationally excited benzene: Overtone spectroscopy and intramolecular dynamics of C6H6, C6D6, and partially deuterated or substituted benzenes

Abstract

Ultrasensitive dye laser photoacoustic spectra have been obtained for CH and CD stretch overtone excitations of gas phase benzene (C6H6, C6D6, C6HD5), toluene, and fluorobenzenes (C6H5F, C6HF5) in the visible and near infrared spectral regions. Interpretations of the spectral observables (band positions, intensities, and widths) clarify the nature and dynamics of highly vibrationally excited states in benzene and related polyatomic molecules: (1) the states we initially prepare by direct one-photon absorption involve highly localized and selective excitations of CH (CD) bond mode sites; (2) these localized excitations cause dynamical symmetry lowering of the molecular systems; and (3) ultrarapid (i.e., subpicosecond) temporal evolution occurs by highly nonstatistical intramolecular rate processes. Analyses of the energy and state dependence of overtone bandwidths support a mechanistic picture in which sequential state-to-state decays are the preferred dynamical pathways for intramolecular vibrational energy redistribution.