Two-photon spectroscopy of jet-cooled benzonitrile: New results for a charge transfer molecule

We compare the first jet-cooled two-photon fluorescence excitation spectrum of benzonitrile with its one-photon spectrum measured by resonance-enhance d multi-photon ionization (REMPI). Both spectra include the pure electronic transition and one vibrationally induced transition. The inducing mode is the v,, in the one-photon spectrum and the V-27 in the two-photon spectrum. This is similar to benzene, where the corresponding vibration Vg induces a transition in the one-photon and the corresponding vibration v,, induces a transition in the two-photon spectrum. The one-photon spectrum shows no vi bronic bands above 2160 cm(-1). This is ascribed to small Franck-Condon fac tors for the second progression members of totally symmetric vibrations. Th e two-photon fluorescence excitation spectrum has a cut off at the higher e xcess energy of 3000 cm(-1) pointing to a nonradiative relaxation channel s imilar to benzene. No indication for a charge transfer process in the first electronic state of benzonitrile was found up to an excess energy of 3000 cm(-1).