Channel-three decay in benzene a picosecond fluorescence investigation

Fluorescence quantum yields, decay times and spectra have been measured following excitation of benzene vapour above the channel-three threshold. Experiments have been carried out on the static gas and on molecules jet-cooled to a vibrational temperature of 160 K. The excitation source was a recently developed high-power, picosecond, narrow-bandwidth laser continuously tunable from 224 to 252 nm. Channel-three decay was observed for all vibrational levels, even those not excited via line-broadened transitions. In addition it was observed that fluorescence decay curves resulting from excitation just above the channel-three threshold were non-exponential. It is proposed that the cause of absorption line broadening — intramolecular vibrational redistribution — is not directly related to channel-three, which is postulated to be coupling to a state X, which could be an isomeric form of benzene or a hidden singlet electronic state.