Laser photolysis of benzene. V. Formation of hot benzene

Abstract

Flash photolysis of gaseous benzene has been carried out with a KrF laser (248.4 nm) as an excitation source. Two new transients have been detected, one of which has a structured absorption in the wavelength region from 235 to 260 nm. It has a rise time of 80 ns and a long decay time (≳25 μs). The precursor of this transient has also been detected. It appears as a shoulder at 225 nm and has a lifetime of 80 ns. This species is postulated to be a highly excited vibrational state of the ground electronic state, namely, a hot benzene molecule. This species is quenched by ground state benzene as well as by 12 different foreign gases, including oxygen and nitrogen, which have similar quenching rates. The decay of the hot benzene is accompanied by an increase in the temperature of the sample, which leads in turn to formation of the species with the structured absorption. The temperature of the equilibrated sample was determined by comparing the thermalized spectra in the nanosecond time scale with the steady-state absorption spectra at higher temperatures. The temperature rises by about 50° when 20 Torr of benzene are excited with an energy of 33 mJ/cm2. This temperature increase indicates that the yield of hot benzene is 0.6±0.2. The Tn←T1 absorption spectrum has been measured and the intersystem crossing yield is estimated to be 0.2±0.1. The major nonradiative channel in benzene is internal conversion.