JET SPECTROSCOPY AND EXCITED-STATE DYNAMICS OF BENZYL AND SUBSTITUTED BENZYL RADICALS

Benzyl and its p-fluoro and p-methyl derivatives are produced by the ArF laser (193 nm) photolysis of their chlorides in the supersonic free jet. The spectroscopy and excited state dynamics of these radicals are studied by the laser induced fluorescence (LIF) method under the collision free condition. The assignments of vibronic bands are carried out from the LIF excitation and dispersed spectra and the vibrational energies of the D1 state are determined. The excitation spectrum of p-fluorobenzyl shows quite similar vibrational structure to that of p-fluorotoluene up to about 1000 cm-1 from the 0(0)0 band, which indicates that D2 of p-fluorobenzyl lies about 1000 cm-1 above D1 and no vibronic coupling exists lower than this energy. On the other hand, benzyl and p-methylbenzyl show very complicated and irregular vibronic structures in excitation spectra, which are not similar to those of toluene and p-xylene. This complication is explained by the D1-D2 vibronic coupling caused by low lying D2 states in these radicals. Time profiles of the emission intensity of p-fluorobenzyl and p-methylbenzyl show single exponential decay and their lifetimes do not indicate significant dependence on vibronic levels. On the other hand, benzyl shows dual exponential decay, which is interpreted by intermediate coupling case behavior.