Specific rate constants for the fragmentation of vibrationally excited benzyl radicals

The competition between collisional deactivation and fragmentation of vibra tionally highly excited benzyl radicals in the ground electronic state has been investigated. Vibrationally highly excited benzyl radicals have been p repared from the dissociation of ethylbenzene after UV photon excitation fo llowed by rapid internal conversion. Subsequently, the benzyl radicals are collisionally deactivated in collisions with the bath gas, argon. After sel ected delay times the benzyl radicals were excited again by absorption of a nother UV-photon, followed by fast internal conversion. A certain fraction of the reexcited benzyl radicals decomposes while the rest is collisionally stabilized. The total loss of benzyl radicals due to reexcitation has been monitored by UV absorption spectroscopy. Absolute values of the specific r ate coefficients for fragmentation have been derived for excitation energie s between 47000 and 53000 cm(-1). These specific rate constants agree well with a deconvolution of the thermal high pressure rate constant obtained fr om previous shock wave experiments.