C-C AND C-H BOND SPLITS OF LASER-EXCITED AROMATIC-MOLECULES .4. SPECIFIC RATE CONSTANTS AND BRANCHING RATIOS FOR THE DISSOCIATION OF THE XYLENES

o-Xylene, m-xylene, and p-xylene were excited under collision-free conditions by nanosecond laser pulses of low intensity at 193 nm. The electronic excitation is followed predominantly by fast internal conversion to the electronic ground state. The CH3 radicals produced in the subsequent dissociation of the vibrationally hot electronic ground state were monitored by 2 + 1 REMPI, 3 + 1 REMPI, and VUV single-photon ionization. Methyl yields were calibrated by means of the photolysis at 193 nm of ethylbenzene and nitromethane. At an excitation energy near 53 000 cm(-1), 15% (+/-3), 11% (+/-3), and 12% (+/-3) of o-, m-, and p-xylene, respectively, were found to dissociate via C-C bond cleavage whereas the dominant dissociation pathway was found to be C-H bond split in one of the methyl groups. Specific rate constants for dissociation were also measured. Analysis of these results by unimolecular rate theory permits to calculate thermal branching ratios between C-H and C-C channels of the xylene fission reactions.