PHOTOFRAGMENTATION OF CHLOROBENZENE - TRANSLATIONAL ENERGY-DISTRIBUTION OF THE RECOILING C1 FRAGMENT

Laser photodecomposition of a C6H5Cl molecular beam was studied by the measurement of time-of-flight (TOF) distributions of the Cl photofragment at 193 and 248 nm. In a 193 nm photolysis of C6H5Cl, a distribution of the total translational energy obtained from the TOF spectrum in a 193 nm photolysis is well explained by the superposition of three distribution functions; one Boltzmann, and two Gaussian distribution functions. While a photolysis of C6H5Cl at 248 nm gave rise to a translational energy distribution which consists of two distributions, one Boltzmann and one Gaussian. In contrast to the above, a 193 nm photolysis of C6F5Cl (pentafluorochlorobenzene) resulted in a remarkably reduced kinetic energy distribution of the Cl fragments whose entire profile can be well expressed by single Boltzmann distribution function. These results can lead to a conclusion that the photodecomposition of the C-Cl bond in chlorobenzene by the 193 nm excitation takes place through three different dissociation channels with probabilities of similar magnitudes; (1) a direct dissociation or very fast predissociation, (2) a channel via vibrationally excited triplet levels, and (3) a channel via highly excited vibrational levels of the ground electronic state (hot molecules). The photodecomposition of C6H5Cl at 248 nm occurs dominantly via the second and third channels. Laser excited C6F5Cl, however, dissociated through hot molecules.