Photofragmentation of chlorotoluenes and dichlorobenzenes: Substituent effects on the dissociation mechanism, and angular distribution of the Cl fragment

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Abstract
Time-of-flight spectra of the Cl photofragments were measured for molecular beams of o-, m-, and p-chlorotoluene (ClC6H4CH3) and o-, m-, and p-dichlorobenzene (ClC6H4Cl) irradiated by a 193 nm excimer laser pulse. The observed translational energy distributions of photofragments revealed that these chlorinated benzene derivatives dissociate via three different channels: (1) very fast predissociation and/or a direct dissociation, (2) predissociation through the triplet state, and (3) predissociation via highly excited vibrational levels of the ground electronic state (hot molecules). The three dissociation channels for dichlorobenzene have similar probabilities (∼0.3) in accord with those for chlorobenzene, indicating no significant change caused by the additional chlorine atom. The methyl substituent on chlorobenzene (chlorotoluene), however, remarkably induces dissociation through triplet states, probably due to the enhanced intersystem crossing by the methyl group. The angular distribution of the photofragment was also measured for p-chlorotoluene and p-dichlorobenzene excited by linearly polarized laser light. Angular distributions of the Cl fragments via the 2nd and 3rd channels were isotropic, while the fastest fragment via the 1st channel has an anisotropic distribution, confirming that the dissociation rate of the 1st channel is shorter than a molecular rotation.