The Photodissociation of CH_3OCl to CH_3O + Cl at 248 nm

We investigate the 248 nm photodissociation of methyl hypochlorite (CH_3OCl), an atmospheric chlorine reservoir species. Photofragment translational spectroscopy with a crossed laser-molecular beam apparatus, coupled with tunable VUV photoionization detection, identifies the primary photodissociation channel as cleavage of the O-Cl bond to produce Cl atoms and CH_3O radicals. This result is consistent with the direct dissociation mechanism suggested by other computational and experimental studies of alkyl hypohalites. The measured recoil kinetic energy distribution of the products shows that the CH_3O product is formed with a very narrow range of internal energies. A simple model predicts from conservation of angular momentum that nearly all of the internal energy is in rotational excitation. CH_3OCl thus serves as a photolytic precursor of CH_3O radicals with high and well-defined rotational and translational energies.