Photodissociation of CH3OCl to CH3O + Cl at 248 nm

This study examines the 248 nm photodissociation of methyl hypochlorite (CH3OCl), a molecule that serves as an atmospheric chlorine reservoir. The data show that the primary photodissociation channel is cleavage of the O−Cl bond to produce Cl atoms and CH3O radicals, a result consistent with the direct dissociation mechanism found by other computational and experimental studies of alkyl hypohalites. Photofragment translational spectroscopy with a crossed laser-molecular beam apparatus, coupled with tunable VUV photoionization detection, identified the momentum-matched products at m/e = 35 (Cl+ from Cl atoms) and m/e = 29 (the CHO+ daughter ion of CH3O). Products were formed with a narrow range of recoil kinetic energies, peaking at 48 kcal/mol in the center-of-mass reference frame, with a full-width half-maximum of 4 ± 2 kcal/mol. This kinetic energy distribution shows that the CH3O products are formed with a very narrow range of internal energies, and a simple model shows that to conserve angular momentum this internal energy, near 20 kcal/mol, is partitioned primarily to rotational energy. Thus CH3OCl could serve as a photolytic precursor of CH3O radicals with high and well-defined rotational and translational energies.