Unraveling the dissociation of dimethyl sulfoxide following absorption at 193 nm

We have studied the photodissociation of dimethyl sulfoxide, DMSO-h(6) and DMSO-d(6), at 193 nm using the technique of photofragment translational spectroscopy with a tunable vacuum ultraviolet product probe provided by undulator radiation on the Chemical Dynamics Beamline at the Advanced Light Source. In contrast to previous investigations we have found the dissociation to proceed via a stepwise mechanism involving multiple reaction channels. The primary dissociation, S-C bond cleavage to eliminate a methyl radical, was found to have two competing channels with distinct translational energy distributions. The translational energy distribution for the major primary dissociation channel suggests that it proceeds in a statistical manner on the ground electronic surface following internal conversion. In competition with this channel is a primary dissociation that exhibits a translational energy distribution suggestive of dissociation on an excited electronic surface with most of the available energy partitioned into translational and electronic degrees of freedom, Secondary decomposition of the CD3SO intermediate was found to proceed exclusively via C-S bond cleavage, CD3SO-->CD3 + SO. However, secondary decomposition of the CH3SO intermediate was found to exhibit competition between CH3SO-->CH3+SO and CH3SO-->CH2SO+H. The dissociation to CH3 and SO was the major secondary decomposition channel with the translational energy distribution indicating a barrier to recombination of >8 kcal/mol. While a minor hydrogen atom elimination channel was found to play a role in secondary decomposition of CH3SO intermediates, no analogous secondary C-D bond cleavage was detected from the CD3SO intermediates indicating the importance of tunneling in the secondary decomposition of CH3SO. (C) 1997 American Institute of Physics.