Roaming is the dominant mechanism for molecular products in acetaldehyde photodissociation

Reaction pathways that bypass the conventional saddle-point transition state (TS) are of considerable interest and importance. An example of such a pathway, termed 'roaming,' has been described in the photodissociation of [H.sub.2]CO. In a combined experimental and theoretical study, we show that roaming pathways are important in the 308-nm photodissociation of C[H.sub.3]CHO to C[H.sub.4] + CO. The C[H.sub.4] product is found to have extreme vibrational excitation, with the vibrational distribution peaked at [approximately equal to] 95% of the total available energy. Quasiclassical trajectory calculations on full-dimensional potential energy surfaces reproduce these results and are used to infer that the major route to C[H.sub.4] + CO products is via a roaming pathway where a C[H.sub.3] fragment abstracts an H from HCO. The conventional saddle-point TS pathway to C[H.sub.4] + CO formation plays only a minor role. H-atom roaming is also observed, but this is also a minor pathway. The dominance of the C[H.sub.3] roaming mechanism is attributed to the fact that the C[H.sub.3] + HCO radical asymptote and the TS saddle-point barrier to C[H.sub.4] + CO are nearly isoenergetic. Roaming dynamics are therefore not restricted to small molecules such as [H.sub.2]CO, nor are they limited to H atoms being the roaming fragment. The observed dominance of the roaming mechanism over the conventional TS mechanism presents a significant challenge to current reaction rate theory. reaction dynamics | roaming mechanisms | photochemistry | quasiclassical trajectories | transition state