State-correlated DC slice imaging of formaldehyde photodissociation: roaming atoms and multichannel branching

Abstract

High-resolution slice imaging methods allow for detection of single product quantum states with sufficient velocity resolution to infer the full correlated product state distribution of the undetected fragment. This is a level of detail not available in previous studies of formaldehyde photodissociation, and in this application it reveals startling new aspects of unimolecular decomposition. The CO rotational distributions from near ultraviolet dissociation of formaldehyde are bimodal, and the imaging experiments allow us to decompose these into two dynamically distinct components: the conventional molecular dissociation over a high exit barrier, and a novel ‘roaming atom’ reaction in which frustrated radical dissociation events lead to intramolecular H abstraction, bypassing the transition state entirely. In probing the dynamics of the conventional molecular dissociation over the barrier, we use the complete v_H2-j_CO correlation to model the exit channel dynamics in new detail. Furthermore, these state-correlated measurements provide insight into radical–radical reactions and the underlying dynamics and energy dependence of the roaming pathway.

Acknowledgements

We would like to thank Dave Townsend and Suk Young Lee for their contributions to the initial work. We thank Gregory E. Hall and Anatoly Komissarov for the finite slicing inversion program, as well as Wen Li for the IMACQ and IMAN programs. We acknowledge J. Bowman and his group (Emory University), and F. Suits (IBM) for their contributions. This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences, of the US Department of Energy and under the Contract Number: DE-FG02-04ER15593.