Nonadiabatic reactive scattering in atom+triatom systems: Nascent rovibronic distributions in F+ H O→HF+OH

Crossed supersonic jet studies of F+ H O→HF+OH (Π2 3/2, Π2 1/2) have been performed under low density, single collision conditions at Ecom =6 (2) kcal/mol, yielding rotational, vibrational, and spin-orbit state distributions in the nascent OH product by laser induced fluorescence methods. The lowest reaction barriers on the ground and first excited electronic surfaces are ΔE≈4 kcal/mol and ΔE≈25 kcal/mol, correlating with OH (Π2 3/2) and OH (Π2 1/2), respectively. Although only reactions on the ground state potential are Born-Oppenheimer allowed at the experimental collision energies, both ground and excited spin-orbit OH products are observed in a Π2 3/2: Π2 1/2 =69 (1) %:31 (1) % branching ratio. This indicates the presence of strong nonadiabatic surface hopping interactions, in agreement with previous results for the F+ D O→DF+OD reaction. Despite clear differences in the rotational distributions between F+ H O and F+ D O isotopic reactions, the overall electronic branching into spin-orbit manifolds is nearly identical for both OH and OD products. Furthermore, when plotted versus total electronic+rotational energy, the nascent OH and OD populations each lie on single curves, with pronounced kinks in the Boltzmann plots suggestive of microscopic branching in the reaction dynamics. Such an equivalence of electronic and rotational energy release in the OH/OD products is consistent with predominantly nonadiabatic processes taking place in the immediate post-transition state region rather than asymptotically in the exit channel. © 2009 U.S. Government.