Product-state-resolved dynamics of the elementary reaction of atomic oxygen with molecular hydrogen, O(3P) + D2 → OD(X2Π) + D

Elementary three-atom systems provide stringent tests of the accuracy of ab initio theory. One such important reaction, O(3P) + H2 → OH(X2Π) + H, has eluded detailed experimental study because of its high activation barrier. In this reaction, both the ground-state reactant atom and product diatomic molecule have open-shell character, which introduces the intriguing complication of non-Born–Oppenheimer effects in both the entrance and the exit channels. These effects may be probed experimentally in both the fine-structure and the Λ-doublet splittings of the OH product. We have used laser-induced fluorescence to measure OD internal product-state distributions from the analogous reaction of O(3P) with D2, enabled by a unique high-energy O(3P) source. We find that the OD (ν′ = 0) product is rotationally highly excited, in excellent agreement with earlier theoretical predictions. However, the distributions over the OD(X2Π) fine-structure and Λ-doublet states, diagnostic of electronic non-adiabaticity in the reaction, challenge the prevailing theoretical understanding. The reaction O(3P) + H2 → OH(X2Π) + H has, until now, eluded detailed experimental investigation. Now, a laser-induced fluorescence study of the deuterated analogue has revealed product-state distributions that defy the current descriptions of non-Born–Oppenheimer mixing on coupled potential energy surfaces, issuing new challenges to theory.