Reactive and nonreactive quenching of OH(A²∑⁺) in collisions with H atoms

We use trajectory surface hopping methods to calculate cross sections and rate coefficients for reactive and nonreactive quenching of OH(A ²∑⁺) in collisions with H atoms. All calculations are based on multireference configuration interaction potential surfaces for the 1¹A′ and 2¹A′ surfaces of water, using a diabatic representation to describe electronic transitions. The overall rate of quenching plus reaction is in good agreement (lower by 25-30%) with earlier estimates at 300 and 1500 K. Analysis of the trajectories shows that this rate is dominated by a capture mechanism on the 2¹A′ state of water, with a nearly unity probability for hopping to the 1¹A′ state following formation of a short-lived collision complex. Both reactive and nonreactive decay of the complex are significant, with branching to H + OH (nonreactive quenching), H + OH (atom exchange with quenching), and O(¹D) + H₂ (reaction) accounting for approximately 45%, 35%, and 20%, respectively, of the product yield. The cross section for atom exchange without quenching is only 1-3% of the total quenching/reaction cross section. In contrast to what is often assumed in energy transfer studies, we find that the quenching and reactive cross sections are very weakly dependent on OH rotational quantum number for quantum numbers in the range 0-15. A statistical model provides only a rough description of product branching and product energy partitioning, indicating that the intermediate complexes are too short-lived to show statistical dynamics. We find that pure rotational energy transfer cross sections are small (few percent) compared with quenching/reaction.