Low-Temperature Experimental and Theoretical Rate Constants for the O( 1 D) + H 2 Reaction.

In the present joint experimental and theoretical study, we report thermal rate constants for the O(1 D) + H2 reaction within the 50-300 K temperature range. Experimental kinetics measurements were performed using a continuous supersonic flow reactor coupled with pulsed laser photolysis for O(1 D) production and pulsed laser-induced fluorescence in the vacuum ultraviolet wavelength range (VUV LIF) for O(1 D) detection. Theoretical rate constants were obtained using the ring polymer molecular dynamics (RPMD) approach over the two lowest potential energy surfaces 11 A' and 11 A″, which possess barrierless and thermally activated energy profiles, respectively. Both the experimental and theoretical rate constants exhibit a weak temperature dependence. The theoretical results show the dominant role of the 11 A' ground state and that contribution of the 11 A″ excited state to the total thermal rate decreases dramatically at lower temperature. Agreement between the experimental and theoretical results is good, and the discrepancy does not exceed 25%. It is argued that these differences are likely to be due to nonadiabatic couplings between the 11 A' and 21 A' surfaces.