Kinetics of the radical - Radical reaction, O(³P_J) + OH(X²∏_Ω) → O₂ + H, at temperatures down to 39 K

The kinetics of the reaction between O atoms and OH radicals, both in their electronic ground state, have been investigated at temperatures down to ca. 39 K. The experiments employed a CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) apparatus to attain low temperatures. Both reagents were created using pulsed laser photolysis at 157.6 nm of mixtures containing H₂O and O₂ diluted in N₂ carrier gas. OH radicals were formed by both direct photolysis of H₂O and the reaction between O('D) atoms and H₂O. O(³P) atoms were formed both as a direct product of O₂ photolysis and by the rapid quenching of O(¹D) atoms formed in that photolysis by N₂ and O₂. The rates of removal of OH radicals were observed by laser-induced fluorescence, and concentrations of O atoms were estimated from a knowledge of the absorption cross-section for O₂ at 157.6 nm and of the measured fluence from the F₂ laser at this wavelength. To obtain a best estimate of the rate constants for the O + OH reaction, we had to correct the raw experimental data for the following: (a) the decrease in the laser fluence along the jet due to the absorption by O₂ in the gas mixture, (b) the increase in temperature, and consequent decrease in gas density, as a result of energy released in the photochemical and chemical processes that occurred, and (c) the formation of OH(v = 0) as a result of relaxation, particularly by O₂, of OH radicals formed in levels v > 0. Once these corrections were made, the rate constant for reaction between OH and O(³P) atoms showed little variation in the temperature range of 142 to 39 K and had a value of (3.5 ± 1.0) × 10^-11 cm ³ molecule^-1 s^-1. It is recommended that this value is used in future chemical models of dense interstellar clouds. © 2006 American Chemical Society.