Kinetics of O(1D) + H2O and O(1D) + H2: absolute rate coefficients and O(3P) yields between 227 and 453 K

Abstract

The rate coefficients for the crucial atmospheric reactions of O(¹D) with H₂O and H₂, k₁ and k₂, were measured over a wide temperature range using O(¹D) detection based on the chemiluminescence reaction of O(¹D) with C₂H. Analyzing the decays of the chemiluminescence intensities yielded a value for k₁(T) of (1.70 × 10⁻¹⁰exp[36 K/T]) cm³ s⁻¹. Multiplying or dividing k₁(T) by a factor f(T) = 1.04 exp(5.59(|1 K/T − 1/287|)), gives the 95% confidence limits; our new determination, in good agreement with previous studies, further reduces the uncertainty in k₁. An extended study of k₂ yielded a temperature independent rate constant of (1.35 ± 0.05) × 10⁻¹⁰ cm³ s⁻¹. This precise value, based on an extended set of determinations with very low scatter, is significantly larger than the current recommendations, as were two other recent k₂ determinations. Secondly, the fractions of O(¹D) quenched to O(³P) by H₂O and H₂, k_1b/k₁ and k_2b/k₂, were precisely determined from fits to chemiluminescence decays. A temperature-independent value for k_1b/k₁ of 0.010 ± 0.003 was found. For the quenching fraction k_2b/k₂ a value of 0.007 ± 0.007 was obtained at room temperature. Both determinations are significantly smaller than values and upper limits from previous studies.