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

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, k1b/k1 and k2b/k2, were precisely determined from fits to chemiluminescence decays. A temperature-independent value for k1b/k1 of 0.010 ± 0.003 was found. For the quenching fraction k2b/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.