A temperature dependence kinetic study of O(1D) + CH4: overall rate coefficient and product yields

Abstract

Using a recently-developed chemiluminescence technique for monitoring O(¹D), the rate coefficient, k₁, of the important atmospheric reaction O(¹D) + CH₄ → products has been determined over a wide temperature range, 227 to 450 K. The rate coefficient was shown to be independent of temperature, having a value of (1.91 ± 0.08) × 10⁻¹⁰ cm³ s⁻¹; the quoted uncertainties are with 95% confidence. This highly precise value, based on an extended set of determinations with very low scatter, is significantly greater, 26%, than current recommended values. Secondly, the fraction of O(¹D) quenched to O(³P) by CH₄, k_1q/k₁, was precisely determined from chemiluminescence decays over the temperature range 236 to 340 K. A temperature independent value for k_1q/k₁ of 0.002 ± 0.003 was found. Finally, LIF detection of OH has been applied to accurately determine the product branching fraction to OH of O(¹D) + CH₄ at room temperature. Our value, k_1a/k₁ = 0.76 ± 0.08 (95% confidence), is in line with recent determinations by other groups.