A discharge-flow study of the self-reaction of IO

Abstract

A discharge-flow tube equipped with resonance-fluorescence detection for I atoms and chemiluminescence detection for O atoms has been used to investigate the kinetics of the reaction IO + IO → products (1) at T = 296 ± 1 K and P = 1.9–2.2 Torr. The rate constant was found to be k₁ = (9.3 ± 1.9) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, where −d[IO]/dt = 2k₁[IO]² and the errors are 95% confidence limits of the mean of the experimental rate constants combined with an estimated further 20% error in other parameters. The determination of the rate constant for reaction (1) also required the investigation of the kinetics of the reaction IO + CF₃ → products (2). The rate constant was found to be k₂ = (1.6 ± 0.7) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, where errors are 95% confidence limits of the slope of the regression line of a second-order plot, combined with systematic errors of 20% for the calibration of [CF₃]. I atoms are generated in reactions (1) and (2)IO + IO → 2I + O₂→OIO + ICF₃ + IO → CF₃O + I

The yields have been quantified, and indicate that k_1a/k₁ = 0.56 ± 0.20, with k_1b/k₁ = (1 − k_1a/k₁); and k_2a/k₂ = 0.4 ± 0.1. Our experiments differ from others previously reported in that the results do not rely on a value for the cross section for absorption of IO in the visible region. Atmospheric modelling studies show that the IO self-reaction plays a minor role in ozone depletion in the marine troposphere and that further work is required to quantify the fate of OIO, a product of this reaction.