| Abstract
| - The absorption cross sections of the gas-phase IO radical and the kinetics and products of the IO self-reactionhave been measured using the technique of laser photolysis with time-resolved UV−vis absorption spectroscopy.The IO absorption cross section at the peak of the (4,0) vibronic band of the (A2Π ← X 2Π) transition at427.2 nm, determined using the reaction O(3P) + CF3I to form IO and calibrated relative to the O3 crosssection was found to be (1.9 ± 0.17) × 10-17 cm2molecule-1 at 295 K and 1.13 nm fwhm spectral resolution.The IO cross sections were found to exhibit a negative temperature dependence. The kinetics of the IO self-reaction were measured using the reaction O(3P) + I2 to form IO, and the self-reaction rate coefficient k1,determined from the loss of IO radicals in the absence of ozone, was found to be (8.2 ± 1.3) × 10-11molecules-1 cm3 s-1 at 295 K and 760 Torr. The self-reaction rate coefficient was found to be independentof pressure between 100 and 760 Torr, and to display a negative temperature dependence between 222 and325 K, described by k1 = (4.1 ± 3.4) × 10-11 exp{(220 ± 230)/T} molecules-1 cm3 s-1. All errors are 2σ.Four potential product channels exist for the IO self-reaction: IO + IO → 2I + O2 (1a), IO + IO → I2 + O2(1b), IO + IO → OIO + I (1c), IO + IO + M → I2O2 + M (1d). No direct measurement of I atom productionwas performed. I2 formation was observed, but attributed to IO-catalyzed I atom recombination (I + IO +M → I2O + M; I + I2O → I2 + IO). OIO formation was observed and shown to result from the IO self-reaction. Formation of a broadband absorbing product underlying the IO absorption at low (λ < 400 nm)wavelengths was observed, and tentatively attributed to I2O2. The OIO cross sections and yield from theIO + IO reaction were determined via measurement of OIO production from the IO + BrO reaction whichallowed limits to be placed on the branching ratio for OIO formation in the IO self-reaction at 295 K and760 Torr. Branching ratios for all reaction channels were found to lie in the ranges 0.07 ≤ k1a/k1 ≤ 0.15,k1b/k1 ≤ 0.05, 0.30 ≤ k1c/k1 ≤ 0.46, and 0.42 ≤ k1d/k1 ≤ 0.55 at 295 K and 760 Torr. The results are comparedwith previous studies of the IO self-reaction, and their implications for atmospheric iodine chemistry areconsidered.
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