Kinetics and Products of the IO Self-Reaction

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 105; no. 33; pp. 7840 - 7854
• Main Authors: Bloss, William J, Rowley, David M, Cox, R. Anthony, Jones, Roderic L
• Format: Journal Article
• Language: English
• Published: American Chemical Society 23.08.2001
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp0044936

The absorption cross sections of the gas-phase IO radical and the kinetics and products of the IO self-reaction have 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 (A 2Π ← X 2Π) transition at 427.2 nm, determined using the reaction O(3P) + CF3I to form IO and calibrated relative to the O3 cross section 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 k 1, determined from the loss of IO radicals in the absence of ozone, was found to be (8.2 ± 1.3) × 10-11 molecules-1 cm3 s-1 at 295 K and 760 Torr. The self-reaction rate coefficient was found to be independent of pressure between 100 and 760 Torr, and to display a negative temperature dependence between 222 and 325 K, described by k 1 = (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 production was 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 the IO + IO reaction were determined via measurement of OIO production from the IO + BrO reaction which allowed limits to be placed on the branching ratio for OIO formation in the IO self-reaction at 295 K and 760 Torr. Branching ratios for all reaction channels were found to lie in the ranges 0.07 ≤ k 1a/k 1 ≤ 0.15, k 1b/k 1 ≤ 0.05, 0.30 ≤ k 1c/k 1 ≤ 0.46, and 0.42 ≤ k 1d/k 1 ≤ 0.55 at 295 K and 760 Torr. The results are compared with previous studies of the IO self-reaction, and their implications for atmospheric iodine chemistry are considered.