Kinetics and products of the IO self-reaction

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(2)Pi <-- X (2)Pi) transition at 427.2 nm, determined using the reaction O(P-3) + CF3I to form IO and calibrated relative to the O-3 cross section was found to be (1.9 +/- 0.17) x 10(-17) cm(2)molecule(-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 IO(P-3) + I-2 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) x 10(-11) molecules(-1) cm(3) 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) x 10(-11) exp{(220 +/- 230)/T} molecules(-1) cm(3) s(-1). All errors are 2 sigma. Four potential product channels exist for the IO self-reaction: IO + IO --> 2I + O-2 (1a), IO + IO --> I-2 + O-2 (1b), IO + IO --> OIO + I (1c), IO + IO + M --> I2O2 + M (1d). No direct measurement of I atom production was performed. I-2 formation was observed, but attributed to IO-catalyzed I atom recombination (I + IO + M --> I2O + M; I + I2O --> I-2 + 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 (lambda < 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 less than or equal to k(1a)/k(1) less than or equal to 0.15, k(1b)/k(1) less than or equal to 0.05, 0.30 less than or equal to k(1c)/k(1) less than or equal to 0.46, and 0.42 less than or equal to k(1d)/k(1) less than or equal to 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.