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 Π r X 2 Π) transition at 427.2 nm, determined using the reaction O( 3 P) + CF 3 I to form IO and calibrated relative to the O 3 cross section was found to be (1.9 ( 0.17) × 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 selfreaction were measured using the reaction O( 3 P) + 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) × 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) × 10 -11 exp{(220 ( 230)/T} molecules -1 cm 3 s -1 . All errors are 2σ. Four potential product channels exist for the IO self-reaction: IONo direct measurement of I atom production was performed. I 2 formation was observed, but attributed to IO-catalyzed I atom recombination (I + IO + M f I 2 O + M; I + I 2 O f I 2 + IO). OIO formation was observed and shown to result from the IO selfreaction. Formation of a broadband absorbing product underlying the IO absorption at low (λ < 400 nm) wavelengths was observed, and tentatively attributed to I 2 O 2 . 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 e k 1a /k 1 e 0.15, k 1b /k 1 e 0.05, 0.30 e k 1c /k 1 e 0.46, and 0.42 e k 1d /k 1 e 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.

The flash photolysis/UV absorption technique has been used to study the self-reaction of BrO radicals over the temperature range 222-298 K and the pressure range 100-760 Torr of N 2 or O 2 . Two chemical sources of BrO radicals were used: photolysis of Br 2 in the presence of excess ozone and photolysis of O 2 in the presence of excess Br 2 . The overall rate constant, k 1 , for the BrO self-reaction (defined by -d[BrO]/dt ) 2k 1 [BrO] 2 ) was found to be temperature and pressure independent at T g 250 K, with k 1 ) (2.88 ( 0.20) × 10 -12 cm 3 molecule -1 s -1 . At temperatures below 250 K, k 1 was found to be pressure dependent, due to the emergence of a new termolecular channel of the BrO self-reaction 1c, -1c forming the BrO dimer, Br 2 O 2 (BrO + BrO + M h Br 2 O 2 + M). Channel-specific rate constants were determined for the two bimolecular channels of the BrO self-reaction above 250 K, giving for (1a) (BrO + BrO f 2Br + O 2 ) k 1a ) (5.31 ( 1.17) × 10 -12 exp{(-211 ( 59)/T} cm 3 molecule -1 s -1 and for (1b) (BrO + BrO f Br 2 + O 2 ) k 1b ) (1.13 ( 0.47) × 10 -14 exp{(983 ( 111)/T} cm 3 molecule -1 s -1 . Below 250 K, the overall rate coefficient of the two bimolecular channels is reduced as the dimer forming channel emerges. At 235 and 222 K, rate constants for the formation (k 1c ) and decomposition (k -1c ) of Br 2 O 2 were recorded. Using the values for K 1c , ∆H r for reaction 1c was estimated as -58.6 ( 0.1 kJ mol -1 . A UV absorption spectrum attributed to Br 2 O 2 was also recorded over the wavelength range 300-390 nm. The cross section of the smooth Br 2 O 2 spectrum was found to be 1.2 × 10 -17 cm 2 molecule -1 at 320 nm. These results are rationalized in terms of a mechanism of the BrO self-reaction that shows competition, at low temperatures, between collisional quenching and unimolecular dissociation of an excited BrOOBr* intermediate. The rate constant for the reaction of oxygen atoms with molecular bromine was also determined in the course of these experiments [O + Br 2 f BrO + Br (5)], giving k 5 ) (5.12 ( 1.86) × 10 -13 exp{(989 ( 91)/T} cm 3 molecule -1 s -1 . All errors are 1σ. The atmospheric implications of these results are discussed.