Kinetics and products of the IO plus BrO reaction

The kinetics and products of reaction I between gas-phase BrO and IO radica ls have been studied using the technique of laser photolysis with time-reso lved UV-vis absorption spectroscopy. The O + IBr reaction 8, used as one so urce of and BrO radicals, was found to produce predominantly IO radicals at 295 K. The rate coefficient for reaction 8 is correlated with the branchin g ratio for IO production (channel 8a). Using a value of 0.7 for the branch ing ratio for IO production (k(8a)/k(8)), k(8) was found to be (3.6 +/- 2.4 ) x 10(-11) cm(3) molecule(-1) s(-1) at 295 K, with no significant temperat ure dependence between 210 and 333 K. Sensitivity tests using a kinetic mod el showed that, in addition to (1), decay traces were also sensitive to the rate coefficient for reaction 12: 1 + BrO --> Br + IO. This rate coefficie nt was found to be (1.3 +/- 1.2) x 10(-11) molecule(-1) cm(3) s(-1) at 295 K, with no significant temperature dependence between 235 and 333 K. The ti tle reaction (1), IO + BrO --> products was found to have a rate coefficien t (8.5 +/- 1.4) x 10(-11)cm(3) molecule(-1) s-1 at 295 K. Reaction I exhibi ted a negative temperature dependence between 210 and 333 K, adequately des cribed by ki = (6.7 +/- 0.8) x 10(-12) exp ((760 +/- 30)/T) molecules(-1) c m(3) s(-1). No pressure dependence to k(1) was found between 100 and 760 To rr. All errors are 2 sigma. Five potential products exist for the IO + BrO reaction: IO + BrO --> I + Br + O-2 (1a), IO + BrO --> IBr + O-2 (1b), IO BrO --> OIO + Br (1c), IO + BrO --> OBrO + I (1d), and 10 + BrO --> IBrO2 (1e). No direct measurement of I or Br formation was performed. IBr was obs erved as a minor product (k(1b)/k(1) < 0.2). OIO formation was observed and shown to result from the 10 + BrO reaction for the first time in this work . No evidence for OBrO formation was observed (k(1d)/k(1) < 0.15). No evide nce for the formation or existence of IBrO2 was observed. The results obtai ned here, in conjunction with other published work, were used to constrain the branching ratio, alpha, for OIO production in the IO + IO reaction, giv ing {0.30 +/- 0.05 less than or equal to alpha less than or equal to 0.46 /- 0.08}. This constraint allowed the absorption cross section of OIO to be constrained giving (1.29 +/- 0.22) greater than or equal to sigma (OIO) X 10(17) greater than or equal to 0.87 +/- 0.15) molecules(-1) cm(2) at the ( 5, 1, 0) peak at 549 nm, 295 K, and 760 Torr. Results are compared with pre vious studies of the 10 + BrO reaction, and the atmospheric implications ar e briefly discussed.