Laser Flash Photolysis Studies of Radical-Radical Reaction Kinetics: The HO2 + IO Reaction

Laser flash photolysis of Cl2/CH3OH/O2/I2/NO2/SF6/N2 mixtures at 308 nm has been coupled with simultaneous time-resolved detection of HO2 (by infrared tunable diode laser absorption spectroscopy) and IO (by visible absorption spectroscopy) to investigate the kinetics of the atmospherically important reaction HO2 + IO f products over the temperature range 274-373 K in N2 buffer gas at pressures of 12 and 25 Torr. All experiments were performed under near pseudo-first-order conditions with HO2 in excess over IO. At 298 K, the rate coefficient was determined to be (9.7 ( 2.9) × 10-11 cm3 molecule-1 s-1, with the primary source of uncertainty being knowledge of the infrared line strength(s) required to convert measured HO2 absorbances to absolute concentrations. The temperature dependence of the HO2 + IO rate coefficient was found to be adequately described by the Arrhenius expression k ) 9.3 × 10-12 exp(680/T) cm3 molecule-1 s-1. The results reported in this study are compared with other recent studies of HO2 + IO kinetics, and the potential roles of this reaction in atmospheric chemistry are discussed.