Absorption Spectrum and Reaction Kinetics of the HO2 Radical in the Gas Phase

HO2 radicals were produced in the gas phase by flash photolysis of water vapor (3%) in an atmosphere of hydrogen, helium, or argon containing ∼ 2% oxygen. Water is dissociated in the first continuum to H and OH, and O2 converts the H atoms to HO2. Hydrogen nearly doubles the amount of HO2 produced by converting OH to H. The absorption spectrum of HO2 is a broad band with a peak at 2050 Å. The molar extinction coefficient, εmax, based on measurement of the H2O2 formed in the hydrogen system, is 1770 ± 150 M−1· cm−1. The rate constant for the bimolecular combination reaction, HO2+HO2 → H2O2 + O2, was evaluated as 5.7 ± 0.5 × 109 M−1· sec−1 at 298°K and for the reaction HO2+OH→ H2O+O2, k = 1.2 ± 0.2 × 1011M−1. sec−1. From auxiliary measurements of the rate of O3 formation it was also found that, in the flash photolysis of O2 (2%) in H2, hot O atoms react with H2 to form OH and H which are then converted to HO2.