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 $\text{∼ 2\%}$ 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, ε_max, based on measurement of the H₂O₂ formed in the hydrogen system, is ${\text{1770 ± 150 M}}^{\text{−1}}·{\mathrm{cm}}^{\text{−1}}$ . The rate constant for the bimolecular combination reaction, ${\mathrm{HO}}_2+{\mathrm{HO}}_2\to {\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}}_2$ , was evaluated as ${\text{5.7 ± 0.5 × 10}}^9{M}^{\text{−1}}·{\sec }^{\text{−1}}$ at 298°K and for the reaction ${\mathrm{HO}}_2+\mathrm{OH}\to {\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2{\text{, k = 1.2 ± 0.2 × 10}}^{11}{M}^{\text{−1}}.{\sec }^{\text{−1}}.$ 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.