Spectroscopic and kinetic properties of HO₂ radicals and the enhancement of the HO₂ Self reaction by CH₃OH and H ₂O

The line center absorption cross sections and the rate constants for self-reaction of hydroperoxy radicals (HO₂) have been examined in the temperature range of 253-323 K using pulsed laser photolysis combined with tunable diode laser absorption in the near-IR region. The transition prohed was in the 2v₁ OH overtone transition at 1506.43 nm. The temperature dependence of the rate constant (k) for the HO₂ + HO₂ reaction was measured relative to the recommended value at 296 K, giving k = (3.95 ± 0.45) × 10-13 × exp[(439 ± 39)/7]cm3 molecule-1 s-1 at a total pressure of 30 Torr (N₂ + O₂). After normalizing our determination and previous studies at low pressure, we recommend k = (2.45 ± 0.50)× 10-13 × exp[(565 ± 130)/T]cm3 molecule-1 s-1 (0 < P < 30 Torr, 95% confidence limits). The observed rate coefficient, k_obs, increases linearly with CH ₃OH concentration, and the enhancement coefficient (k'), defined by k_obs = k + k;[CH₃OH], is found to be (3.90 ± 1.87) × 10-35 × exp[(3849 ± 135)/T]cm6 molecule-2 s-1 at 30 Torr. The analogous water vapor enhancement coefficient (k′) is (1.16 ± 0.58) × 10-36 × exp[(4614 ± 145)/T]cm6 molecule-2 s-1. The pressure-broadened HO₂ absorption cross section is independent of temperature in the range studied. The line center absorption cross sections at 1506.43 nm, after correction for instrumental broadening, are (4.3 ± 1.1) × 10-19, (2.8 ± 0.7) × 10-19, and (2.0 ± 0.5) × 10-19 cm2/molecule at total pressures of 0, 30, and 60 Torr, respectively (95% confidence limits).