Spectroscopic and Kinetic Properties of HO2 Radicals and the Enhancement of the HO2 Self Reaction by CH3OH and H2O

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 probed was in the 2v(1) 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) x 10(-13) x exp[(439 +/- 39)/T] cm(3) molecule(-1) s(-1) at a total pressure of 30 Torr (N-2 + O-2). After normalizing our determination and previous studies at low pressure, we recommend k = (2.45 +/- 0.50) x 10(-13) x exp[(565 +/- 130)/T] cm(3) molecule(-1) s(-1) (0 < P < 30 Torr, 95% confidence limits). The observed rate coefficient, k(obs), increases linearly with CH3OH concentration, and the enhancement coefficient (k'), defined by k(obs) = k + k'[CH3OH], is found to be (3.90 +/- 1.87) x 10(-35) x exp[(3849 +/- 135)/T]cm(6) molecule(-2) s(-1) at 30 Torr. The analogous water vapor enhancement coefficient (k '') is (1.16 +/- 0.58) x 10(-36) x exp[(4614 +/- 145)/T]. cm(6) 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) x 10(-19), (2.8 +/- 0.7) x 10(-19), and (2.0 +/- 0.5) x 10(-19) cm(2)/molecule at total pressures of 0, 30, and 60 Torr, respectively (95% confidence limits).