Falloff curves of the reaction H + O₂(+ M) = HO₂ (+ M) over extended temperature and pressure ranges

Previous studies of the transition from third to second order for the reaction H + O₂ (+ M) → HO₂ (+ M) led to partly controversial results: room temperature measurements up to bath gas pressures of 200 bar allowed for an extrapolation to the second order limit with a rate constants 7.5 × 10^-11 cc/molecule-sec, which is in good agreement with theoretical predictions based on an ab initio potential of HO₂ (9.6 × 10^-11 cc/molecule-sec. Calculations for 1300 K gave 1.8 × 10^-10 cc/molecule-sec, in contrast to extrapolations shock wave studies which showed deviations from third order at unexpectedly low pressures. To better characterize the temperature dependence of the falloff curves, beatable high pressure flow reactors were constructed, allowing for measurements over the ranges 200-900 K and 1-1000 bar. At the same time, falloff calculations employing classical trajectories on ab initio potentials were extended. The new measurements in the high pressure flow cell, confirmed the earlier results for the transition from third to second order which were obtained in a static high pressure cell with M = Ar, N₂, and CH₄. Possible influences of non-statistical lifetime distributions of HO₂* on the width of falloff curves, which may have become observable in the available experimental studies, were discussed. Original is an abstract.