Pressure dependence of the reaction H + O 2 (+Ar) → HO 2 (+Ar) in the range 1–900 bar and 300–700 K

The reaction H + O2 (+Ar) → HO2 (+Ar) was studied in a high pressure flow cell in the bath gas argon at pressures between 1 and 900 bar and temperatures between 300 and 700 K. H atoms were generated by laser flash photolysis of NH3 at 193.3 nm, HO2 radicals were monitored by light absorption at 230 nm. The results are consistent with experimental low pressure rate constants k0 = [Ar] 2.5 × 10−32(T/300 K)−1.3 cm6 molecule−2 s−1 and theoretical high pressure rate constants k∞ = 9.5 × 10−11(T/300 K)+0.44 cm3 molecule−1 s−1 from the literature. The intermediate falloff curve was found to be best represented by k/k∞ = [x/(1 + x)]Fcent1/(1+(a+logx)2/(N ± ΔN)2) with x = k0/k∞, a ≈ 0.3, N ≈ 1.05, ΔN ≈ 0.1 (+ΔN for (a + log x) < 0 and −ΔN for (a + log x) > 0), and Fcent(Ar) ≈ 0.5 independent of the temperature. A comparison with literature data between 300 and 1200 K does not confirm major deviations from third order kinetics in earlier medium pressure experiments.