Pressure dependence of the reaction H + O₂ (+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^-1s^-1 from the literature. The intermediate falloff curve was found to be best represented by k/k_∞ = [x/(1 + x)]F_cent^{1/(1 + (a + log x)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 F_cent(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.