Pressure and Temperature Dependence of the Gas-Phase Reacombination of Hydroxyl Radicals

Article abstract of DOI:10.1021/j100325a038

Rate constants for the reaction OH+OH+M->H2O2+M (M=N2, H2O) have been determined by using flash photolysis of H2O vapor in combination with quantitative OH resonance spectrometry.For M=N2 experiments were performed at 253, 298, and 353 K and at pressures between 26 and 1100 mbar.Under these conditions the reaction is found to be primarily in the low-pressure limit with k_1,N2⁰(T=298 K)=(6.9_-2.5^+1.4)*10^-31 cm⁶/s and a temperature dependence of T^-0.8.Both the absolute value of k_1,N2⁰ and its temperature variation are in very setisfactory agreement with theoretical predictions and extrapolations from high-temperature dissociation data.A pressure falloff of k_1,N2 is also observed.On the basis of a theoretical analysis of the falloff behavior, a high-pressure limiting rate coefficient of k₁^infinite=1.5*10^-11 cm³/s, independent of temperature, is predicted.From experiments in N2/H2O mixtures with x_H2O=0.11 at pressures up to 140 mbar a low-pressure limiting rate coefficient for H2O as a third body of k_1,H2O⁰ (T=298 K)=(4.0_-2.0^+1.3)*10^-30 cm⁶/s is obtained.