Pressure and temperature dependence of the gas-phase recombination of hydroxyl radicals

Abstract

Rate constants for the reaction OH + OH + M ..-->.. H/sub 2/O/sub 2/ + M (M = N/sub 2/, H/sub 2/O) have been determined by using flash photolysis of H/sub 2/O vapor in combination with quantitative OH resonance spectrometry. For M = N/sub 2/ 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/sub 1,N/sub 2///sup 0/ (T = 298 K) = (6.9/sub -2.5//sup +1.4/) x 10/sup -31/ cm/sup 6//s and a temperature dependence of T/sup -0.8/. Both the absolute value of k/sub 1,N/sub 2// and its temperature variation are in very satisfactory agreement with theoretical predictions and extrapolations from high-temperature dissociation data. A pressure falloff of k/sub 1,N/sub 2// is also observed. On the basis of a theoretical analysis of the falloff behavior, a high-pressure limiting rate coefficient of k/sub 1/infinity = 1.5 x 10/sup -11/ cm/sup 3//s, independent of temperature, is predicted. From experiments in N/sub 2//H/sub 2/O mixtures with /kappa cur//sub H/sub 2/O/ = 0.11 at pressures up to 140 mbar a low-pressure limiting rate coefficient for H/sub 2/O as a third bodymore » of k/sub 1,H/sub 2/O//sup 0/ (T = 298 K) = (4.0 /sub -2.0//sup +1.3/) x 10/sup -30/ cm/sup 6//s is obtained.« less