The pressure dependence of the rate constant of the reaction of OH radicals with CO

Absolute rate constants for the reaction OH+CO+M have been determined in the gas phase, at 298 K, in the pressure range 20–700 Torr of He, N2, CF4, and SF6 and 0–20 Torr of H2O. Hydroxyl radicals were generated by flash photolysis of H2O vapor in the vacuum UV, and monitored by time-resolved resonance absorption at 308.2 nm OH (A 2Σ+→X 2Π). The second-order rate constant was found to depend on the pressure and identity of the third body M. Approximate relative efficiencies of the gases as third bodies were estimated to be: H2O: SF6: CF4: N2: He≊ 1.0: 0.5: 0.3: 0.1: 0.02. In the presence of small amounts of O2 the OH decay became nonexponential and slower. Computer simulation of our system indicated that the effects of O2 are adequately explained by reactions of HO2 radicals formed in the presence of O2. A mechanism of the reaction which is consistent with the pressure effect is discussed. The values of the rate constant obtained with M=N2 are recommended for use in modeling atmospheric processes.