Low-temperature kinetics of reactions between neutral free radicals. Rate constants for the reactions of OH radicals with N atoms (103 ⩽T/K ⩽ 294) and with O atoms (158 ⩽T/K ⩽ 294)

Abstract

Rate constants have been determined for the reaction between OH radicals and N atoms at temperatures down to 103 K and for the reaction between OH and O atoms down to 158 K. Discharge-flow methods were used to generate known steady-state concentrations of N and O atoms in a cryogenically cooled flow of gas. OH radicals were produced, in concentrations much smaller than those of the atomic radicals, by pulsed laser photolysis of a small concentration of HNO₃ introduced into this gas flow, and their first-order kinetic decays were observed using the time-resolved laser-induced fluorescence technique. The rate constants for both reactions increase monotonically down to the lowest temperatures achievable in the present experiments but do not exhibit any simple functional dependence on temperature. It is suggested that expressions of the form: k(T)=F_elec(T)(T/298)ⁿk′(298) are used to fit the data on these reactions at temperatures ⩽ 515 K. Here F_elec(T) is the temperature-dependent ratio of the electronic degeneracy of the lowest potential surface, leading adiabatically from reagents to products, to the product of the electronic partition functions for the reagents, n has the value –0.17 for the N + OH reaction and –0.24 for O + OH, and the values of k′(298) are 2.0 × 10^–10 cm³ molecule^–1 s^–1 for N + OH and 3.7 × 10^–10cm³ molecule^–1 s^–1 for O + OH, and correspond to rate constants for reaction on the lowest potential-energy surface at 298 K. Because the value of F_elec(T) for both reactions is essentially constant for T⩽ 50 K, it is suggested that expressions of the form k(T)=AT^B are used in chemical models of interstellar clouds with A= 2.0 × 10^–10 cm³ molecule^–1 s^–1 and B=– 0.17 for N + OH and A= 3.7 × 10^–10 cm³ molecule^–1 s^–1and B=– 0.24 for O + OH.