Determination of the limiting low pressure rate constants of the reactions of CH with N2 and CO:: a CRESU measurement at 53 K

The pulsed laser photolysis, time-resolved laser-induced fluorescence technique has been implemented in a CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) apparatus to measure the rates of the association reactions of CH radicals with N-2 and CO at 53 K and five different densities of the carrier gas Ar. The pressure-dependent rate constants have been combined with those reported previously for the temperature range 202 less than or equal to T/K less than or equal to 584 (R. A. Brownsword, L. B. Herbert, I. W. M. Smith and D. W. A. Stewart, J. Chem. Sec., Faraday Trans., 1996, 92, 1087) to yield, via a global fitting procedure, the following expressions for the two rate constants in the limit of low pressure: CH + N-2: k(0)(T) [M = Ar] = 1.6 x 10(-31) x (T/298)(-2.2) [Ar] cm(3) molecule(-1) (s)-(1) (R1) CH + CO: k(0)(T) [M = Ar] = 4.1 x 10(-30) x (T/298)(-2.1) [Ar] cm(3) molecule(-1) s(-1) (R2) These expressions are very similar to those deduced previously by Brownsword et al., demonstrating that the analysis of rate constants for association reactions by the methods proposed by Tree (J. Chem. Phys., 1977, 66, 4758) can be applied over a wide range of temperature. These low temperature kinetic data, especially those for CH + N-2, have some relevance to the chemistry of the atmospheres of some planets and planetary moons.