Kinetic investigation of the third-order rate processes between K + O2+ M by time-resolved atomic resonance absorption spectroscopy

Abstract

Absolute reaction rate constants (k3) for the third-order processes K + O2+ M → KO2+ M have been determined by time-resolved resonance absorption measurements on ground-state atomic potassium at λ= 768 nm (42PJâ†� 42S1/2) in the “single-shot mode” following the flash photolysis of KI vapour. The reactions were investigated at the temperatures T= 753 and 873 K, k3 exhibiting a small negative temperature dependence, fitted to the form k3=AT–1, and yielding the following results for the third bodies M = He, N2 and CO2: M k3/cm6 molecule–2 s–1, He (9.8 ± 1.5)× 10–28T–1, N2(1.7 ± 0.6)× 10–27T–1, CO2 4 x 10–27T–1. The rate constants, k3, for K + O2+ M are found to be greater than those for Na + O2+ M, obtained hitherto by time-resolved resonance absorption measurements, by approximately a factor of two. These results for both K and Na, obtained from monitoring the atoms following pulsed irradiation, are approximately three orders of magnitude greater than those derived previously from flame measurements. The rate measurements further demonstrated a kinetic component, second-order overall in [K] and [O2], which did not arise from direct O-atom abstraction but is tentatively attributed to a mechanism involving the species “O2–I” and which can readily be extracted in the analysis that leads to the quantitative determination of k3. The rates of diffusion of K(42S) in the gases He, N2 and CO2 were also studied in detail in this investigation and the resulting diffusion coefficients for this atom, as well as those for atomic sodium obtained previously using the “long-time solution” of the diffusion equation for a cylinder, are compared with diffusion coefficients for Na and K reported from measurements on the burnt gases (H2+ H2O + N2) of fuel-rich flames.