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 (4²P_Jâ†� 4²S_1/2) 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.