Temperature dependence of the absolute third-order rate constant for the reaction between Na + O2+ N2 over the range 571–1016 K studied by time-resolved atomic resonance absorption spectroscopy

Abstract

We present a detailed investigation of the absolute third-order reaction rate constant (k₁) for the process Na + O₂+ N₂→ NaO₂+ N₂(1) measured over the temperature range T= 571–1016 K. Na(3 ²S_1/2) was generated by the pulsed irradiation of NaI vapour and monitored in the presence of O₂ and N₂ in the ‘single-shot’ time-resolved mode by atomic resonance absorption spectroscopy of the unresolved D-line doublet at λ= 589 nm [Na(3 ²P_1/2,3/2)â†� Na(3 ²S_1/2)]. A new experimental system has been specially designed for this temperature-dependent investigation. The diffusion coefficient, D(Na–N₂), has been measured in this temperature range, demonstrating a dependence on T^1.52±0.24 and yielding a value of D(Na–N₂) at s.t.p. of 0.15 ± 0.05 cm² s^–1. A limited body of data from similar measurements in neon yields D(Na–Ne) at s.t.p. = 0.19 ± 0.04 cm² s^–1. The data for k₁(Na + O₂+ N₂) within the temperature range 571–1016 K are described by the form k₁=(1.11 ± 0.08)× 10^–23T^{–2.47±0.14} cm⁶ molecule^–2 s^–1. However, for extrapolation to temperatures outside the measured regime the formalism of the unimolecular rate theory of Tröe, with a dependence of 〈ΔE〉 on T^–0.7, has been quantitatively employed to yield k₁(200 K) and k₁(2000 K)= 8.8 × 10^–30 and 6.8 × 10^–32 cm⁶ molecule^–2 s^–1, respectively. The extrapolated rate data are of particular relevance to the role of reaction (1) in the measosphere and in flames. A limited body of rate data is reported for the effect of neon in the analogue of reaction (1) yielding k₁(Na + O₂+ Ne)=(3.5 ± 0.2)× 10^–31 and (2.7 ± 0.1)× 10^–31 cm⁶ molecule^–2 s^–1 at T= 726 and 844 K, respectively.