Kinetic study of the reactions of the sodium dimer (Na2) with a range of atmospheric species

Abstract

The reactions of Na₂ with a series of atmospheric constituents were studied using a fast flow tube with detection of Na₂ by laser induced fluorescence at 656.2 nm [Na₂(A¹Σ⁺_u − X¹Σ⁺_g)]. The resulting rate coefficients at 298 K for the reactions of Na₂ with OH, O₂, NO₂, NO, O₃, H, H₂ and H₂O are: (1.01^+0.35_−0.25) × 10⁻¹⁰, (2.95 ± 0.46) × 10⁻¹¹, (1.79^+0.51_−0.31) × 10⁻¹⁰, (1.33 ± 0.16) × 10⁻¹¹, (8.0⁺²⁴_−3.0) × 10⁻¹¹, ≤6 × 10⁻¹², ≤4 × 10⁻¹⁵, and ≤3 × 10⁻¹³ cm³ molecule⁻¹ s⁻¹, respectively. The quoted uncertainties include measurement imprecision at the 1σ level, and systematic errors. The reaction between Na₂ and OH produces chemiluminescence at 589 nm [Na(3²P_J – 3²S_1/2)], with a measured branching ratio of (7.6^+15.0_−3.7) × 10⁻³. The reaction enthalpies are calculated using quantum theory at the Complete Basis Set (CBS-Q) level; all reactions except Na₂ + H₂O and Na₂ + H₂ are exothermic. The surprisingly slow reaction of Na₂ with OH is explained using trajectory calculations and consideration of the splitting between the covalent and ionic surfaces involved in the reaction, coupled with the Landau–Zener formalism. The small upper limit to the rate coefficient for the strongly exothermic reaction Na₂ + H appears to be a striking example of the light atom anomaly where the reaction is kinematically constrained.