A study of the reaction of N ⁺ with O ₂: Experimental quantification of NO ⁺(a ³∑ ⁺) production (298-500 K) and computational study of the overall reaction pathways

The product branching ratios for NO ⁺(X ¹∑ ⁺) and NO ⁺(a ³∑ ⁺) produced from the reaction of N ⁺ with O ₂ have been measured at 298 and 500 K in a selected ion flow tube. Approximately 0.5% of the total products are in NO ⁺(a) at both temperatures, despite the fact that the reaction to form NO ⁺(a) is 0.3 eV exothermic. High-level ab initio calculations of the potential energy surfaces for the N ⁺ + O ₂ reaction show that the reaction from N ⁺( ³P) + O ₂( ³∑ _g) reactants starts with an efficient early stage charge transfer to the N( ²D) + O ₂ ⁺(X ²∏) channel, which gives rise to the O ₂ ⁺(X ²∏) product and, at the same time, serves as the starting point for all of the reaction channels leading to NO ⁺ and O ⁺ products. Pathways to produce NO ⁺(a ³∑ ⁺) are found to be less favorable than pathways leading to the major product NO ⁺(X ¹∑ ⁺). Production of N( ²D) has implications for the concentration of NO in the mesosphere.