The product branching ratios for NO (X ∑ ) and NO (a ∑ ) produced from the reaction of N with O 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 reaction show that the reaction from N ( P) + O ( ∑ ) reactants starts with an efficient early stage charge transfer to the N( D) + O (X ∏) channel, which gives rise to the O (X ∏) 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 ∑ ) are found to be less favorable than pathways leading to the major product NO (X ∑ ). Production of N( D) has implications for the concentration of NO in the mesosphere. © 2006 American Chemical Society. + 1 + + 3 + + + + + + 3 3 2 + 2 + 2 + + + 3 + + 1 + 2 2 2 2 g 2 2