Impact of electronic excitation on dissociation in high-temperature oxygen, nitrogen, and air mixtures

The role of molecular electronic excitation in the dissociation of O2, N2, and NO in oxygen, nitrogen, and air mixtures is studied using an electronic state-resolved kinetic model of N2-O2-Ar mixtures. The longstanding disagreement between ab initio O2(X3Σg-)-O and experimental O2-O dissociation rate coefficients is explained mechanistically for the first time, including a clear explanation of why this effect is only observed in collisions with O and not with O2 or Ar. Using electronic state-resolved simulations of a recent set of shock tube experiments, a simulated inference of the O2-O rate coefficient is computed and shown to reproduce all experimental data within uncertainty. The model is then validated using ground and excited electronic state measurements of N2 from several shock tube experiments. Next, the validated model is used to study how electronic excitation affects N2 dissociation more broadly, with the N2(A3Σu+) state being shown to significantly enhance N2 dissociation within both nitrogen and air mixtures. The role of excited electronic states in NO dissociation is evaluated and shown to be negligible, owing to the low dissociation rate coefficients of the excited states. Key experiments needed to further validate and constrain the model predictions are identified, with priority given to direct measurements of dissociation in N2-O2 mixtures, and high spatial and temporal resolution measurements of N2(A3Σu+) in the near-shock region. Effective rate coefficients incorporating the influence of electronic excitation are extracted from state-resolved calculations and are compared with existing rate expressions, providing guidance for the treatment of electronic excitation effects in hypersonic computational fluid dynamics codes.