Electronic structure calculations of NO potential curves and associative ionization (AI) cross sections and their impact on atmospheric entry modeling

Associative ionization of N+O to form NO⁺ and an unbound electron is a significant ionization initiation reaction during hypersonic entry of spacecrafts into Earth’s atmosphere. Despite the importance of AI, the high temperatures realized in these non-equilibrium flows make it challenging to probe state-specific kinetics for the metastable atomic states experimentally and evolving theory can be an invaluable tool for determining the rate coefficients routinely used in fluid dynamics and radiation studies.

In this work, we calculate potential energy curves for the ground and excited electronic states of NO and NO⁺ using the newly developed improved internally contracted multi-reference configuration interaction method (i²cMRCI) and compare the new curves to computations from the literature and spectroscopic data. The adiabatic potential curves are transformed to a diabatic representation, which is used in time-dependent wave packet calculations for each electronic and vibrational state. Based on the computed wave functions for nuclear motion, T-matrix and cross sections for the DR and AI are computed, including the low energy metastable atomic states. The effect of coupling between the electronic states is explored. DR and AI rate coefficients are compared with available experimental data and the rate coefficient expressions used in radiation calculations. The impact of the ionization rate coefficients on the predicted UHF communication attenuation by the plasma is also shown.