Electron-Impact Excitation and Ionization in Air

In a partially ionized hypersonic flow field, electron collision plays a significant role in energy re-distribution and in the production of excited states of atoms and molecules that can subsequently radiate. Thus electron collision data is part of the database used in modelling high speed entry flows. First principles computational methods for the calculation of electron-impact data are reviewed. Their merits are discussed, based on the requirements of reliability and the ability to handle high-lying excited states. Three processes are considered: the production of electrons by electron-impact ionization, the removal of electrons by radiative recombination and dielectronic recombination, and the production of electronic excited states by electron impact. Electronnitrogen atom collisions are used as an illustrative example. The study clarifies the role of high-lying electronic excited states in the ionization process. It shows that the recombination processes can produce sizable radiative heat load in the VUV region. It also shows examples using a scaling method to improve the frequently used Born approximation for electron-impact excitations.