Rate of Ionization Behind Shock Waves in Air. II. Theoretical Interpretations

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Abstract
The problem of spontaneous ionization (i.e., no externally applied electromagnetic fields, nor hard radiation) in the reaction zone behind strong normal shock waves in air has been treated concurrently with the problem of dissociation and vibrational relaxation. Through a comparison of specific ionization rates, one may conclude that up to a shock velocity of 9 km/sec (about 27 times the speed of sound at room temperature), the predominant electron production process would be atom—atom ionizing collisions. This would be followed in an approximately decreasing order of importance by photoionization, electron impact, atom—molecule collisions, and molecule—molecule collisions. The charge exchange reactions, while not contributing directly to the electron production process, were found to have a small but noticeable indirect effect on the resultant electron density distribution at some distance behind the shock due to their continuous shifting of the relative population between atomic and molecular ions (which recombine with the electrons at different rates). The specific rate constants for the atom—atom processes required to interpret all existing experimental results appear to be consistent with a simple extrapolation of the low‐temperature rate constants according to the crossing‐point model of Bates and Massey for atom—atom ionizing collisions.