Abstract
The rates of ionization behind strong shock waves in argon, krypton, and xenon are observed by a transverse microwave probe over a range of electron densities low enough that atom-atom inelastic collisions are the rate determining mechanism. Shocks of Mach number 7.0 to 10.0 propagate down a 5 cm square aluminum shock tube into ambient gases at pressures of 3 to 17 mm Hg, heating them abruptly to atomic temperatures of 5500°K to 9600°K. The subsequent relaxation toward ionization equilibrium is examined in its early stages by the reflection, transmission, and phase shifts of a 24.0 Kmc (1.25 cm) transverse microwave beam propagating between two rectangular horns abreast a glass test section. The data yield effective activation energies of 11.9 +/- 0.5 ev for argon, 10.4 +/- 0.5 ev for krypton, and 8.6 +/- 0.6 ev for xenon. These coincide, within experimental error, with the first excitation potentials, rather than the ionization potentials of the gases, indicating that in this range, ionization proceeds via a two-step process involving the first excited electronic state. Within experimental error, the pressure dependence is found to be proportional to the number density squared.
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