The rate of ionization behind strong shock waves in argon, krypton, and xenon, is 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 2-in. sq. aluminum shock tube into ambient gases at pressures of 3.0 to 17.0 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 Gc/sec (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 ± 0.5 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 states
of which the excitation step is rate controlling
