Initial Ionization Rates in Shock‐Heated Argon, Krypton, and Xenon

• Published in: The Physics of fluids (1958) 1964-02, Vol.7 (2), p.214-222
• Main Authors: Harwell, Kenneth E., Jahn, Robert G.
• Format: Article
• Language: English
• ISSN: 0031-9171; 2163-4998
• DOI: 10.1063/1.1711135

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.