Atom—Atom Ionization Cross Sections of the Noble Gases—Argon, Krypton, and Xenon

Abstract

An experimental investigation of the initial phase of shock produced ionization in argon, krypton, and xenon has been conducted in order to elucidate the atom—atom ionization reaction and to determine the atom—atom ionization cross sections. A high‐purity shock tube was employed to heat these gases to temperatures in the range from 5000° to 9000°K at neutral particle densities of 4.4×10¹⁷, 7.0×10¹⁷, and 13.3×10¹⁷ cm⁻³, and impurity levels of approximately 10⁻⁶. A K‐band (24‐GHz) microwave system situated so that the microwave‐beam propagation direction was normal to the shock tube, monitored the ionization relaxation process occurring immediately after the passage of the shock front. Electron density was calculated from the microwave data using a plane‐wave—plane‐plasma slab interaction theory corrected for near field effects associated with the coupling of the microwave energy to the plasma. These data, adjusted to compensate for the effects of shock attenuation, verified that the dominant electron‐generation process involve a two‐step, atom—atom ionization reaction, the first step (excitation to the first excited states) being rate determining. The quadratic dependence on neutral density associated with this reaction was experimentally demonstrated (with an uncertainty of ±15%). The cross section, characterized as having a constant slope from threshold (first excited energy level), represented as the cross‐sectional slope constant C, was found to be equal to 1.2×10⁻¹⁹±15% cm²/eV, 1.4×10⁻¹⁹±15% cm²/eV, and 1.8×10⁻²⁰±15% cm²/eV for argon, krypton, and xenon, respectively. The electron—atom elastic momentum‐exchange cross sections derived from the microwave data correlated quite well with Maxwell‐averaged beam data, the agreement for the case of argon being ±20%; krypton, ±30%; and xenon, within a factor of 2.