NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.An experimental investigation of the initial phase of shock produced ionization in argon, krypton, xenon, and argon-xenon mixtures has been conducted in order to elucidate the atom-atom ionization reaction and to determine the atom-atom ionization cross sections for the gases noted. A high-purity shock tube was employed to heat these gases to temperatures in the range 5000[degrees]K to 9000[degrees]K at neutral particle densities of 4.41x10[superscript 17] cm[superscript -3], 6.96x10[superscript 17] cm[superscript -3], and 13.76x10 [superscript 17] cm[superscript -3], and impurity levels of around 10[superscript -6]. A K-band (24 gcps) 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 involves 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 per cent). 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 1.2x10[superscript -19] [...] 15 per cent cm[superscript 2]/ev, 1.4x10 [superscript -19] [...] 15 per cent cm[superscript 2]/ev, 1.8x10 [superscript -20] [...] 15 per cent cm[superscript 2]/ev for argon, krypton, and xenon, respectively. The C factor for argon ionizing xenon was determined to be equal with an uncertainty of [...] 20 per cent to the xenon-xenon C factor, i.e., 1.8x10[superscript -20] cm[superscript 2]/ev. This would imply that, for atom-atom processes in the noble gases at about 1 atmosphere pressure and temperature of about 1 ev, the ionization cross section is independent of the electronic structure of the projectile atom. 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 per cent; krypton, [...] 30 per cent; and xenon, within a factor of 2.
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