Structure in the Ionization Near Threshold of Rare Gases by Electron Impact

Ionization efficiency curves for xenon, krypton, and argon have been studied with an electron energy analyzer. The electron energy distribution was measured and the absolute voltage scale determined in each experiment. The results of these studies (1) favor a linear threshold ionization law over a 1.127 power law, and (2) show that the data cannot be explained simply by ionization processes with onsets at the ${}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$ and ${}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ ground states of the ion, but can be well fitted by a series of linear processes. The ionization potentials obtained by extrapolating according to a linear threshold law agree with spectroscopic values to within 0.02 ev. New onsets in argon were observed at about 0.64 v and 1.27 v above threshold. The observed structures in the rare gases are not readily explained by auto-ionization and no alternative explanation is offered. The structures observed in these experiments are compared with the results obtained by other "high-resolution" techniques. This comparison is complicated by the disparity in the published data on onset energies, and by the even greater disagreement on the relative probabilities for the various ionization processes. An independent check on consistency of data was made by comparison with "low-resolution" data obtained on a conventional mass spectrometer. The present data are in excellent agreement with the lower resolution data, while some of the other "high-resolution" data are not.