Ionization Probability Curves for Krypton and Xenon near Threshold

The shapes of the ionization probability curves of krypton and xenon have been determined near the ionization potential by measuring the ionization produced by nearly monoenergetic electrons in a mass spectrometer. Ionization processes associated with the ${}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ and ${}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$ ground states of the ions are clearly resolved. Ions are observed which are attributed to auto-ionization of atoms excited to the higher states having the ${}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ core configuration.