Threshold Behavior of the Cross Section for Ionization of He and Ar by Mono-Energetic Electrons

Abstract

A new apparatus using crossed-beam techniques with a cylindrical electrostatic electron velocity selector for the study of ionization probability curves of gases is described. Results for ${\mathrm{He}}^{+}$ and ${\mathrm{Ar}}^{+}$ in the first eV above the ionization threshold are given. New computer techniques determine structure and near-threshold behavior. A simple power law yields a good representation of the experimental results for each ion state, with a different power in each case. For He a power law with power 1.16 is found. The results for Ar may be represented by two power laws with powers 1.3 and 1.34, respectively, joining at the energy of the ${}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ level of ${\mathrm{Ar}}^{+}$.