Metastable Ions of the Noble Gases

It is shown that ions of the noble gases in metastable excited states may be detected by their greater ability, with respect to the normal ion, to eject electrons from an atomically clean metal surface. The metastable ions are produced by a single electron impact with the parent normal atom. The ratio of cross sections for formation of the metastable and normal ions for argon, krypton, and xenon has been determined as a function of bombarding electron energy. At its maximum this ratio is about 0.02. The metastable ion must be observed over the "background" of normal ions in this method of detection, which is its inherent limitation. The relative number of helium ions formed in the $2{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ metastable state is below the detection sensitivity. It has been necessary to consider the nature of the interaction of a metastable ion with a metal surface, and an argument is given which indicates that the electron yield for the metastable singly-charged ion should be approximately equal to that measured for the doubly-charged ion.