Excitation of Metastable Levels in Helium near Threshold

The excitation of the metastable levels in helium ($2{}_{}{}^3{}_{}{}^{}S$ and $2{}_{}{}^1{}_{}{}^{}S$) was studied as a function of electron energy near threshold. The excitation due to essentially monoenergetic electrons was obtained by using the retarding potential method. The metastables were detected by their emission of electrons from a gold-plated cylinder. It was found that the excitation function for the $2{}_{}{}^3{}_{}{}^{}S$ level is linear from the onset of excitation to approximately 0.4 ev above onset except for a small amount of tailing near threshold. The excitation function peaks at 0.6 ev above the excitation threshold. The value of the cross section of the $2{}_{}{}^3{}_{}{}^{}S$ level is in reasonable agreement with the value given by Maier-Leibnitz.