Evidence of Low-Binding-Energy Electrons in a Low-Temperature Helium Afterglow

Moderate, pulsed microwave heating of the free electrons in a weakly ionized helium afterglow plasma at 9.5 Torr at a gas temperature of 77^∘{}K produces an unusual modulation of the visible afterglow light. Observations of a second (probing) microwave signal indicate an accompanying significant increase in free-electron density under the assumption that there is no significant anomalous phase shift or absorption of the probing signal.