Electron-impact excitation of He: Threshold region

Abstract

Differential cross sections for excitation to the $n=2$ states of He are studied with an electron-impact spectrometer from threshold to 4 eV above. We observe pronounced angle-dependent structures in threshold excitation to the $2^{3}P$ state and in near-threshold excitation in the $2^{3}S$ state. Together, these experimental findings and recent theoretical considerations suggest the existence of a new $^{2}P$ resonance near 20.8 eV.