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 2P3 state and in near-threshold excitation in the 2S3 state. Together, these experimental findings and recent theoretical considerations suggest the existence of a new P2 resonance near 20.8 eV.