Electron-impact excitation of He: Threshold region

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 2³P state and in near-threshold excitation in the 2³S state. Together, these experimental findings and recent theoretical considerations suggest the existence of a new ²P resonance near 20.8 eV.