Observation of a Broad Resonance in the 23S Excitation of Helium by Electron Impact

We have observed a broad $P$-wave resonance at about 50 eV primary energy in the angular distribution of electrons scattered after excitation of the ${2}^{3}S$ state of helium. The resonance has a width of about 15 eV and affects the differential excitation cross section by more than 3 orders of magnitude. It is suggested that this resonance is associated with the temporary formation of triply excited states of the negative helium ion.