The energies of the triply excited n = 2 intrashell He− resonances 2s22p and 2s2p2 revisited

Using a recently constructed high-resolution crossed-beam apparatus consisting of a hemispherical electron monochromator and a quadrupole mass spectrometer, we have measured in detail the He+ ionization cross-section function in the electron energy range from 56–59 eV. By fitting the two structures corresponding to the presence of the two triply excited n = 2 intrashell He− resonances (2s22p)2P and (2s2p2)2D with the theoretical natural line shape, we have deduced that their energy positions lie at 57.06 ± 0.05 and 58.15 ± 0.05 eV, respectively. Between these two states, an additional previously unseen structure has been observed and tentatively linked to the doubly excited autoionizing (2s2)1S state of He. When comparing the presently deduced resonance energies with previous data, there is good agreement within the error bars with those experiments which used both a similar experimental technique to detect these states and a data analysis based on the Fano line shape. Experimental results using other methods to detect and characterize these states (electron scattering, electron excitation) appear to lie at slightly higher energies; nevertheless, the energy separation between the two states is the same for almost all experiments