Cross sections and threshold effects for electron-impact excitation of the(2s2)S1and(2s2p)P3states of helium

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Abstract
The energy and intensity of scattered electrons resulting from excitation of the (2s2)S1 and (2s2p)P3 autoionizing states of helium have been measured as a function of incident electron energy. For incident energies only slightly higher than the threshold for (2s2)S1 excitation, post-collision interactions between the scattered and ejected electrons cause the slowly moving scattered electron to lose an amount of energy proportional to Eex−1.25±0.05 (where Eex is the energy the scattered electron would have had in the absence of a post-collision interaction). This energy dependence compares well with an energy gain of the ejected electron proportional to Eex−1.2, obtained by previous workers. Measurements of the intensity of scattered electrons show that both S1 and P3 total cross sections peak at threshold and have magnitudes about (2.0 and 4.0) × 10−20 cm2, respectively, then rise to broad maxima of magnitudes about (2.5 and 5) × 10−20 cm2 about 4.0 eV above their thresholds. A subsidiary peak at 59.0 eV in the P3 cross section is attributed to decay of the previously unidentified (2s2p2)S2 He− state into the P3 channel. The magnitude, energy, and width of this resonant structure in the P3 channel agrees with theoretical predictions.