Photoionization of helium above theHe+(n=2) threshold: Autoionization and final-state symmetry

The partial cross section, the satellite branching ratio, and the angular-distribution asymmetry parameter for simultaneous photoionization and excitation to the n=2 states of the ${\mathrm{He}}^{+}$ ion have been measured in the 67.5–90-eV photon-energy range. In the nonresonance regions (67.5 eV≤hν≤69.5 eV and 75 eV ≤hν≤90 eV), the asymmetry-parameter values have been used to infer the ratio of the 2p cross section to the 2s cross section. These results indicate that the ${\mathrm{He}}^{+}$(n=2) satellite is predominantly 2p near threshold, in agreement with the experimental and most of the theoretical results reported to date. In the region below the ${\mathrm{He}}^{+}$(n=3) threshold (69.5 eV≤hν≤73.0 eV), the effects of a series of autoionizing Rydberg levels on the n=2 cross section, branching ratio, and asymmetry parameter have been measured, this being the first detailed measurement of the angular distribution of a satellite over an autoionization resonance. In addition, qualitative information concerning the total cross section and the 1s partial cross section has been obtained for the first member of this series (3s3p), disagreeing with previous experimental and theoretical results for the total cross section, but in agreement with recent photoemission measurements of the 1s cross section. The present results suggest that the qualitative shapes of the total and 1s cross sections over the 3s3p resonance are similar to the profile of the n=2 cross section for this resonance. To illustrate quantitative methods for the interpretation of autoionization phenomena, the derivation from the resonance data of several parameters defining the autoionization process is described.