Photoionization phenomena near the double-ionization threshold of helium

A sensitive electron spectrometer has been used to study photoionization processes in the region of the double-ionization threshold of helium. Analysis of the cusp observed at the He 2+ potential in a threshold photoelectron spectrum indicates that this structure would support the predictions of Wannier theory with a range of validity of 2.0+or-0.5 eV. Asymmetry parameters ( beta ) have been obtained for near-threshold excitation of He + *(n) satellite states up to and above n=10 at photoelectron energies of 0.14 and 0.25 eV. For n)6, beta was found to have a roughly constant value of about -0.5 and did not approach the predicted value of -1. Similarly, for the double-ionization process, beta is approximately constant at -0.4 in the energy range from 0.25 to 2.0 eV above the He 2+ potential and does not display the steep rise from beta =-1 to positive values which is predicted to occur in this energy range.