THE DOUBLE-IONIZATION CUSP IN THE THRESHOLD PHOTOELECTRON-SPECTRUM OFHELIUM

New measurements of the zero-energy electron yield in the region of th e photodouble ionization threshold in helium are presented, showing fo r the first time a Wannier-type cusp with a small asymmetry of the exc itation/ionization wing amplitudes, similar to the electron impact res ults in the region of the first ionization threshold. The observed pre ssure dependence of the cusp amplitude asymmetry and other second-orde r effects are discussed in an attempt to explain the discrepancy with the earlier photoionization results of Hall et al.