The double ionization cusp in the threshold photoelectron spectrum of helium

New measurements of the zero-energy electron yield in the region of the photodouble ionization threshold in helium are presented, showing for the first time a Wannier-type cusp with a small asymmetry of the excitation/ionization wing amplitudes, similar to the electron impact results in the region of the first ionization threshold. The observed pressure dependence of the cusp amplitude asymmetry and other second-order effects are discussed in an attempt to explain the discrepancy with the earlier photoionization results of Hall et al. (author)