Electron correlation effects near threshold for electron-impact ionization of helium

Using a modification of the trapped-electron technique, we have measured the total intensity of electrons inelastically scattered from helium as a function of both incident and final energies near the ionization threshold. We find that a "cusp," which appears in the scattered electron intensity at the ionization threshold for electrons of near-zero final energy, decreases in visibility as the energy of the scattered electrons increases. For scattered-electron energies greater than about 2.0 eV, the intensity is smooth through the ionization threshold. The physical reason for the cusp is the formation of states with two excited and strongly correlated electrons. These experiments define the range of excess energies above the ionization threshold for which correlation effects are important, and hence measure the energy range of validity of the Wannier law of threshold ionization.