THRESHOLD ANGULAR-DISTRIBUTIONS OF (E, 2E) CROSS-SECTIONS OF HELIUM-ATOMS

Cross sections for the electron-impact ionization of helium atoms in a coplanar symmetric geometry are computed by including in the Coulomb-Born approximation a normalization factor which approximately incorporates electron-electron interactions in the final-state wave function. This approximation yields excellent agreement with recent experimental observations at impact energies between 0.5 and 5 eV above the ionization threshold. It is shown that the angular distributions given by the present theory are in agreement with Wannier's threshold theory.