Origin of dips and peaks in the absolute fully resolved cross sections forthe electron-impact double ionization of He

Using a multiparameter multicoincidence spectrometer, we have measured the coplanar (e,3e) angular distributions following the double ionization of he lium at an incident energy of approximate to 5.6 keV and under a small proj ectile's scattering angle of 0.45 degrees. The two ejected electrons have b een detected with equal energies E-b = E-e= 10 eV. The absolute value of th e cross section is determined with an accuracy of 25%. The origin of dips a nd peaks in the spectra is exposed by analyzing the corresponding theoretic al calculations. These calculations have been done using a four-body final- state wave function for the three electrons moving in the field of He2+ . T he dipolar limit is investigated and the manifestation of the deviation fro m this limit are pointed out. General features and possible trends for othe r targets are proposed. [S1050-2947(99)02805-X].