Origin of dips and peaks in the absolute fully resolved cross sections for the 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 helium at an incident energy of ≈5.6 keV and under a small projectile's scattering angle of 0.45^∘. The two ejected electrons have been detected with equal energies Eb=Ec=10 eV. The absolute value of the cross section is determined with an accuracy of $25%$. The origin of dips and peaks in the spectra is exposed by analyzing the corresponding theoretical calculations. These calculations have been done using a four-body final-state wave function for the three electrons moving in the field of He²⁺. The dipolar limit is investigated and the manifestation of the deviation from this limit are pointed out. General features and possible trends for other targets are proposed.