Double ionization of helium by electron-impact: Complete pictures of the four-body breakup dynamics

The dynamics of He double ionization by 2 keV electron impact is studied ex perimentally for a momentum transfer of 0.6 a.u, at excess energies of 10 a nd 40 eV. Complete sets of fivefold differential cross sections are present ed for all electron emission angles in coplanar geometry. Contributions bey ond the first Born approximation are identified comparing experimental data with first order convergent close-coupling calculations which are in consi derably better agreement with the present experiment than with the earlier measurement of Kheifets et al.