Perturbation theory for (e, 3e) on helium

The contributions of order e 4 to the T matrix for electron impact double ionization of atomic helium have been evaluated. The resulting fivefold differential cross sections are characterized by the interference of the amplitudes associated with the different combinations of binary interactions of the active electrons. They show strong artefacts due to neglect of high-order post-collisional interactions, but also remarkable agreement with current experimental data if one of the atomic electrons is ejected into the forward half-plane. The results allow a discussion of the validity of the 'reaction mechanism' and 'photoionization limit' approaches to the interpretation of experimental (e, 3e) data.