Identification of higher order contributions in three-dimensional (e,2e) cross-sections for helium

Fully differential cross-sections for single ionization of helium by 102 eV and by 1 keV electron impact have been obtained using an advanced reaction microscope. The data cover a large range of emission angles for a low-energy (E ≤ 15 eV) electron and different scattering angles for the fast electron. Significant electron emission out of the projectile scattering plane in between the binary and the recoil lobes is observed. The experimental data are compared with theoretical predictions from a three-Coulomb wave function model.