Importance of non-first-order effects in the (e,3e) double ionization of helium

Angular distributions of the two ejected electrons resulting from the double ionization of helium by electron impact have been measured by means of a multicoincidence multiangle (e,3e) spectrometer at an incident energy of about 0.6 keV and equal outgoing energies E_b=E_c=11 eV. We identify various regimes of kinematical parameters where substantial differences are found with respect to the first-Born convergent close-coupling calculations: an angular shift of the position of the main lobe and the presence of additional lobes. These differences are attributed to high-order contributions in the projectile-target interaction. This conclusion is supported by recent (e,3e) calculations performed within the second-Born approximation.