Predominance of the second-order, two-step mechanism in the electron impact double ionization of helium at intermediate impact energy

The (e,3-1e) four-fold differential cross sections (4DCS) are measured for the double ionization of helium in coplanar asymmetric geometry for a wide range of ejected electron energies and at an incident energy of about 600 eV. The experimental angular distributions of the 4DCS are characterized by large angular shifts of the forward and backward lobes with respect to the momentum transfer direction or its opposite, respectively. This validates our previously published results (Lahmam-Bennani et al 2002 J. Phys. B: At. Mol. Opt. Phys. 35 L59) which were questioned by Götz et al (2003 J. Phys. B: At. Mol. Opt. Phys. 36 L77). A qualitative, kinematical analysis is given which allows relating these shifts and the observed structures in the intensity distributions to the second-order, 'two-step 2' double ionization mechanism, which is shown to predominate over the first-order 'shake-off' and 'two-step 1' mechanisms under the present kinematics.