Electron impact double ionization of rare gases: A kinematical analysis of the non-first order effects at intermediate impact energy

The (e,3-1e) four-fold differential cross sections (4DCS) are measured for the electron impact double ionization of rare gas atoms (He, Ne, Ar) in coplanar asymmetric geometry for a wide range of ejected electron energies (5 to 144 eV) and at an incident energy of about 600 - 700 eV. Though we only show here a sample of the results, all 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 Goetz 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 (or sub-lobes) in the cross section 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.