Double ionization of helium by electron impact in the impulsive regime

The dynamics of helium double ionization by 2 keV electron impact has been investigated experimentally and theoretically at large momentum transfer of \q\ = 2 a. u. Fully resolved fivefold differential cross sections (FDCS's) are presented for symmetric and asymmetric energy sharing between the two ejected electrons at excess energies from 10 to 40 eV, and for the coplanar as well as the out-of-plane scattering geometries. Experimentally, a multielectron-recoil-ion coincidence technique has been applied and a large part of the final-state momentum space has been mapped. The presently employed theoretical model treats the interaction between the two slow ejected electrons nonperturbatively using the convergent close- coupling method, whereas the projectile-target interaction is described in the first Born approximation. The experimental and theoretical FDCS's agree well in shape. The cross section is dominated by two pairs of strong peaks. From this pattern it can be concluded that the two-step 1 mechanism, which is due to interelectron interaction after a single ionizing collision, is the dominant ionization process for the present kinematics