Double ionization of helium by fast electron impact

We report on a kinematically complete investigation of He double ionization by 3 keV electron impact. A new experimental approach for experiments on electron impact ionization is applied: in a multi-electron recoil-ion coincidence the momentum vectors of the recoiling ion and of two slowly ejected electrons (Eb,c < 20 eV) are determined. The momentum of the fast scattered electron follows from momentum conservation. By examining the double ionization process as a function of the momentum transferred to the target by the scattered projectile different kinematical regimes are separated. For small values of the momentum transfer the angular correlation and the energy partitioning of the slowly ejected electrons exhibit the characteristics of double photoionization. For Bethe ridge kinematics signatures of the TS 1 ionization mechanism are observed. (orig.)