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 e lectron impact ionization is applied: in a multi-electron recoil-ion coinci dence the momentum vectors of the recoiling ion and of two slowly ejected e lectrons (E-b,E-c < 20 eV) are determined. The momentum of the fast scatter ed electron follows from momentum conservation. By examining the double ion ization process as a function of the momentum transferred to the target by the scattered projectile differed kinematical regimes are separated. For sm all 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.