Electron impact ionization of helium: coplanar, triply differential cross sections at high and intermediate energies

The authors calculate the triply differential cross section (TDCS) for electron impact ionization of helium in the coplanar (Ehrhardt-type) geometry with asymmetric excess energy partitioning. The impact energy of the projectile electron is six to twenty-four times larger than the ionization potential. In generating the state functions they generalize the approach of Brauner et al. (1989) who introduced a two-electron-continuum wavefunction for electron impact ionization of hydrogen which incorporates the electron-electron repulsion and all electron-nucleus interactions. This wavefunction gives the exact repulsion three-body Coulomb boundary condition in the limit of large particle separation. In the authors' application to helium, they account for the distortion of the He + -core by introducing phase shifts for the slow, ejected electron.