ELECTRON-IMPACT IONIZATION OF HELIUM - COPLANAR, TRIPLY DIFFERENTIAL CROSS-SECTIONS AT HIGH AND INTERMEDIATE ENERGIES

We 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 we generalize the approach of Brauner et al, 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 asymptotic three-body Coulomb boundary condition in the limit of large particle separation. In our application to helium, we account for the distortion of the He+-core by introducing phase shifts for the slow, ejected electron. We have also introduced a high-energy approximation for the correlation effect and the fast, scattered electron. Good agreement, within the experimental error, was found with the experimental data at the highest energies considered. At smaller impact energies our model fails to reproduce the full extent to which the peak maxima are shifted.