Electron impact excitation of He

A new approximation is developed for electron collisions with two-electron atoms and is applied to the excitation of He. The core orbital is taken to be that given by equivalent frozen 1s2 orbitals and the valence wavefunction is taken to be that given by a Hartree or Hartree-Fock frozen-core approximation. The resulting orthogonality of the core and valence wavefunctions enables the exact three-electron collision equations to be simplified to a form similar to that for a two-electron problem. The explicit dependence on the core wavefunction being removed, the effect of choosing a non-equivalent core is investigated within the Hartree frozen-core approximation. The collision equations are solved using a number of five-state exchange distorted-wave (EDW) approximations which include various multipole exchange distorting potentials exactly. The authors calculate total collision strengths for all transitions between the 11S, 23S, 21S, 23P and 21P states of He for incident electron energies from just above threshold up to 15.0 Ryd.