Initial-state, final-state and higher-order effects in electron impact ionization of helium atoms

An approximate method to include second-order effects in the treatment of electron impact ionization processes within a combined distorted wave Born approximation (DWBA) + R-matrix approach is presented. The sensitivity of the results for triple differential cross sections for electron impact ionization of helium to the description of the initial and final states, and to the inclusion of second-order effects within the R-matrix box, is analysed. It is found that changes in the representation of the initial bound state have the strongest effect on the theoretical predictions. (author)