Nonperturbative B-Spline R-Matrix with Pseudostates Calculations for Electron Impact Ionization of Helium*

The theoretical and numerical approach used in a recent publication [1] describing a non\-perturbative treatment of ionization and simultaneous ionization plus excitation of helium by electron impact will be discussed. We then present a variety of comparisons between the present predictions, experimental data, and results from other non\-perturbative theories, such as convergent close-coupling and time-dependent close-coupling, for ionization without excitation. The overall excellent agreement with the other available data provides confidence in using the \hbox{$B$-spline} \hbox{$R$-matrix} with pseudo\-states approach for this benchmark system, as well as extending it to more complex situations for which no other non\-perturbative methods are currently available.\\[4pt] [1] O. Zatsarinny and K. Bartschat, Phys. Rev. Lett.~{\bf 107} (2011) 023203.