Ionization and excitation of helium by proton impact



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Cross sections for the simultaneous ionization and excitation of helium by proton impact were calculated using a first Born approximation. The main purpose of this theoretical investigation was to further investigate a thesis by Byron and Joachain that only atomic wave functions containing electron-electron correlation terms are sufficiently accurate to describe double-processes in helium, that is, processes which cause a transition of both electrons. For the final state wave function a symmetric product of a hydrogen-like wave function and a positive energy Coulomb wave function were used. For the initial state three different wave functions were used, a 12-term correlated wave function, an uncorrelated product of hydrogen-like wave functions, and a Green's expansion of the Hartree-Fock wave function which is also uncorrelated. Comparisons were made with the analytical results of Bell and Kingston who investigated the single ionization of helium by proton impact using a correlated wave function; however, in their work they considered the case only for the residual ion in the ground state. The analysis of Mapleton for simultaneous ionization and excitation of helium using uncorrelated wave functions was also used for comparison. Although for single processes (i.e. transition of one electron) correlation effects are not important, the results for multiple processes show significant differences between cross sections using correlated wave functions and cross sections using uncorrelated wave functions.