Excitation of Helium-Like Ions by Electron Impact in the First-Order Exchange Coulomb Approximation

Abstract

Usefulness of the first-order exchange Coulomb approximation has been examined by applying it to the electron impact excitation processes of helium-like ions. Collision strengths for the excitation from the ground state to the n =2 states (2 3 S, 2 1 S, 2 3 P and 2 1 P) for Li + , C 4+ , O 6+ and Si 12+ have been calculated at energies up to five times threshold energies. It is found that the present approximation improves the Coulomb-Born-Oppenheimer approximation remarkably and gives results comparable with the five-state close-coupling calculation of Wyngaarden et al. (1979). It is also found that the short range distortion effect is largely taken care of in the present first-order approximation.