Electron impact excitation of the n=2 level in He+and H at low energies

Abstract

Two Coulomb-exchange distorted-wave (CEDW) approximations are developed and applied to the electron impact excitation of the 1s to 2s and 1s to 2p transitions in He+ and H at low energies. In the first approximation, CEDW1, partial waves are distorted by various direct multiple potentials. In the second approximation, CEDW2, particular partial waves are additionally distorted by an exchange monopole potential. Results for cross sections presented here are compared with those of the three-state close-coupling and Coulomb-Born-Oppenheimer approximations, and with experiment. Despite the unfavourable degeneracy of the 2s and 2p states in hydrogenic atoms, the CEDW2 approximation gives satisfactory agreement with the three-state close-coupling approximation for the 1s to 2s transition and fairly good agreement for the 1s to 2p transition together with a significant reduction in computing time. Thus the author's distorted-wave approach can be useful at low energies for singly-ionised and neutral atoms and is sufficiently promising to justify further development for application to more complex systems, where the authors no longer have the degeneracy of states that we do with hydrogenic atoms.