Electron excitation cross sections for Si XII

Excitation cross sections are calculated for Si XII by the coulomb-Born method, using numerical Hartree-Fock radial functions. The transitions considered are 2s-np, ns-n'd, 2p-nd and 3s-np. Reasonable agreement with the general results of Bely and Petrini is obtained, but an appreciable difference occurs for 2s-7p. The effect of including higher multipoles in the coulomb-Born method is illustrated. For comparison, the plane wave Born approximation is also used to obtain cross sections for the 2s-np transitions, as well as for 2s-ns and 2s-nd, and is found to approach the correct result to within a factor of three for incident energies greater than three times the threshold value.