Electron-Exchange Effect in Electron-Impact Excitation of the n=2 Fine-Structure Levels of Hydrogenic Ions

The electron-impact transitions between the n =2 fine-structure levels of hydrogenic ions are calculated by an improved close coupling scheme that accounts for the electron-exchange effect. It is found that the electron-exchange effect is relatively unimportant in the optically allowed transitions, as expected from its long-range nature, but represents a considerable contribution to the cross-sections of the optically forbidden transitions, where the contribution of low partial waves is more significant. Hence, a close coupling calculation including the exchange effect is required for low partial waves.