Electron-Impact Excitation of Be+ , Mg+ , and Ca+ Ions

The classical binary-encounter model for the ionization of ions by impact of electrons given by Thomas and Garcia has been extended to calculate the excitation cross section of ions. The electron-impact excitation cross sections for the $3s\ensuremath{-}3p$ and the $3s\ensuremath{-}3d$ transitions in ${\mathrm{Mg}}^{+}$, the $2s\ensuremath{-}2p$ transition in ${\mathrm{Be}}^{+}$, and the $4s\ensuremath{-}4p$ transition in ${\mathrm{Ca}}^{+}$ have been calculated. The results are compared with calculations based on the close-coupling and the Coulomb-Born approximations. Our results agree better with close-coupling calculations than with calculations based on the Coulomb-Born approximation.