Electron impact excitation of the resonance lines of alkali-like positive ions

Abstract

Cross sections and collision strengths for electron impact excitation of the resonance lines of Mg+, Ca+, Sr+ and Ba+, calculated in the Coulomb distorted wave, Coulomb Born and Coulomb Bethe approximations, are presented. A technique for calculating contributions to the total collision strength from large values of incident electron angular momenta is discussed. A comparison of the results with three-state close-coupling calculations indicates that the Coulomb Born and Coulomb distorted wave approximations agree well for energies greater than about 8 times threshold, while comparison with recently measured absolute cross sections shows that the Coulomb Born and Coulomb distorted wave results agree well with experiment only for energies above about 80 times threshold.