Electron Impact Excitation of Positive Ions: Application to Ca+ 4s-4p and 3d-4p

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Abstract
A review is given of different approximate methods for computing cross sections for transitions in positive ions produced by electron impact. It is shown that it is necessary to take into account the distortion by the ion field, and, for strong optically allowed transitions, one must allow for strong coupling effects. Application to the 4 s −4 p and 4 p −3 d transitions in singly ionized calcium shows that the neglect of strong coupling effects overestimates the cross sections. However, our results, computed with a Coulomb distorted wave approximation, with allowance for strong coupling effects, are much bigger than those of Jefferies, the reason being that he considered only one value ( l = 1) of the angular momentum of the incident electron, whereas the main contribution is shown to occur for l = 4. The Bethe approximation, with allowance for Coulomb distortion, implies in fact two different approximations: the weak coupling approximation and the “long range approximation”. It is possible to make corrections for strong coupling effects and, for strong allowed transitions such as 4 s −4 p and 4 p −3 d in Ca + , this method gives good results in a very simple way.

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