An Assessment of Collision Strengths for Lithium and Lithium-Like Ions

Abstract

In this paper electron impact collision strengths for atomic lithium and lithium-like ions (up to Z = 28) are intercompared and assessed by taking account of available experimental and theoretical results. A selection of these experimental and theoretical data is presented graphically as plots of effective collision strengths υ, that is, averages of collision strengths over Maxwellian electron velocity distributions, against reduced temperature kT/E0 (where E0 is the excitation threshold energy). The agreement among the various theoretical results is generally much closer than the errors quoted for the experimental results so the final assessment is largely determined by the theoretical values. An important need to improve the experimental methods is identified. Only a few of the theoretical calculations include the effect of resonances and these show that this omission can lead to big disparities in the threshold (low temperature) region. It appears that data for ions for which specific calculations are not available can be derived satisfactorily by interpolation from available and neighboring material. The paper concludes with a set of recommendations for the most reliable values and suggestions for work to improve on the current situation.