Effect of the configuration interaction on the fine-structure-level splittings for quasi-two-electron atoms

A calculational procedure is presented to study the effect of the configuration interaction on the fine-structure-level splittings for a quasi-two-electron atom. A detailed application is carried out to examine the anomaly in the fine-structure-level splittings of the Al i i ${}_{}{}^1{}_{}{}^{}$,3F series which are strongly affected by the presence of the 3p3d ${}_{}{}^3{}_{}{}^{}F$ perturber. Our calculated level splittings are in close agreement with the available experimental values. In particular, our study has identified a direct link between the experimentally observed fine-structure-level splittings and the probability densities of the dominating configuration in the calculated state wave functions.