Ion‐Size Effects on the Energy Levels of F, FA, and FH Centers in Cesium Halides

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Abstract
The energy levels of F centers in cesium halides are calculated variationally using the point‐ion lattice approximation with different sets of trial wavefunctions for the ground and the excited states. The effect of the finite size of the lattice ions on these energy levels is then evaluated self‐consistently within the framework of a local model pseudopotential approach. This method is then used to obtain the changes in the ion‐size effects induced by a substitutional monovalent cation impurity in a nearest neighbour position — the FA center — or a substitutional monovalent anion impurity in the second neighbor position — the FH center. In each case the ion‐size effects are calculated for all the trial functions considered. The calculated transition energies are compared with the observed positions of the corresponding optical absorption bands. It is concluded that the results of optical measurements on all the three centers can be accounted for, at least qualitatively, on the basis of a reasonably simple theory.