Photoionization processes of rare-earth dopant ions in ionic crystals

A systematic study of the photoionization thresholds was able to provide pertinent information on the location of impurity ion states in comparison with the edge of the conduction band. In some divalent rare-earth doped alkaline earth fluorides, it was demonstrated that the delocalization of the electron in the conduction band may result in the formation of impurity-trapped excitons leading to unexpected luminescence. Models have been proposed to evaluate the photoionization energies and to describe exciton-like states.