Nephelauxetic effect in divalent rare earth ions

Recent measurements of the optical spectra of divalent rare earth impurities in alkaline earth halide hosts cannot be explained using only crystal field theory, i.e., by a description of the host as a lattice of point charges. To account for the nephelauxetic effect (the reduction of the electrostatic interaction of the impurity electrons) found in these spectra, it must be assumed that the impurity electrons and the nearby halide electrons form molecular orbitals. The work leading to these conclusions is reviewed here. (U.S.)