Rare-Earth Ions in the Alkali Halides. II. Pseudolocalized Vibrational Frequencies

Vibronic structure observed in the emission and absorption spectra of alkali halides containing either ${\mathrm{Sm}}^{2+}$, ${\mathrm{Eu}}^{2+}$, or ${\mathrm{Yb}}^{2+}$ is reported, and shown to result from pseudolocalized vibrations occurring at the rare-earth defect. A model for the defect is defined, and an analysis of its vibrational frequencies is given for the sample case of a RbCl host lattice. A Green's function formalism is used in the analysis. The calculation, in which a number of approximations are made, yields the qualitative features of the experimental results.