Extra lines in rare earth crystal spectra

We estimate the magnitude of expected shifts that would be observed as 'extra' lines in rare earth sharp line spectra due to an impurity ion near the rare earth. The results ( ∼ 10-20cm-1) support the suggestion and experiment of Faulhaber and Hüfner. In disagreement with these authors we show that the experimental results can not be used to investigate covalency and that symmetry changes do result and can have important consequences. We also suggest that the many extra lines observed at times in electron paramagnetic resonance studies arise for the same reason as in the optical case. © 1969.