1. D. L. Dexter: In Solid State Physics, Vol. 6, ed. by F. Seitz, D. Turnball (Academic Press, New York 1958)
2. G. F. Imbusch: In Luminescence Spectroscopy, ed. by M. D. Lumb (Academic Press, London 1978)
3. A more detailed treatment is found, for example, in B. Di Bartolo: Optical Interactions in Solids (Wiley, New York 1968)
4. G. H. Dieke: Spectra and Energy Levels of Rare Earth Ions in Crystals (Wiley-Interscience, New York 1968)
5. B. G. Wybourne: Spectroscopic Properties of Rare Earths (Wiley-Interscience, New York 1965)