1. Spectra and Energy Levels of Rare Earth Ions in Crystals;Dieke,1968
2. Optical Spectra of Transparent Rare Earth Compounds;Hüfner,1978
3. Luminescent Materials;Blasse,1994
4. Spectroscopic Properties of Rare Earths;Wybourne,1965
5. The Effective Crystal Field Potential,2000