1. E.U. Cordon and G.H. Shortley, “The Theory of Atomic Spectra,” Cambridge University Press, New York, 1935.
2. B.R. Judd, “Operator Techniques in Atomic Spectroscopy,” McGraw-Hill, New York, 1963.
3. B.G. Wybourne, “Spectroscopic Properties of Rare Earths,” Wiley, New York, 1965.
4. M. Tinkham, “Group Theory and Quantum Mechanics,” McGraw-Hill, New York, 1964.
5. G.H. Dieke, “Spectra and Energy Levels of Rare Earth Ions in Crystals,” Wiley, New York, 1968.