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2. C. A. Morrison and R. P. Leavitt, in Handbook of the Physics and Chemistry of Rare Earths, edited by K. A. Gschneidner and L. Eyring (North‐Holland, New York, 1982), Vol. 5, p. 461.
3. B. R. Judd, Operator Techniques in Atomic Spectroscopy (McGraw‐Hill, New York, 1963).
4. B. G. Wybourne, Spectroscopic Properties of Rare Earths (Wiley, New York, 1965).
5. S. Hüfner, Optical Spectra of Transparent Rare Earth Compounds (Academic, New York, 1978).