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The absorption and fluorescence spectra of Eu3+ in single crystals of Y2O3 have been observed and analyzed. The intermediate-coupling free-ion energy levels for Eu3+, including the effects of the 46 singlet states of the 4f6 configuration, are presented for all levels appearing below 41 000 cm—1. The results obtained by including the singlet states in the free-ion energy level calculations are discussed. Values of E1=6217.84 cm—1, E2=38.94 cm—1, E3=630.15 cm—1, and ζ=1343.1 cm—1 are found to give the best over-all agreement between the observed and calculated energy levels.
Chang et al. (Sun,) studied this question.