Fine structure has been observed in the fluorescence and excitation spectra of Eu +2 in CaSO 4 and in the fluorescence spectrum of Eu +2 in BaMg ( SO 4 ) 2 . The fine structure in the fluorescence of Eu +2 in CaSO 4 is identified as a zero phonon line with many sharp phonon replicas. The fine structure in its excitation spectrum is due to the closely spaced energy levels in the excited state produced by a strong exchange interaction splitting of the 4f 6 5d states. The fine structure in the fluorescence of Eu +2 in BaMg ( SO 4 ) 2 is due to the normal splitting of the 4f 7 excited state and the dominance of 4f 7 → 4f 7 transitions.
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Ryan et al. (1974) studied this question.