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The two-photon excitation spectra of CaF 2 (Eu 2+ ) and SrF 2 (Eu 2+ ) have been studied at 300K and at 4.2K in the region between 24100 cm -1 and 28000 cm -1 . The observed spectra consist of broad bands due to 4f 7 to 4f 6 5d transitions coupled with odd-parity phonons, and two groups of sharp lines due to pure 4f 7 to 4f 7 transitions.
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Fritzler et al. (1976) studied this question.
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