Energy transfer between the Sm3+, Eu3+, Tb3+, and Dy3+ ions has been systematically studied in the sodium rare-earth tungstate hosts. Evidence has been obtained for the transfer of excitation energy between all ion pairs of this group. Direct evidence for exchange of energy was established by the method of selective excitation for the Dy3+ → Tb3+, Tb3+ → Eu3+, and Sm3+ → Eu3+ ion pairs. Indirect evidence for energy transfer was found by fluorescence enhancement for the Tb3+ → Sm3+ ion pair and by lifetime measurements for the Dy3+ → Sm3+ and Dy3+ → Eu3+ ion pairs. The results are consistent with the “resonant” exchange mechanism as the dominant process responsible for the transfer of energy between these ions.
W. W. HOLLOWAY, JR. and M. KESTIGIAN, "Energy Transfer between the Sm3+, Eu3+, Tb3+, and Dy3+ Ions in Sodium Rare-Earth Tungstates," J. Opt. Soc. Am. 56, 1171-1171 (1966)