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Absorbance and site-selective fluorescence studies of Eu3+:Sr5(PO4)3F (Eu:SFAP) provide a nearly complete characterization of the Eu3+ energy levels. These results are compared with calculated energies generated by lattice-sum analysis. Eu3+ is found to occupy one site predominantly, but numerous minority sites are also evident; polarization studies of absorption and fluorescence lines provide evidence for the principle site having Cs symmetry. Vibronic transitions occur in spectra as well. Interpretation of the lattice location and charge compensation mechanism for the principal site is discussed in the context of obstacles arising from spectral interpretation, high crystal fields in SFAP, low site symmetry, low J-numbers in Eu3+ energy manifolds, and lattice covalency. Eu2+ emission upon ultraviolet excitation is also demonstrated; valleys in the emission profile are attributed to intracrystal absorption by Eu3+.

Energy transfer and upconversion studies in yttrium-aluminum-garnet crystal doped with thulium and holmium under near-infrared laser excitationAbsorption spectra of Ho3 + ions in yttrium aluminum garnet ( Ho3 + :YAG) are reported between 2.16 and 0.23 ,um at various temperatures between 4 K and room temperature. Laser-excited excitation spectra and emission spectra from the 'F4 and 5S', multiplet manifolds to the ground state manifold 51s were obtained at 4, 20, and 55 K. The majority of Ho" + ions substitute for Y3 + ions in sites of D2 point-group symmetry in the lattice. Over 1000 temperature-dependent transitions (hot bands) establish 280 experimental Stark levels of the 4 f lo zs+ 'LJ) multiplet manifolds. Symmetry labels I't, I',, ( P3, or 14, appropriate to D2 symmetry, have been assigned to Stark levels up to 43 000 cm-'. Experimental levels are compared with results obtained from a theoretical calculation. The model Hamiltonian includes Coulombic, spin-orbit, and interconfiguration interaction terms for the 4 f lo atomic ~configuration of Ho3 + and crystal-field terms in D, symmetry. The Hamiltonian was diagonalized within the 50 lowest "+ 'L, manifolds in a LSJM, basis set that includes 486 calculated levels. Comparison between 151 experimental Stark levels and the corresponding levels of the lowest 199 calculated levels yields an rms deviation of 4.3 cm-'.8183

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