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Optical spectra and energy levels of the sextet, quartet, and doublet states of ${\mathrm{Sm}}^{3+}$ ${(4f}^{5})$ incorporated into single crystals of ${\mathrm{Y}}_{3}{\mathrm{Al}}_{5}{\mathrm{O}}_{12}$ $({\mathrm{Sm}}^{3+}:\mathrm{Y}\mathrm{A}\mathrm{G}),$ where YAG denotes yttrium aluminum garnet, are reported and analyzed at wavelengths between 560 and 280 nm. The analysis of energy (Stark) levels is based on a model Hamiltonian consisting of Coulombic, spin-orbit, and interconfigurational terms for the ${4f}^{5}$ atomic configuration of ${\mathrm{Sm}}^{3+}$ and crystal-field terms in ${D}_{2}$ symmetry (the site symmetry of the ${\mathrm{Sm}}^{3+}$ ions in the garnet lattice). The Hamiltonian also includes contributions arising from the spin-correlated crystal field. Because of the strength of the crystal field, the entire energy matrix is diagonalized within the complete ${4f}^{5}$ ${\mathrm{SLJM}}_{J}$ basis set representing 73 LS states, 198 ${}^{2S+I}{L}_{J}$ multiplets, and 1001 doubly degenerate crystal-quantum states. In ${D}_{2}$ symmetry, all Stark levels are characterized by the same irreducible representation ${(}^{2}{\ensuremath{\Gamma}}_{5}).$ Optimization between 314 calculated-to-observed Stark levels was carried out with a final rms deviation of 10 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Eigenvectors obtained from the crystal-field splitting analysis are used to calculate transition line strengths originating from the ground-state Stark level to Stark levels in excited manifolds. The calculated line strengths are compared with experimental line strengths obtained from the absorption spectrum at 3.8 K. The line-strength analysis is useful in identifying individual excited Stark levels associated with sextet, quartet, and doublet states strongly mixed by the crystal field.

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