Spectra and energy levels of trivalent holmium in strontium fluorapatite

Polarized absorption and fluorescence spectra are reported for Ho3+ (4f10) ions incorporated into single crystals of strontium fluorapatite, Sr5(PO4)3F, also known as SFAP. Site-selective excitation experiments indicate that, to within instrumental resolution, the Ho3+ ions occupy similar lattice sites having Cs symmetry. Spectra were obtained between 1940 and 400 nm at temperatures from 4 K to room temperature on crystals having the hexagonal structure [P63/m(C6h2)]. Individual energy (Stark) levels for the fourteen lowest-energy multiplet manifolds of Ho3+ (4f10) were analyzed in detail. These manifolds include the I52 (ground state), the remaining I5J manifolds, F5J, S52, K38(2), G56, and the G35(2) manifolds. A Hamiltonian consisting of Coulombic, spin-orbit, and crystal-field terms was diagonalized over these manifolds in an LSJMJ basis set, which includes 146 crystal-quantum states, labeled either Γ1 or Γ2, appropriate to the crystal-field symmetry of Cs. By varying the crystal-field parameters, Bnm, and the centroid for each manifold, we obtained agreement between 101 experimental and calculated Stark levels with a rms deviation of 9 cm−1. An additional 15 Stark levels, not included in the original analyses, are predicted to within the rms deviation. The predicted symmetry label of each Stark level agrees with the experimental assignment based on the analysis of the polarized spectra.