SITE-SELECTIVE EXCITATION AND POLARIZED ABSORPTION AND EMISSION-SPECTRA OF TRIVALENT THULIUM AND ERBIUM IN STRONTIUM FLUORAPATITE

Polarized fluorescence spectra produced by site-selective excitation, and conventional polarized absorption spectra were obtained for Tm3+ a nd Er3+ ions individually incorporated into single crystals of stronti um fluorapatite, Sr-5(PO4)(3)F, also known as SFAP. Substitution of th e trivalent rare earth ion for divalent strontium was achieved by pass ive charge compensation during Czochralski growth of the fluorapatite crystals. Spectra were obtained between 1780 and 345 nm at temperature s from 4 K to room temperature on crystals having the hexagonal struct ure [P6(3)/m(C-6h(2))]. The polarized fluorescence spectra due to tran sitions from multiplet manifolds of Tm3+(4f(12)), including D-1(2), (1 )G(4), and H-3(4) to manifolds H-3(6) (the ground-state manifold), F-3 (4), H-3(5), H-3(4), and F-3(3) were analyzed for the details of the c rystal-field splitting of the manifolds. Fluorescence Lifetimes were m easured for Tm3+ transitions from D-1(2), (1)G(4), and H-3(4) at room temperature and from (1)G(4) at 16 K. Results of the analysis indicate that the majority of Tm3+ ions occupy sites having C-s symmetry. A po int-charge lattice-sum calculation was made in which the crystal-field components, A(nm), were determined assuming that trivalent thulium re places divalent strontium in the metal site having C-s symmetry. Resul ts support the conclusion that the nearest-neighbor fluoride (F-) is r eplaced by divalent oxygen (O2-), thus preserving overall charge neutr ality and local symmetry. Crystal-field splitting calculations predict energy levels in agreement with results obtained from an analysis of the experimental data. By varying the crystal-field parameters, B-nm, we obtained a rms difference of 7 cm(-1) between 43 calculated and exp erimental Stark levels for Tm3+(4f(12)) in Tm:SFAP. Absorption and flu orescence spectra are also reported for Er3+ ions in Er:SFAP. Measurem ent of the temporal decay of the room temperature fluorescence from th e I-4(11/2) and I-4(13/2) manifolds yielded fluorescence lifetimes of 230+/-20 mu s and 8.9+/-0.1 ms, respectively. The experimental Stark l evels obtained from an analysis of the spectroscopic data were compare d with a crystal-field splitting calculation. The initial set of B-nm parameters for Er3+(4f(11)) was established from the three-parameter t heory and the final set of B-nm parameters obtained for Tm3+(4f(12)) i n Tm:SFAP. The best overall agreement between calculated and experimen tal Stark levels is 8 cm(-1) for 48 Stark levels, representing 12 obse rved multiplet manifolds of Er3+(4f(11)) in Er:SFAP. (C) 1997 American Institute of Physics.