SrF2 and BaF2 crystals, doped with the Yb3+ ions, have been investigated by electron paramagnetic resonance and optical spectroscopy. As-grown crystals of SrF2 and BaF2 show the two paramagnetic centres for the cubic (Tc) and trigonal (T4) symmetries of the Yb3+ ions. Empirical diagrams of the energy levels were established and the potentials of the crystal field were determined. Information was obtained on the SrF2 and BaF2 phonon spectra from the electron-vibrational structure of the optical spectra. The crystal field parameters were used to analyse the crystal lattice distortions in the vicinity of the impurity ion and the F− ion compensating for the excess positive charge in T4. Within the frames of a superposition model, it is shown that three F− ions from the nearest surrounding cube, located symmetrically with respect to the C3 axis from the side of the ion-compensator, approach the impurity ion and cling to the axis of the centre when forming T4. The F− ion located on the axis of the centre between the Yb3+ ion and ion-compensator, also approaches close to the impurity ion.

Electron paramagnetic resonance, electron-nuclear double resonance, and optical spectroscopy of the tetragonal Yb 3ϩ center in KMgF 3 are reported here. The results of these experiments allow us to conclude that a previously given structural model as well as the interpretation of the optical spectrum of this center are incorrect. A model is presented and experimentally and theoretically justified. In particular, the values of the hyperfine and transferred hyperfine interaction parameters were determined as well as an experiment-based energy-level scheme. Its parametrization is performed by including simultaneously the crystal field and the spin-orbit interaction within the 7 F term. Furthermore, a theoretical analysis of the transferred hyperfine interaction ͑THFl͒ parameters is presented. It is further shown from optics and from microscopic calculations of the THFI parameters that g ʈ and g Ќ have opposite signs and that the rule of correspondence between the cubic g factor and g ϭ 1 3 (g x ϩg y ϩg z) does not depend on the relative magnitude of the cubic and lowsymmetry crystal field acting on the rare-earth ion.

The Yb 3+ paramagnetic center of the trigonal symmetry (''oxygen'' paramagnetic center T 2 ) in CaF 2 and SrF 2 single crystals is studied by EPR and optical spectroscopy. The Stark level energies of the Yb 3+ multiplets are established from absorption, luminescence and excitation luminescence spectra and the crystal field parameters are calculated. r

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