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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.

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A home-built multifunctional computerized spectrometer for optical detection of magnetic resonance (ODMR) is described. Preliminary results of studying Yb 3 + ions in BaF, single crystal by electron paramagnetic resonance, ODMR and optical spectroscopy are presented.

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KMgF 3 and KZnF 3 crystals doped with Nd 3+ ions were studied using EPR and optical spectroscopy methods. Several types of paramagnetic centers of Nd3+: KMgF 3 -two of tetragonal and one of rhombic symmetry, KZnF 3 --one of tetragonal and one of trigonal symmetry were found. Parameters of the corresponding spin Hamiltonians were determined. Using optical spectroscopy paramagnetic centers Nd 2+ and Nd 4+ in KMgF 3 were found.