Intraconfigurational 4f 7 transitions of Eu 2ϩ in KMgF 3 have been investigated by means of two-photon excitation spectroscopy. A well-resolved Stark structure of the 8 S 7/2 → 6 P 7/2,5/2 transitions is demonstrated and interpreted. Additional lines appearing in the excitation spectra are discussed in terms of noncubic perturbations at the substitutional impurity site. 4f 7 excited states of Eu 2ϩ belonging to the 6 D multiplet, inaccessible to single-photon spectroscopy, have been identified in

Polarization dependence of sharp intraconfigurational 4f7 transitions of Eu2+ in KMgF3 have been studied by means of two-photon excitation spectroscopy. A new puzzle appears connected with the site-symmetry distortion at the Eu2+ ions and the weakness of third-order spin-orbit corrections, suggesting a mechanism for intraconfigurational interaction independent from crystal-field strength and from the site symmetry on the rare-earth ion. Intensity and polarization dependencies of the 8S7/2→6P7/2,6P5/2,6D9/2 transitions have been satisfactorily interpreted in terms of second-order perturbation theory.

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A number of peculiar effects caused by the self-induced anisotropy of nonlinear susceptibility a t high excitation levels is revealed in cubic crystals. The self-induced turn of the polarization plane a t resonance of radiation frequency to one of the actual transitions in crystal and the self-induced oscillations of polarization with crystal length when the frequency i s off resonance are of special interest [l t o 31. These effects being symmetry-defined depend substantially on the actual mechanism of the optical nonlinearity and therefore may be used for determining of fine parameters in impure crystals. In the resonant case the self-induced change of light polarization (SICLP) occurs a t extremely low (non-laser) excitation providing that both the traditional spectroscopy and the nonlinear spectroscopy techniques can be used. Such an approach proved t o be most fruitful for the investigation of reorienting centres in alkali halides.As a rule the centres under consideration occupy several equivalent equilibrium positions in the unit cell. Because the resonant absorption of the centre is anisotropic and the reorientation probability in excited electronic state exceeds heavily that in the ground state, the polarized resonant radiation produces easily self-induced optical anisotropy through centre alignment, revealing itself in a polarization change. As microscopic theory shows [a, 61, this change is very sensitive t o a variation of centre parameters, i.e. wavelength and temperature dependent. Here we report on some results of a spectroscopic SICCP investigation in KCl with different colour centres. l ) Prospekt Nauki 115, SU-252650 Kiev, USSR.