NONCUBIC CENTERS IN CUBIC CRYSTALS AND THEIR SPECTRA IN EXTERNAL FIELDS

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Abstract
A short review is given of the general properties of local centers in a cubic lattice which possess an inherent noncubic point symmetry. The subjects considered are : (1) classification of noncubic centers into 7 types in accordance with the class of their symmetry and orientation in the lattice ; (2) the number of energetically-equivalent positions of noncubic centers differing in orientation in the crystal ("orientational") degeneracy) ; (3) oscillator models for optical transitions in noncubic centers and "latent" optical anisotropy of cubic crystals with noncubic centers. A general approach to the study of noncubic centers is discussed consisting in applying an anisotropic perturbation to a crystal which would act selectively on individual groups of centers. Also considered are characteristic properties of the splitting pattern of spectral lines of noncubic centers with the following perturbations applied to the crystal : (1) elastic uniaxial strain (a "piezo-spectroscopic" effect) ; (2) external electric field (a linear Stark effect for centers which do not possess inversion ; (3) External magnetic field. Information concerning the properties of centers and of optical transitions obtained in the study of spectra under application of external perturbations is reviewed. Examples are given of an experimental investigation of the strain-induced splitting and of the linear Stark effect for a number of zero-phonon lines in the spectra of colored lithium fluoride

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