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d group-theoretical method is suggested for determining allowed type states of pairs and complexes of impurity ions in ionic crystals. Two cases are discussed in detail. In the first case the energy of the electrostatic interaction Vy,t between electrons of different impurity ions is greater than that of the internal magnetic interactions H F and in the second case VFnt is less than HF. For both cases V&t is assumed to be smaller than the total energy of the internal electrostatic interactions and the energies of a crystal field produced by the remaining ions of the crystal. As an example the I-V order pairs of the equivalent ions Cr3+ in corundum are examined.

The method of effective Hamiltonian (EFH) is applied to the excited state (2E4A2) fine structure of Cr3+–Cr3+ fourth order pairs in ruby. Comparison with experimental data illustrates the importance of both the anisotropic and antisymmetric exchange interactions, and that part of isotropic exchange interaction, which is nondiagonal in the orbital variables. Various microscopical mechanisms responsible for terms in the effective Hamiltonian are discussed.

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It has been made an extention of the conventional theory based on the assumption of the well isolated Zhang-Rice singlet to be a first electron-removal state in dielectric copper oxide. One assumes the photohole has been localised on either small (pseudo)Jahn-Teller polaron or large non-adiabatic polaron enclosed one or four to five CuO 4 centers, respectively, with active one-center valent ( 1 A 1g − 1,3 E u ) manifold. In the framework of the cluster model we have performed a model microscopic calculation of the k-dependence of the matrix element effects and photon polarization effects for the angle-resolved photoemission in dielectric cuprate like Sr 2 CuO 2 Cl 2 . We show that effects like the "remnant Fermi surface" detected in ARPES experiment for Ca 2 CuO 2 Cl 2 may be, in fact, a reflection of the matrix element effects, not a reflection of the original band-structure Fermi surface, or the strong antiferromagnetic correlations. The measured dispersion-like features in the low-energy part of the ARPES spectra may be a manifestation of the complex momentum-dependent spectral line-shape of the large PJT polaron response, not the dispersion of the well-isolated Zhang-Rice singlet in antiferromagnetic matrix.

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AbstractsGroup-theoretical methods for determining permitted terms of electrons in polyatomic molecules and complexes in the MO and HL methods are proposed. Essentially, the permutational symmetry of the electron system is utilized. Some concrete examples of the operation of these methods are considered.On propose des mtthodes provenantes de la theorie des groupes pour dtterminer les termes d'tlectrons permis dans les molecules et les complexes polyatomiques dans le cadre des methodes MO et HL. On utilise principalement la symktrie permutationnelle du syst6me d'klectrons. On examine quelques exemples concrets pour illustrer les mtthodes propostes.Gruppentheoretische Methoden fur die Bestimmung der in mehratomigen Molekulen und Komplexen erlaubten Elektronenterme werden vorgeschlagen; diese Methoden konnen sowohl in der MO-als auch in der HL-Methode angewendet werden. Man benutzt wesentlich die Permutationssymmetrie der Elektronensysteme. Einige konkrete Beispiele werden betrachtet.