Symmetry and anisotropic optical spectrum of charge-transfer excitons in the model of CuO2 plane

A method of group-theoretical classification of excitonic states with a cha rge transfer from a unit cell to neighboring ones is suggested in a tetrago nal two-dimensional model of a CuO2 plane. The orientation (anisotropy) and polarization (pleochroism) dependences of the intensities of excitonic dip ole-forbidden optical transitions are determined. The phenomenon of initiat ing dipole-forbidden transitions in an external electric field is theoretic ally considered. General dependences of the probabilities of such transitio ns on the value and direction of the electric field and on the light wave p olarization are found. The dependences obtained can be used to identify wea k absorption bands in dielectric cuprates containing CuO2 planes. (C) 2000 MAIK "Nauka/ Interperiodica".