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 charge transfer from a unit cell to neighboring ones is suggested in a tetragonal two-dimensional model of a CuO₂ plane. The orientation (anisotropy) and polarization (pleochroism) dependences of the intensities of excitonic dipole-forbidden optical transitions are determined. The phenomenon of initiating dipole-forbidden transitions in an external electric field is theoretically considered. General dependences of the probabilities of such transitions on the value and direction of the electric field and on the light wave polarization are found. The dependences obtained can be used to identify weak absorption bands in dielectric cuprates containing CuO₂ planes.