  A semi-quantitative cluster approach is developed to describe the charge
transfer (CT) electron-hole excitations in insulating cuprates in a rather wide
energy range up to 10- 15 eV. It generalizes the Zhang-Ng (ZN) model of CT
excitons by considering the complete set of Cu3d and O2p orbitals within the
CuO4 embedded molecular cluster method and by introducing one-center
(intra-center) Frenkel-like and two-center (inter-center) excitons. Special
attention is paid to the transition matrix element effects both in optical and
electron energy loss spectra (EELS). In the latter case we obtain the momentum
dependence of matrix elements both for intra-center and inter-center
transitions. We are able to give a semi-quantitative description of the optical
and EELS spectra for a large number of 0D (like CuB2O4), 1D (Sr2CuO3) and 2D
(like Sr2CuO2Cl2) insulating cuprates in a unifying manner. By comparing our
analysis with the experimental data we find that the CT gap in insulating
cuprates is determined by nearly degenerate intra-center localized excitations
and inter-center CT excitons. The corresponding EELS intensity is found to be
strongly k-dependent: even for isolated exciton it sharply decreases by
approaching the Brillouin zone (BZ) boundary. It is shown that the interaction
of two-center excitons can result in destructive interference effects with an
intensity compensation point.
