Character of charge transfer excitons in Sr 2 CuO 2 Cl 2

High resolution electron energy loss spectroscopy (EELS) measurements of the insulating, layered cuprate compound ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}$ have been performed with a transferred momentum near $\mathbf{k}=0$ and in a wide energy range up to 20 eV. To interpret the data a semiquantitative embedded cluster approach is applied to describe the electron-hole excitations in insulating cuprates. The theory includes the complete set of Cu $3d$ and O $2p$ orbitals and predicts a rather wide $(2\textdiv{}13 \mathrm{eV})$ spectral range of the main dipole-allowed one- and two-center charge transfer (CT) transitions whose positions are in good quantitative agreement with the experimental spectra. Both, theory and the experimental data show that the CT gap is determined by a superposition of nearly degenerate one-center, localized CT excitations and two-center CT excitons with sizable dispersion.