RESONANT X-RAY RAMAN-SPECTRA OF CU DD EXCITATIONS IN SR2CUO2CL2

We present resonant x-ray Raman scattering results on Sr2CuO2Cl2, a mo del compound for high-T-c superconductors. We demonstrate that the dd excitations can be observed and show that the polarization dependence can be used to identify the dd excitations. We find the transition fro m the d(x2-y2) ground state to the d(xy) excited state at 1.35 eV and to the degenerate d(xz) and d(yz) excited states at 1.7 eV. From analy sis of the polarization dependence we conclude that the d(3z2-r2) orbi tal energy is at 1.5 eV and not in the midinfrared (0.5 eV) as recentl y suggested. We use recent theoretical arguments to show that the d(3z 2-r2) excitation is accompanied by a local spin flip resulting in a sh ift upwards of 0.2 eV due to the exchange interaction with the neighbo ring spins.