Resonant Inelastic X-Ray Scattering from Valence Excitations in Insulating Copper-Oxides.

We report resonant inelastic x-ray measurements of insulating La sub 2 CuO sub 4 and Sr sub 2 Cu O sub 2 Cl sub 2 taken with the incident energy tuned near the Cu K sub (alpha) absorption edge. We show that the spectra are best described by a shakeup process in 3rd order perturbation theory which exhibits both incoming and outgoing resonances, and demonstrate how to extract a spectral function from the raw data. We show q dependent measurements of the charge transfer gap which suggest that momentum information can be probed with resonant scattering.