Polarization dependence and symmetry analysis in indirect K -edge RIXS

We present a study of the charge-transfer excitations in undoped Nd ₂CuO₄ using resonant inelastic x-ray scattering (RIXS) at the CuK -edge. At the Brillouin zone center, azimuthal scans that rotate the incident-photon polarization within the CuO₂ planes reveal weak fourfold oscillations. A comparison of spectra taken in different Brillouin zones reveals a spectral weight decrease at high-energy loss from forward- to back-scattering. We show that these are scattered-photon polarization effects related to the properties of the observed electronic excitations. Each of the two effects constitutes about 10% of the inelastic signal while the " 4p -as-spectator" approximation describes the remaining 80%. Raman selection rules can accurately model our data, and we conclude that the observed polarization-dependent RIXS features correspond to E_g and B _1g charge-transfer excitations to non-bonding oxygen 2p bands, above 2.5 eV energy-loss, and to an E_g d→d excitation at 1.65 eV.