Fig. 4

RIXS spectra showing distinct dd-excitations for the interfacial and bulk-like Cu ions. RIXS spectra at 18 K measured a in π-polarization and b σ-polarization of the incident light at Q_∥ = −0.33 r.l.u. for incident energies between 929.8 and 933.3 eV. c, d Corresponding RIXS spectra at 931.2 and 930.6 eV that are representative of the response of the bulk-like and the interfacial Cu ions, respectively. Note that the p1 peak at ΔE = −0.7 eV is only observed for incident x-ray energies close to 930.6 eV