Understanding 2p core-level excitons of late transition metals by analysis of mixed-valence copper in a metal-organic...

The L_2,3-edge X-ray absorption spectra of late transition metals such as Cu, Ag, and Au exhibit absorption onsets lower in energy for higher oxidation states, which is at odds with the measured spectra of earlier transition metals. Time-dependent density functional theory calculations for Cu²⁺/Cu⁺ reveal a larger 2p core-exciton binding energy for Cu²⁺, overshadowing shifts in single-particle excitation energies with respect to Cu⁺. We explore this phenomenon in a Cu⁺ metal-organic framework with ∼12% Cu²⁺ defects and find that corrections with self-consistent excited-state total energy differences provide accurate XAS peak alignment.