Photoelectron Spectra of Gd2O2− and Non-Monotonic Photon-Energy Dependent Variations in Populations of Close-Lying Neutral States

Photoelectron spectra of Gd₂O₂⁻ obtained with photon energies from 2.033 eV to 3.495 eV exhibit numerous close-lying neutral states with photon-energy-dependent relative intensities. Transitions to states falling within the electron binding energy window of 0.9 and 1.6 eV are attributed to one- or two-electron transitions to the ground and low-lying excited neutral states. An additional, manifold of electronic states observed in the 2.1 to 2.8 eV window cannot be assigned to any simple one-electron transitions. Because of the relatively simple electronic structure from the half-filled 4f⁷ subshell occupancy in Gd₂O₂^–, the numerous transitions observed in the spectra are fairly well-resolved, allowing a detailed view of the changes in relative intensities of individual transitions with photon energy. With supporting calculations on the numerous close-lying electronic states, we suggest a description of strong photoelectron-valence electron interactions that result in the photon-energy dependent shake-up transitions and switching between ferro- and antiferromagnetic coupling.