Prediction and Measurement of Resonant and Nonresonant Shake Effects in the Core-Level X-ray Emission Spectra of 3d⁰ Transition Metal Compounds

Shake effects, resulting from sudden core potential changes during photoexcitation, are well-known in X-ray photoelectron spectroscopy (XPS) and often produce satellite peaks due to many-body excitations. It has been thought, however, that they are negligible in core-to-core X-ray emission spectroscopy (CTC-XES), where the difference in core-hole potentials upon radiative decay is rather small. We demonstrate that shake effects are significant in Kα XES from 3d transition metal systems with nominally zero valence electrons. We show that valence level shake satellites are amplified via interference due to a resonance between the 2p_3/2-hole (Kα₁) plus valence level shake state and the 2p_1/2-hole (Kα₂) state. Additionally, while the Kα₂ shake satellite is indeed predicted to be weak, we observe it experimentally, providing further independent verification of our model. This prediction includes a detailed analysis of 2p to 1s Kα XES using density functional theory (DFT)-augmented multiplet ligand field theory (MLFT).