Iron Kβ X-ray Emission Spectroscopy: The Origin of Spectral Features from Atomic to Molecular Systems Using Multi-configurational Calculations

Abstract

Kβ X-ray emission spectroscopy (XES) is widely used to fingerprint the local spin of transition-metal ions, including in pump−probe experiments, to identify excited states or in chemical and biological reactions to characterize short-lived intermediates. In this study, the spectra of ferrous and ferric complexes for various spin states were measured experimentally and described theoretically through restricted active space (RAS) calculations including dynamic correlations. Through the RAS calculations from simple atomic models to complex molecular systems, spectral effects such as the exchange interactions, crystalfield strength, and covalent orbital mixing were evaluated and discussed. The calculations find that only the spectral features of low-spin cases show a dependence on the crystal-field strength, particularly for ferrous low spin. The effect of the covalent orbital mixing strength on the first moment of the Kβ 1,3 main line and the Kβ 1,3 −Kβ′ energy splitting is quantitatively described. Clear relationships are found within a given nominal spin but less between different spin states, which calls for careful selection of reference spectra in future experiments. This study further advances our understanding of the correlation between changes in experimental spectral features and their corresponding electronic structure information.

Time-Resolved X-ray Emission Spectroscopy and Synthetic High-Spin Model Complexes Resolve Ambiguities in Excited-State Assignments of Transition-Metal Chromophores: A Case Study of Fe-Amido Complexes

“…Thus, the logical simplicity of this approach can belie the complexity of excited-state properties and prevent a true (or reliable) delineation of the dynamics involved. Time-resolved iron Kβ X-ray emission spectroscopy (trXES), on the other hand, allows for the ultrafast dynamics of a coordination complex’s excited states to be probed directly from the perspective of the metal. − As the Kβ main line fluorescence signal is predominantly sensitive to the 3d spin moment, the technique can enable a more robust differentiation of the character of excited states, which may be involved in a decay cascade (Figure b) . Here, we demonstrate how trXES in conjunction with high-spin synthetic models can overcome ambiguities in excited-state assignments via the optical TA/spectroelectrochemistry approach and conclusively reassign the optically active long-lived excited state in a series of highly covalent Fe-amido complexes, ( R L 1 ) 2 Fe (Figure c). , …”