Theoretical X-Ray Spectroscopy of Transition Metal Compounds

X-ray spectroscopy is one of the most powerful tools to access structure and properties of matter in different states of aggregation as it allows to trace atomic and molecular energy levels in course of various physical and chemical processes. X-ray spectroscopic techniques probe the local electronic structure of a particular atom in its environment, in contrast to UV/Vis spectroscopy, where transitions generally occur between delocalized molecular orbitals. Complementary information is provided by using a combination of different absorption, emission, scattering as well as photo- and autoionization X-ray methods. However, interpretation of the complex experimental spectra and verification of experimental hypotheses is a non-trivial task and powerful first principles theoretical approaches that allow for a systematic investigation of a broad class of systems are needed. Focussing on transition metal compounds, L-edge spectra are of particular relevance as they probe the frontier d-orbitals involved in metal-ligand bonding. Here, near-degeneracy effects in combination with spin-orbit coupling lead to a complicated multiplet energy level structure, which poses a serious challenge to quantum chemical methods. MCSCF theory has been shown to be capable of providing a rather detailed understanding of experimental X-ray spectroscopy. However, it cannot be considered as a 'blackbox' tool and its application requires not only a command of formal theoretical aspects, but also a broad knowledge of already existing applications. Both aspects are covered in this overview.