Theoretical Explorations in X-ray Spectroscopies and Ultrafast Dynamics*

Probing and controlling the flow of charge and dynamics within a molecule following photoexcitation is a fundamental requirement for the targeted synthesis of the next generation of molecules and materials designed for artificial light-harvesting, photochemical energy conversion, and photocatalysis. X-rays, by virtue of their atomic specificity, are ideal probes to study ultrafast processes including electron and proton transfers and couplings in molecules in non-equilibrium conditions and emerging X-ray free electron laser (XFEL) sources offer novel probes of chemical systems, in the gas and condensed phases, with unprecedented spatial and temporal resolutions. Extracting microscopic details from these state-of-the-art X-ray experiments hinges on our ability to simulate the underlying electronic and structural dynamics and our ability to interpret and predict core-level spectroscopic observables. This is essential for the design of sophisticated multi-pulse experiments and for their interpretation. Over the last two decades, both real-time and linear-response time-dependent density functional theory, despite various theoretical challenges, has become a computationally attractive and versatile framework to study excited-state spectra including X-ray spectroscopies. In this talk, our theoretical efforts based on linear-response time-dependent density functional theory based calculations, covering static and transient X-ray absorption and emission, resonant inelastic X-ray scattering, and X-ray circular dichroism signals, will be presented. These studies will be illustrated with applications to solvated transition metal complexes, mixed-valence transition metal complexes, intramolecular proton transfer in organic hydrogen bonded complexes, and chiral molecular systems. Comparisons to experiments will be made where applicable and theoretical predictions will be covered.