Description
Ultrafast EUV and X-ray sources open a regime where electronic relaxation, charge migration, proton transfer, and nuclear fragmentation occur on comparable femtosecond or attosecond time scales. In this contribution, I will discuss how ab initio theory can help translate such experiments into molecular mechanisms, and how, conversely, EUV/X-ray measurements provide stringent benchmarks for computational photodynamics. Examples from our work will include ionized water and hydrated molecular systems, proton-transfer-mediated charge rearrangement, Auger and intermolecular Coulombic decay, disruptive probing of early ionization dynamics, and the birth of the hydrated electron observed through core-hole-clock concepts. A recurring theme is the coupling of electronic and nuclear motion across environments: from gas-phase molecules and clusters, where the dynamics can be resolved in detail, to liquid-phase systems, where solvation, friction, charge delocalization, and statistical averaging reshape the observable response. I will emphasize both the promise and the current limitations of ab initio approaches, including nonadiabatic dynamics, multireference electronic structure, constrained density functional theory, and time-dependent quantum chemistry. The goal is to outline a theoretical framework in which EUV and X-ray experiments are not only interpreted, but actively used to improve and benchmark the predictive power of molecular simulations.
