Probing core level ionization in solvated transition metal complexes using X-ray pump, X-ray probe absorption spectroscopy: Computations and proposed XFEL experiment

Femtosecond X-ray pump X-ray probe experiments are currently possible at free electron lasers such as the Linac Coherent Light Source (LCLS), which opens new opportunities for studying systems in condensed and solvated phases. In order to make the most effective use of these experiments, it is necessary to determine which chemical properties an X-ray pump pulse will measure. We combine Monte-Carlo electron cascade calculations and excited-state time-dependent density functional theory calculations to first predict the initial state prepared by an X-ray pump and then determine the subsequent X-ray absorption spectra at the Fe K-edge in the solvated model transition metal complexes, K₄Fe^II(CN)₆ and K₃Fe^III(CN)₆. Final states with 3p holes produce unique shifts from the Fe K-β energy as a function of 3p and 3d electron interactions. The initial hole in a t_2g orbital of the Fe^III complex leads to more highly ionized probed states, and alters the distribution of 3p transitions relative to the Fe^II complex. We find several key spectral features that report on the ligand-field splitting, solvent-solute interactions, and 3p -3d electron interactions. Finally, we show how these features could be measured in an experiment.