The newly realized ability to generate multi-pulse multi-color X-ray experiments at Free Electron Lasers (FELs) provides an opportunity to design and establish new experiments in the femto- and attosecond regimes for solvated systems. Current limits on photon energy separation between pulses restrict the experimental design to focus on individual atoms. Fortunately, the near-future capacity to separately generate tender and hard X-rays will enable experiments that locally excite and then probe separate regions of a molecule with atomic specificity to better understand how charge moves throughout a system.

We report the early data from a recently developed two-color X-ray pump X-ray probe experimental technique that directly probes core-valence interactions, demonstrated in a model metal-ligand charge transfer system [Fe^II(CN₆)^4-]. The first pulse generates a 1s hole of the metal center, which is subsequently filled by the ensuing Auger-Meitner cascade, resulting in a distribution of holes in both the 3p and valence shells (T_2g molecular orbital). Informed by Monte-Carlo simulations of the cascade and TD-DFT calculations for the predicted ensemble of electronic states, the second pulse (delayed by experimental time T_delay) is tuned resonantly with the 1s - 3p (K-β) transition. The 3p energy shifts in response to the interactions between 3p and valence orbitals. Loss of electron density in the 3p and T2g orbitals increase the K-β energy. We directly measure those shifts by spectrally monitoring the drop in transmission of the probe as a function of pump fluence. This technique benefits from utilizing dipole allowed transitions to report on vacancies that would otherwise have weak transition intensities.

Calculations predict that each hole in the valence results in the K-β blue shifting by ~2.1 eV relative to the atomic K-β transition. A larger shift (~6 eV) results from additional holes in the 3p. These shifts report directly on the strength of interaction between the valence electrons and the high lying core electrons. We share data collected at T_delay = 0 fs showing multiple peaks to the blue of the K-β transition, in agreement with the predictions of additional hole formation in the T_2g orbitals. These early data indicate promising results for tracking the time evolution of core-valence interactions in similar systems.

Sebastien Boutet

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