Matter subjected to intense and/or high energy laser pulses can end up in complicated superposition states that result in coherent electronic dynamics that occur faster than nuclear motion. Drawing on the ability to create bright attosecond X-ray pulses at free electron lasers, attosecond X-ray transient absorption spectroscopy (AXTAS) has become a promising tool for measuring these processes. In AXTAS, the elemental specificity, and corresponding spatial localization, of X-ray absorption can be exploited to probe the time-evolving electronic density in a particular region of a molecule or material. Interpretation of these spectra can be complex, however, and requires a means of relating absorption modulations to the underlying dynamics. In this talk, I will demonstrate how real-time time-dependent density functional theory (RT-TDDFT) simulations can be used to "reverse" from time delay to real-time, allowing one to reconstruct electron dynamics from measured transient spectra. Following a brief overview of the technical challenges specific to simulating these processes, results will be presented for a range of molecular (CO, O<sub>2</sub>, aminophenol, a tetracyanoethyle/benzene dimer) and solid-state systems (SiO<sub>2</sub>, diamond) excited with infrared, ultraviolet, and X-ray pumps. We observe a universal trend for soft X-ray probes: the optical density decreases and the frequency blue-shifts at the times when there is an increased electron density around a particular absorbing atom. Additionally, X-ray pump/X-ray probe schemes are predicted to be most sensitive when using different heteroatoms (e.g., pump@N, probe@O).

First-principles simulations of X-ray transient absorption for probing attosecond electron dynamics

ACS Spring 2022

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