First-Principles Simulations of X-ray Transient Absorption for Probing Attosecond Electron Dynamics.

X-ray transient absorption spectroscopy (XTAS) is a promising technique for measuring electron dynamics in molecules and solids with attosecond time resolution. In XTAS, the elemental specificity and spatial locality of core-to-valence X-ray absorption is exploited to relate modulations in the time-resolved absorption spectra to local electron density variations around particular atoms. However, interpreting these absorption modulations and frequency shifts as a function of time delay in terms of dynamics can be challenging. In this paper, we present a first principles study of attosecond XTAS in a selection of simple molecules based on real-time time-dependent density functional theory (RT-TDDFT) with constrained DFT to emulate the state of the system following interaction with a ultraviolet pump laser. In general, there is a decrease in optical density and a blue-shift in frequency with increasing electron density around the absorbing atom. In carbon monoxide (CO), modulations in the O K-edge occur at the frequency of the valence electron dynamics, while for dioxygen (O2) they occur at twice the frequency, due to the indistinguishably of the oxygen atoms. In 4-aminophenol (H2NC6H4OH) likewise there is a decrease in OD and blue shift in frequency for the oxygen and nitrogen K-edges with increasing charge on the O and N, respectively. Additionally, there are pre-edge features corresponding to core transitions to depopulated orbitals. These potentially offer a background-free signal that only appears in pumped molecules.