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

Abstract

X-ray transient absorption spectroscopy (XTAS) is a promising technique for measuring electron dynamics in molecules and solids with attosecond time resolutions. 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 the 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 the interaction with a ultraviolet pump laser. In general, there is a decrease in the optical density and a blue shift in the 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 (O 2 ) they occur at twice the frequency, due to the indistinguishability of the oxygen atoms. In 4-aminophenol (H 2 NC 6 H 4 OH), likewise, there is a decrease in the optical density and a blue shift in the frequency for the oxygen and nitrogen K-edges with increasing charge density on the O and N, respectively. Similar effects are observed in the nitrogen K-edge for a long-range charge-transfer excitation in a benzene (C 6 H 6 )–tetracyanoethylene (C 6 N 4 ; TCNE) dimer but with weaker modulations due to the delocalization of the charge across the entire TCNE molecule. Additionally, in all cases, 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.

X-ray transient absorption reveals the 1Au (nπ*) state of pyrazine in electronic relaxation

“…X-rays deliver a unique element specificity because they involve the transitions from localized core orbitals, whose energies in different types of atoms are often separated by hundreds of electron volts. Moreover, core-level transitions are sensitive to the local chemical environment and can report on shifts in electron density in the proximities of the probed atoms 28 , 32 – 34 . With recent advances in high-order harmonic generation (HHG), water-window X-ray table-top setups for transient absorption are available for solving photochemistry problems 30 , 35 .…”

X-Ray Transient Absorption Reveals the 1Au (Nπ*) State of Pyrazine in Electronic Relaxation

“…Moreover, core-level transitions are X-ray energy Delay sensitive to the local chemical environment and can report on shifts in electron density in the proximities of the probed atoms. 11,[15][16][17] With recent advances in high-order harmonic generation (HHG), water-window X-ray table-top setups for transient absorption are available for solving photochemistry problems. 13,18 Excited states of pyrazine have been extensively investigated both experimentally [19][20][21][22][23] and theoretically.…”

X-Ray Transient Absorption Reveals the 1Au (Nπ*) State of Pyrazine in Electronic Relaxation

“…Moreover, core-level transitions are X-ray energy Delay sensitive to the local chemical environment and can report on shifts in electron density in the proximities of the probed atoms. 11,[15][16][17] With recent advances in high-order harmonic generation (HHG), water-window X-ray table-top setups for transient absorption are available for solving photochemistry problems. 13,18 Excited states of pyrazine have been extensively investigated both experimentally [19][20][21][22][23] and theoretically.…”