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

Min Chen,Kenneth Lopata

doi:10.1021/acs.jctc.0c00122

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 (O2) they occur at twice the frequency, due to the indistinguishability of the oxygen atoms. In 4-aminophenol (H2NC6H4OH), 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 (C6H6)–tetracyanoethylene (C6N4; 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.

Highlights

Measuring subfemtosecond electron dynamics in molecules and materials has been the focus of much recent interest
In this paper, we present a methodology for using RT-time-dependent density functional theory (TDDFT) to directly compute X-ray attosecond transient absorption (ATA) spectra in molecules, for the case of measuring coherent electron dynamics
For the O Kedge peak at 523 eV, the optical density (OD) decreases by around 33%, and the peak blue shifts by about 0.1 eV. These results demonstrate that the N and O K-edges can act as two separate but complementary probes of the electron density, with the OD modulations being related to the dynamics in a straightforward way

Summary

Introduction

Measuring subfemtosecond electron dynamics in molecules and materials has been the focus of much recent interest. These dynamics, which result from a coherent superposition of states, occur at time scales faster than nuclear motion.[1−8] These dynamics form the initial stages of photochemical and light-harvesting processes, which are still poorly understood. Molecular and solid-state attosecond experiments are poised to shed light on these processes, but progress hinges on developing methods capable of measuring electron density changes with attosecond time resolutions. The choice of the technique is system- and process-dependent

Methods

Results

Conclusion