First-principles nonequilibrium Green's function approach to transient photoabsorption: Application to atoms

TL;DR

This paper introduces a first-principles NEGF method for calculating transient photoabsorption spectra in atoms, effectively addressing electron correlation effects and explaining experimental observations in helium and krypton.

Contribution

The paper develops a novel NEGF-based approach to simulate transient photoabsorption, capturing dynamical correlation effects and handling arbitrary pump fields.

Findings

Explains bending of Autler-Townes peaks with pump-probe delay.

Identifies spectral features as signatures of dynamical correlation.

Captures retardation effects in krypton ion absorption onset.

Abstract

We put forward a first-principle NonEquilibrium Green's Function (NEGF) approach to calculate the transient photoabsorption spectrum of optically thin samples. The method can deal with pump fields of arbitrary strength, frequency and duration as well as for overlapping and nonoverlapping pump and probe pulses. The electron-electron repulsion is accounted for by the correlation self-energy, and the resulting numerical scheme deals with matrices that scale quadratically with the system size. Two recent experiments, the first on helium and the second on krypton, are addressed. For the first experiment we explain the bending of the Autler-Townes absorption peaks with increasing the pump-probe delay $\t$, and relate the bending to the thickness and density of the gas. For the second experiment we find that sizable spectral structures of the pump-generated admixture of Kr ions are fingerprints of {\em dynamical correlation} effects, and hence they cannot be reproduced by time-local self-energy approximations. Remarkably, the NEGF approach also captures the retardation of the absorption onset of Kr$^{2+}$ with respect to Kr$^{1+}$ as a function of $\t$.