Ultrafast Charge Migration in XUV Photoexcited Phenylalanine: A First-Principles Study Based on Real-Time Nonequilibrium Green's Functions

The early-stage density oscillations of the electronic charge in molecules irradiated by an attosecond XUV pulse takes place on femto- or subfemtosecond time scales. This ultrafast charge migration process is a central topic in attoscience because it dictates the relaxation pathways of the molecular structure. A predictive quantum theory of ultrafast charge migration should incorporate the atomistic details of the molecule, electronic correlations, and the multitude of ionization channels activated by the broad-bandwidth XUV pulse. We propose a first-principles nonequilibrium Green's function method fulfilling all three requirements and apply it to a recent experiment on the photoexcited phenylalanine amino acid. Our results show that dynamical correlations are necessary for a quantitative overall agreement with the experimental data. In particular, we are able to capture the transient oscillations at frequencies 0.15 and 0.30 PHz in the hole density of the amine group as well as their suppression and the concomitant development of a new oscillation at frequency 0.25 PHz after ~14 fs.