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 timescales. This ultrafast charge migration process is a central topic in attoscience as 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. In this work we propose a first-principles Non Equilibrium Green's Function method fulfilling all three requirements, and apply it to a recent experiment on the photoexcited phenylalanine aminoacid. 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 0.15PHz and 0.30PHz in the hole density of the amine group, as well as their suppression and the concomitant development of a new oscillation at frequency 0.25PHz after about 14 femtoseconds.