Probing ultrafast electron correlations in high harmonic generation

We show here that electron-electron (e-e) interactions that evolve dynamically on ultrafast timescales can be imprinted onto the high harmonic generation (HHG) spectrum driven by intense laser fields, even far from any resonant multielectronic behavior of the medium and over a wide energy range. Specifically, we find that in bi-elliptical HHG, the ellipticities of the bichromatic pumps that maximize/minimize the HHG yield are sensitive to the level of theory describing the process, and considerably shift upon inclusion of e-e interactions. We explore this phenomenon by performing ab initio calculations with varying levels of theory on noble gas (Ar, Kr, and Xe) and molecular (carbon monoxide) systems. Interestingly, the sensitivity to e-e interactions in atomic systems arises due to mean-field Coulomb repulsion, while in molecules the strong laser field also excites ultrafast dynamical correlations. Our approach can be used to benchmark theories for multielectron systems interacting with strong fields. For instance, it can be used to determine the validity of various approximations, such as, the adiabatic exchange-correlation approximation in time-dependent density functional theory.