Revealing Coulomb time shifts in high-order harmonic generation by frequency-dependent streaking

When an atom is ionized by a strong laser pulse, the field can drive the released electron back to the parent ion and trigger the emission of a high-order harmonic photon with a frequency that is a function of the time of ionization. The attractive Coulomb potential causes a slowdown of the outgoing electron that must be compensated by an earlier release into the accelerating field to produce the same harmonic frequency as without the Coulomb force. By numerical solution of the time-dependent Schr??dinger equation for a helium model atom, we demonstrate that such a subtle time shift of about 35 attoseconds is measurable by streaking with a weak, orthogonally polarized field combined with a complex-time trajectory interpretation. A comparison of results for high and low streaking frequency shows that only the high-frequency method measures the Coulomb shift well. This is confirmed by a classical trajectory model and by the analytical R-matrix theory.