Terahertz field induced time shifts in atomic photoemission

Extreme ultraviolet (XUV) pump – terahertz (THz) probe experiments were carried out at the free-electron laser in Hamburg. At a photon energy of 59.4 eV, neon valence 2p, inner-shell 2s and valence shake-up photoelectrons were emitted into a THz probe field of 152 \textmu m wavelength. Using a reaction microscope, the 3-dimensional photoelectron momentum vectors were measured as a function of the pump – probe delay. A relative time shift between the emission of low-energetic valence shake-up photoelectrons and direct 2p photoelectrons was observed. The measurement agrees with predictions for Coulomb-probe-field coupling induced time shifts in atomic photoemission.