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Abstract:
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 2p valence shake-up photoelectrons were emitted into a THz-probe field of 152 µm wavelength. Using a reaction microscope,
the 3-dimensional photoelectron momentum vectors were measured as a function of
the pump-probe delay. An observation of a delay-dependent oscillation of the complete photoelectron momentum distribution along the THz polarization axis was
possible and the shape of the entire THz probe field was mapped. Furthermore,
a relative time shift of (-8.7 +/- 4.9) fs between the emission of low-energetic Neon
valence shake-up photoelectrons and direct Neon 2p photoelectrons was observed.
The measurements agree within the error bars with predictions of classical formulas
which account for Coulomb-probe-field coupling induced time shifts in atomic
photoemission.
