Time delays for atomic photoemission obtained in streaking or reconstruction of attosecond bursts by interference of two-photon transitions experiments originate from a combination of the quantum mechanical Wigner time and the Coulomb-laser coupling. While the former was investigated intensively theoretically as well as experimentally, the latter attracted less interest in experiments and has mostly been subject to calculations. Here, we present a measurement of the Coulomb-laser coupling-induced time shifts in photoionization of neon at 59.4 eV using a terahertz (THz) streaking field ($\lambda =152\mu \mathrm{m}$). Employing a reaction microscope at the THz beamline of the free-electron laser in Hamburg (FLASH), we have measured relative time shifts of up to 70 fs between the emission of $2p$ photoelectrons ($\sim 38\mathrm{eV}$) and low-energetic ($<1\mathrm{eV}$) photoelectrons. A comparison with theoretical predictions on Coulomb-laser coupling reveals reasonably good agreement.

• Received 9 September 2018

DOI:https://doi.org/10.1103/PhysRevLett.122.073001