The dynamics of image-potential states on Cu(119) have been studied with two-photon photoemission. Direction-dependent quasielastic scattering processes with large momentum transfer are attributed to the finite terrace-width distribution on the stepped surface. This effectively couples image-potential states via interband scattering and leads to an asymmetry of the decay rate. Electrons in the first image-potential state live apparently longer when running upstairs.

User operation at the European X-ray Free-Electron Laser Facility started at the SASE1 undulator beamline in fall 2017. The majority of the experiments utilize optical lasers (mostly ultrafast) for pump-probe-type measurements in combination with X-ray pulses. This manuscript describes the purposedeveloped pump-probe laser system as installed at SASE1, implemented features and plans for further upgrades. research papers J. Synchrotron Rad. (2019). 26, 328-332 Guido Palmer et al. Pump-probe laser system at FXE and SPB/SFX 329

Ultrafast X-ray spectroscopies require flexible X-ray properties together with high temporal and spectral resolution. Here, we demonstrate simultaneous sub-20 fs and sub-eV resolutions for pump/probe experiments, without the need for additional photon arrival-time monitors.

The Si͑100͒-͑2 ϫ 2͒-Ga surface was used to investigate time-dependent Ga͑3d͒ core-level shifts by pumping electrons from the valence to the conduction band. The pump-probe experiments were done by exciting carriers with 1.59 eV laser pulses and probing the Ga͑3d͒ core level with higher harmonics. The higher harmonics were generated by focusing laser pulses with 1.4 mJ energy, 30 fs pulse duration, and 779 nm wavelength from a multipass amplifier at a repetition rate of 1 kHz into argon. For the 23rd harmonic, the time resolution of the experiment was ϳ400 fs after a grating monochromator. The band bending of about 110 meV of the p-doped Si͑100͒-͑2 ϫ 2͒-Ga surface is completely lifted by illumination of the surface with the 1.59 eV laser pulses. The Ga͑3d͒ core level shows a slow time-dependent shift attributed to the rise ͑ϳ1 ns͒ and decay ͑ϳ100 ns͒ of the photovoltage. The Ga͑3d͒ core-level shift and broadening in the femtosecond range are determined to be Ͻ15 meV at the used pump-pulse intensity of 19 mJ/ cm 2 . Experiments with pump pulses of 3.18 eV photon energy showed similar results.

Pump–probe experiments at X-ray free-electron laser (XFEL) facilities are a powerful tool for studying dynamics at ultrafast and longer timescales. Observing the dynamics in diverse scientific cases requires optical laser systems with a wide range of wavelength, flexible pulse sequences and different pulse durations, especially in the pump source. Here, the pump–probe instrumentation available for measurements at the Single Particles, Clusters, and Biomolecules and Serial Femtosecond Crystallography (SPB/SFX) instrument of the European XFEL is reported. The temporal and spatial stability of this instrumentation is also presented.