The diagnostics and laser infrastructure at the Soft X-ray Port of the European XFEL

The diagnostics and laser infrastructure of the Soft X-ray Port (SXP) at the European X-ray Free Electron Laser are presented. Designed as a supplementary open port, SXP aims to enhance the research capabilities alongside the existing baseline instruments at the SASE3 soft X-ray undulator exploiting the high repetition rate operation enabled by superconducting acceleration cavities. These instruments already support studies in atomic, molecular and non-linear optics (SQS), as well as condensed matter physics (SCS). The inception of SXP is largely motivated by the community involved in time-resolved X-ray photoelectron spectroscopy (tr-XPES), focusing on material science dynamics at surfaces and interfaces. The dedicated endstation equipped with a time-of-flight momentum microscope allows for simultaneous recording of energy- and angle-resolved photoelectron distributions in a pump-probe setting, employing two synchronized femtosecond laser systems meant to cover a broad spectrum from the infrared to the extreme ultraviolet region. The alignment laser system, pump-probe laser transport, including the in-coupling optics, and the installed photon arrival-time monitor are described. Under the present operating conditions, however, the photon arrival-time monitor does not yet provide routine single-shot timing corrections, so the currently demonstrated temporal benchmark is obtained in situ at the tr-XPES endstation. Measurements on a patterned Au reference sample show that the laser spot sizes can be characterized directly and that sidebands yield an X-ray/optical cross-correlation of about 58 fs (FWHM). These measurements define the spatial and temporal instrument response currently achieved for pump-probe experiments at SXP.