Development of arrival-time diagnostic tool for X-ray pump-probe experiments at Shanghai Soft X-ray Free Electron Laser.

X-ray free electron lasers (XFELs) serve as advanced light sources and have become essential for investigating ultrafast dynamic phenomena in physics and materials with extraordinary resolution. Owing to the XFEL's ultrafast characteristics and short wavelengths, an arrival-timing tool is crucial for pump-probe experiments. To address this, we have developed a timing diagnostic tool employing both spectral-encoding and spatial-imaging methods at the SBP beamline for the newly constructed Shanghai Soft X-ray Free Electron Laser Facility (SXFEL). This timing tool was experimentally validated, proving that the spectral-encoding technique could achieve single-pulse measurement with an accuracy of under 40 fs [root mean square (RMS)], and exhibited a timing-jitter measurement of 90.3 fs (RMS) at the CSI endstation of SXFEL. Furthermore, the spatial-imaging approach used both polished- and rough-surface GaAs crystals, which simplifies implementation in X-ray pump-probe experiments, and allows for the characterization of X-ray pulse arrival times at the endstation without rotating the sample stage. These findings confirm that the timing diagnostic tool provides dependable high-precision temporal characterization of X-ray pulses at SXFEL, facilitating high-accuracy X-ray pump-probe experiments.