## 4 Time-Resolved X-ray Diffraction Measurement  
As a demonstration, we performed time-resolved X-ray diffraction (tr-XRD) measurement of photo-excited bismuth (Bi), which has been used as a standard sample for evaluation of pump-probe measurements using XFEL and femtosecond optical laser [6], at EH2. The schematic of the experimental setup is illustrated in Figure 2. A Bi thin film with a 53-nm thickness on Si (111) substrate was mounted on a diffractometer (5042, HUBER Diffraktionstechnik GmbH & Co. KG, Rimsting, Germany) with a Bragg angle of Bi (111) (\(\sim\)9 degrees). The monochromatized XFEL beam at 10 keV was focused on the Bi sample by compound refractive lenses to the spot size of \(\sim\)100 \(\upmu\)m in diameter (FWHM) [37]. The diffracted signal of Bi (111) was captured by a multi-port CCD (MPCCD) detector [38]. The optical laser beam was propagated to the Bi sample with a crossing angle of 3 degrees toward the XFEL beam through a prism shape mirror. We focused the optical laser beam to a spot size of about 1 mm in diameter on the sample with a 1-m focal length lens and adjusted the fluence at 1.0 mJ/cm\({}^{2}\) with a variable neutral density filter. The beam spot of the optical laser was matched to that of the XFEL by tuning the motorized mirror and lens holders. The sample condition irradiated by the optical laser and the XFEL beams was observed by long working distance microscopes. The temporal overlap between the XFEL and the optical laser pulses at the sample position was roughly evaluated by monitoring both signals with a fast photodiode (G4176-03, Hamamatsu Photonics K.K., Hamamatsu, Japan) and a high precision oscilloscope (MSOV164A, Keysight Technologies, Santa Rosa, CA, USA) with several tens-of-picosecond accuracies. Finally, we optimized spatiotemporal overlap between the XFEL and the optical laser beams with the diffracted signal from the Bi sample. Figure 16a shows the total  
Figure 15: (**a**) Camera image of the timing monitor and vertical projection trace in the irradiated area by the XFEL (indicated by pink lines); (**b,c**) statistical data of the relative arrival timing between the XFEL and the optical laser pulses; (**b**) histogram of 300 shots; (**c**) trends of averages (red) and standard deviations (blue) in 24 h.  
intensity of the diffracted X-rays as a function of delay, which was normalized by the incident XFEL intensity. The red trace indicates raw data of the delay scan with a 33-fs step, while the blue trace shows post-processed data for the jitter error correction with the timing monitor. We observed the coherent phonon excited by the optical laser in Bi even without the post-processing because the optical laser was precisely synchronized using the BOMPD with a jitter of 50 fs. As a comparison, Figure 16b shows the results of the previous experiment with the same setup, which was conducted when the optical laser was synchronized by a commercial locking system using a fast photo-detector with about 0.3-ps jitter (Synchrolock AP, Coherent Inc., Santa Clara, CA, USA). The diffraction signal changes gently as scanning the delay, as shown in the blue trace, while the oscillation appears after the jitter error correction, as shown in the red trace. Note that the fluence of the optical laser was increased up to 2.0 mJ/cm\({}^{2}\) to make a contrast of the oscillation in the previous experiment.