## 2 Materials and Methods  
WO\({}_{3}\) (particle size \(<\)200 nm) and BiVO\({}_{4}\) (particle size ca. 200 nm) were prepared as described elsewhere [28; 30; 31]. The nanoparticles were dispersed in distilled water. The concentrations of the suspensions were adjusted to obtain an appropriate optical length for the pump laser to pass through the suspensions. A typical concentration was ca. 1 mmol L\({}^{-1}\). Figure 1 shows the pump-probe XAFS setup at NW14A in the Photon Factory Advanced Ring (PF-AR) [9; 20]. PF-AR is a 6.5 GeV electron storage ring and its ring current is typically 50 mA. PF-AR is always operated in a single bunch mode, i.e., only a single electron bunch circulates in the storage ring. This isolated electron bunch creates an X-ray pulse of which the pulse duration is ca. 100 ps with a 1.26 us interval. The NW14A beamline is dedicated to the time resolved X-ray sciences. Besides pump-probe XAFS measurements [19; 20; 21; 22; 32; 33], X-ray liquid scattering [34; 35], X-ray diffraction [36], and X-ray single crystallography [37] techniques have been developed and have contributed to significant scientific achievements. There are two femtosecond lasers available to excite samples for pump-probe XAFS measurements. One is a Ti-sapphire laser with a regenerative amplifier that provides laser pulses up to 800 uJ/pulse at 800 nm with a repetition rate of 946 Hz. The other is a high-repetition rate laser of which the fundamental wavelength is 1030 nm. The laser can be synchronized with each X-ray pulse in the PF-AR. This laser enables high repetition data acquisition experiments to be conducted, which significantly improves the S/N ratio of transient XAFS signals [38]. This high repetition scheme is effective for the measurement of the pump-probe extended X-ray absorption fine structure (EXAFS) and transient XAFS of extremely dilute samples or samples with a small X-ray absorption cross section, such as L\({}_{1}\) edge XAFS. Frequency-doubled laser pulses of the Ti-sapphire laser (400 nm) were used to excite the sample in the W L\({}_{3}\)-edge XAFS experiments. The focal size of the laser on the sample was ca. 0.6 mm and its pulse duration was approximately 1 ps, and the laser fluence was estimated to be 280 mJ\(\cdot\)cm\({}^{-2}\). A photomultiplier tube with a plastic scintillator was employed as an X-ray detector and two boxcar integrators were used to separately accumulate pumped and unpumped XAFS signals. The WO\({}_{3}\) suspension was circulated continuously using a magnet-gear pump and produced a 500 um liquid  
Figure 1: Schematic illustration of the pump-probe X-ray absorption fine structure (XAFS) setups at **(A)** NW14, Photon Factory Advanced Ring (PF-AR), and **(B)** EH2, BL3 at SPring-8 Angstrom Compact free electron LASer (SALCA). jet on the sample position where the pump laser and the probe X-ray source were overlapped with each other. A high repetition rate measurement scheme was employed for the W L\({}_{1}\)-edge and Bi L\({}_{3}\)-edge XAFS experiments. The third harmonics (ca. 343 nm) of the high repetition laser was used to excite the sample with a fluence of ca. 35 mJ-cm\({}^{-2}\). The X-ray was focused using an X-ray capillary lens to 0100 \(\upmu\)m and the sample was a 300 \(\upmu\)m liquid jet. The pump-probe experiments in the femtosecond and picosecond regions were conducted in the EH2 unit of BL3, Spring-8 Angstrom Compact free electron LASer (SALCA) [39; 40]. SACLA generates intense X-ray pulses with a pulse duration less than 10 fs at a repetition rate of 60 Hz [41]. These X-ray pulses are delivered evenly to BL2 and BL3, and consequently, the repetition rate of XFEL pulses in the pump-probe XAFS experiments is 30 Hz. The low repetition rate of the SACLA has an advantage in the use of a commercial optical parametric amplifier for the excitation light source so that the excited wavelength can be easily selected.