## 3 Results  
### Photoexcited States of WO\({}_{3}\) in Femtosecond and Nanosecond Regions  
Figure 2 shows the W L\({}_{3}\)-edge normalized XAFS spectrum of ground-state WO\({}_{3}\)[20]. WO\({}_{3}\) is known as a wide-bandgap semiconductor, of which the bandgap is ca. 2.4 eV. WO\({}_{3}\) can promote the oxygen evolution reaction because of its valence band position [51], and this catalytic property is applied to a visible-light sensitive photocatalyst in the Z-scheme mechanism [7; 8]. The peak at the X-ray absorption edge (called the white line) could be assigned to the transition from the W  
2p\({}_{3/2}\) to 5d orbitals. A strong white line appeared because W is in the 6-valent state, i.e., the d\({}^{0}\) state. After excitation, the white line intensity decreased, as shown by the red curve in Figure 2 (XAFS at a delay of 150 ps). The change in the X-ray absorption intensity was recovered at around 10 ns. This change could be simply explained by photoexcitation of the electron of O 2p at the valence band to the conduction band mainly composed of W 5d; however, this simple interpretation was rejected due to the following reason. The second derivative of W L\({}_{3}\)-edge XAFS spectrum gave two peaks that correspond to the crystal field d-orbital splitting. If the local structure of W in WO\({}_{3}\) was assumed to have an octahedral structure, then the 5d orbitals were split into t\({}_{\text{2g}}\) and e\({}_{\text{g}}\) orbitals, which appeared as two peaks in the second derivative [52]. The bottom of the conduction band was mainly constructed from the t\({}_{\text{2g}}\) states. The spectral change seemed to be the occupation of the higher e\({}_{\text{g}}\) orbitals with the photoexcited electrons because the e\({}_{\text{g}}\) orbitals in the second derivative and the photoinduced change peak top coincided well, as shown in Figure 2. However, it was not possible to excite the electron at the valence band to the higher e\({}_{\text{g}}\) orbital, because the energy difference between the two was much greater than the photon energy of the pump laser. We could not explain why the photoinduced electron was not located at the bottom of the conduction band but at the higher level. The transient XAFS at a delay of 150 ps should be a secondary state after the structural change and the rearrangement of the density of states. The primary process could not be observed with the time resolution of the PF-AR. Picosecond-order pump-probe XAFS experiments were conducted with the SACLA to observe the hidden primary process. Difference spectra at several delays are shown in Figure 3. The time resolution was ca. 500 fs due to the presence of time jitter [42; 50]. Three distinctive peaks were observed in the difference spectra, as shown in Figure 3a. A rise peak A at 11,206 eV and a bleach peak C at 11,211 eV appeared at a delay of 500 fs together with a smaller peak B in-between. The rise peak at A was not observed in the previous experiments conducted at the PF-AR. The rise and bleach features arose from the spectral shift to lower energy. When the pump laser excited an O 2p valence electron to the conduction band consisting of W orbitals, the W valence state should be reduced to 5+, which induces the W L\({}_{3}\) edge shift. The first derivative multiplied by the energy shift ( \(\Delta E\) = 0.4 eV), \(\Delta\mu=\frac{\text{d}\mu}{\text{d}E}\times\Delta E\), agreed well with the difference spectrum at the peak positions at A and C [21]. However, peak A was smaller than the peak height expected from the first derivative. The smaller intensity than that expected could be explained by filling of the t\({}_{\text{2g}}\) state with the photoexcited electron which was located at the bottom of the conduction band. In the following process, the magnitude of peak A decreased  
Figure 2: W L\({}_{3}\) edge XAFS spectra for WO\({}_{3}\) before (black) and after (red) photoexcitation (top panel), and difference XAFS spectrum of photoexcited WO\({}_{3}\) and a 2 nd derivative of the XAFS of WO\({}_{3}\) (blue in bottom panel).