## 4 Summary and Perspective  
### Photoexcited States of Photocatalysts  
The photoexcited states of WO\({}_{3}\) and BiVO\({}_{4}\) photocatalysts were observed using the pump-probe XAFS method. The stage of the photoexcitation process earlier than 500 fs was successfully observed using ATM. The photoexcited electrons in a higher energy level than the bottom of the conduction level induced a shift of the W L\({}_{3}\) edge to the negative side. The photoexcited electrons then reached the bottom of the conduction band around 700 fs where the lowest-lying W d orbitals were present. The electrons at the bottom of conduction band induced an anisotropic structural distortion to produce the metastable state up to 150 ps, as demonstrated by theoretical calculations of the W L\({}_{1}\) edge XANES. On the other hand, in the photoexcited state of BiVO\({}_{4}\), there was no edge shift observed in the Bi L\({}_{3}\)-edge XAFS, which indicates that not the Bi 6s, but rather, the O 2p orbitals dominantly contributed to the valence band top and the photoabsorption. However, the Bi L\({}_{3}\) edge could be used to monitor the structural changes. At several tens of picoseconds, the BiVO\({}_{4}\) reached the metastable state to stabilize the photoexcited electrons at V 3d. The time constants were dependent on the excitation wavelength, which reflected the different rate of release of the extra energy. In both systems, electron transfer from O to the cation and dynamic phonon processes (coherent vibration) occurred in less than 1 ps, followed by the structural changes in the 10-100 ps region to produce a metastable state and to stabilize the photoexcited electrons. These electrons may still have mobility and could contribute to photocatalytic reactions [68].  
Structural changes around the photocarriers have previously been reported from the pump-probe XAFS methods [15; 26; 27; 69; 70]. [PERSON], [PERSON], and colleagues reported structural changes around carrier trap sites in TiO\({}_{2}\)[26] and [PERSON] et al. reported those in ZnO [15] by pump-probe XAFS using SRs. [PERSON] et al. studied TiO\({}_{2}\) by the ultrafast pump-probe XAFS and observed the absorption edge shift at 170 fs by electron trapping in TiO\({}_{2}\) followed by structural distortion near the surface at ca. 400 fs after the photoexcitation in the SACLA [27]. It has been assumed that the photocarriers are localized at the trap sites (defects and/or surface). We consider that photocarriers can be located at the crystal lattice sites accompanied by local structure distortion such as a polaron. A structural distortion  
Figure 6: Bi L\({}_{3}\) XANES spectrum of the ground-state and difference XANES spectra (\(\Delta\)XAS) of the excited (XAS\({}_{\text{ge}}\)) and ground-states (XAS\({}_{\text{gs}}\)), with the corresponding delay time displayed beside each spectrum. (**b**) X-ray absorption intensities at points A, B, and C displayed in (**a**) as a function of time (excited at 480 nm).  
in \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) was observed by [PERSON] et al., which was related to polaron formation [69], and pump-probe Fe 3p absorption spectroscopy revealed that the polaron formation in \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) occurred within 1 ps after the photoexcitation [70].