## 3 Results
### Photoexcited States of BiVO\({}_{4}\)  
Bismuth vanadate (BiVO\({}_{4}\)) is a potential candidate for the fabrication of useful \(Z\)-scheme photocatalysts and/or photoelectrode systems due to the band gap of 2.4-2.6 eV and the valence band position [30; 31; 55; 56; 57; 58; 59]. Apart from improving the performance of BiVO\({}_{4}\) by the development of novel synthetic methods [30; 31; 55; 56; 57; 58], the nature of its photocatalytic activity has been investigated [60; 61; 62; 63; 64; 65; 66]. For example, density functional theory (DFT) calculations have been performed to gain insight into the ground-state electronic structure of BiVO\({}_{4}\) and to elucidate the properties of excited-state photocarriers [60; 61] and the behavior of photocarriers in femtosecond-microsecond time scales was investigated by transient spectroscopic methods [62; 63; 64; 65; 66]. However, there is still an open question of the electronic structure of BiVO\({}_{4}\), in particular, the contribution of Bi atoms. It has been considered that Bi atoms contribute to the top of the valence band, which makes the bandgap narrower and results in higher photocatalytic efficiency. However, there has been no direct evidence that indicates the contribution of Bi atoms. XAFS can be used to elucidate the dynamics of Bi atoms in BiVO\({}_{4}\) after photoexcitation because it monitors the Bi atoms selectively. In this context, pump-probe XAFS experiments on the photoexcited state of BiVO\({}_{4}\) were performed with the PF-AR and SACLA. Figure 5 shows Bi L\({}_{3}\) edge XANES of the ground-state BiVO\({}_{4}\) (upper panel) and its difference spectra after photoexcitation. The main change at a delay of 2 ps appeared around 13,443 eV, denoted as B in Figure 5, which subsequently exhibited gradual growth to achieve a maximum at 60 ps (Figure 5b). The changes around 13,430 and 13,460 eV (denoted as peaks A and C, respectively) increased afterwards with some delay from the change of peak B and reached a maximum around a delay of 60 ps, i.e., a fast X-ray absorption change was observed with a delay time of 1 ps at peak B, while little change was evident in peaks A and C. The photoexcitation of BiVO\({}_{4}\) comprises at least two distinct processes, i.e., the fast depletion at peak B followed by slow magnitude growth of all peaks A, B, and C up to 60 ps. Figure 5b shows kinetic traces of the X-ray absorption coefficients for the A, B, and C peaks. The second slow processes were fitted with a single exponential function, \(y=\alpha\cdot\exp(-t/\tau)+\beta\). The time constants \(\tau\), for peaks A, B, and C were estimated to be \(13\pm 5\) ps, \(14\pm 2\) ps, and \(13\pm 7\) ps, respectively. The excited state was relaxed to the ground-state with a time constant of 40 ns, which was measured at the PF-AR. According to a previous report [60; 61], Bi 6s electrons are assumed to be excited to the conduction band by photoabsorption. If this were the case, then the Bi L\({}_{3}\) edge XANES should have been shifted to higher energy. However, the difference XAFS at 2 ps could not be reproduced using the difference spectrum (\(\delta\mu_{\rm{Eshift}}\)) between Bi L\({}_{3}\)-edge XAFS spectrum of the ground-state BiVO\({}_{4}\) shifted to higher energy by \(\Delta E\), \(\mu^{\rm{gs}}\)L\({}_{3}\)(\(E+\Delta E\)), and the original one, \(\mu^{\rm{gs}}\)L\({}_{3}\)(\(E\)), \(\delta\mu_{\rm{Eshift}}=\mu^{\rm{gs}}\)L\({}_{3}\)(\(E+\Delta E\)) \(-\mu^{\rm{gs}}\)L\({}_{3}\)(\(E\)) [22]. This indicates that the electron density on Bi atoms is not varied to a significant extent after the primary photoabsorption process. In previous papers, coherent oscillations were observed in the early stage (\(<\)1 ps) of the photoexcitation of BiVO\({}_{4}\) by optical spectroscopic measurements [62; 63], which induced a dynamic local structural change around Bi atoms [62; 63; 64; 59]. Therefore, it was inferred that the coherent oscillations could create peak B in the Bi L\({}_{3}\) edge difference XAFS spectrum.