## 3 Results
### Photoexcited States of WO\({}_{3}\) in Femtosecond and Nanosecond Regions
(The figure is reproduced from [20] with permission of Chemical Society of Japan). with the increase in the magnitude of peak C by 200 ps, which suggests that the structural change formed a metastable WO\({}_{3}\) state. Peak C gradually recovered to its initial value with a similar decay rate of nanosecond order as observed with the PF-AR. The W L\({}_{1}\) edge is sensitive to the structural change; therefore, an attempt was made to measure the W L\({}_{1}\) pump-probe XAFS, even though its X-ray cross section was low. Figure 3b shows the W L\({}_{1}\)-edge spectrum of the ground-state WO\({}_{3}\) (upper panel) and its difference spectra (lower panel) at several delays. Two peaks, denoted as (A) and (B), appeared in the difference spectra. The bottom of the difference spectra labeled as the energy shift corresponded to the calculated difference spectra between that with and without the negative 1 eV energy shift where the two peaks appeared at the corresponding positions of (A) and (B). Consequently, peaks (A) and (B) were mainly caused by the edge shift due to the photoexcited electron transfer from O\({}^{2-}\) to W\({}^{6+}\). However, the intensity ratio between peak (A) and peak (B) changed significantly with time after the photoexcitation. The intensity of peak (A) became greater than that of peak (B) at 150 ps. The origin for the intensity ratio changes in peaks (A) and (B) could be attributed to the formation of the metastable state at 150 ps. Peak X in the upper panel of Figure 3b was identified as the electron transition from the 2s to the 5d orbital. Although the 2s\(\rightarrow\)5d transition was dipole-forbidden, the distortion from the O\({}_{\text{h}}\) symmetry allowed the hybridization of 5d with 6p orbitals and a strong dipole transition occurred at peak X. Peak X is a good indicator for the structural distortion around W [53]. Additionally, peak X was shifted to lower energy just after the photoabsorption and increased by the subsequent distortion; therefore, the lower energy peak (A) in the difference spectrum should increase at 150 ps. Although the difference spectra in Figure 3b could be interpreted based on the knowledge of XAFS, it was difficult to elucidate how the local structure of WO\({}_{3}\) changes after photoexcitation. The difference in the W L\({}_{1}\)-edge XAFS at 150 ps was analyzed by conducting full-potential multiple scattering calculations using an FPMS code to understand more details of the local structural change of photoexcited WO\({}_{3}\)[54]. The position of the central W atom was moved by 0.1 A from its original position along the three axes as shown in Figure 3c. The difference of the spectra was calculated based on the structures between distorted and original. The short bonds W-O(1) (1.756 A) and W-O(2) (1.767 A) were aligned along the c and b axes together with long W-O(5) (2.107 A) and W-O(6) (2.173 A), respectively. The +c or +b movement corresponded to the W-O(1) or W-O(2) contraction and indicated the formation of a more distorted structure, while -c or -b corresponded to a less distorted structure. Peak (A) became larger only when the W moved in the +c or +b directions, which indicates enhancement of the distortion of WO\({}_{3}\) as expected. Figure 3: (**a**) W L\({}_{3}\) edge and (**b**) W L\({}_{1}\) edge difference spectra between the photoexcited (pumped) and the ground-state (unpumped) WO\({}_{3}\) measured at SACLA. The upper panels of (**a,b**) show the raw spectra of ground-state WO\({}_{3}\). (**c**) Theoretically calculated L\({}_{1}\) edge difference spectra using FPMS code with different model structures. (The figure was reproduced from [21] with permission of John Wiley and Sons and from [22] with permission of the Royal Chemical Society). Combining the results of L\({}_{3}\) and L\({}_{1}\) edge XAFS, the bottom of the conduction band mainly composed of W d states was partially filled with photoexcited electrons at less than 1 ps.