## 4 Summary and Perspective
### Advantage of Pump-Probe XAFS Techniques and Perspectives  
The pump-probe XAFS technique is unique compared to the other pump-probe methods in that XAFS can give a local picture of a specific element. Optical pump-probe methods typically provide information such as the life-time of photocarriers in the conduction and valence bands in k-space based on their absorption kinetics [71; 72; 73]. It is also possible to obtain transient lattice behavior from vibrational information [62]. On the other hand, XAFS can reveal the behavior of photoexcited materials in r-space. This can give local information around an X-ray absorbing atom with regard to the photoinduced changes of the structure and the electronic state. The absorbing atom can be regarded as an atomic-scale detector of photocarriers embedded in the material. It is thus possible that multi-edge measurements could be used to identify the roles and functions of each component. Although we measured only XAFS of heavy elements in this paper, it is feasible to observe transient XAS of light elements such as oxygen with a time resolution of 100-200 fs using soft XFELs [74; 17; 18]. Oxygen atoms mainly contribute to the valence band of photocatalysts, and it is possible to capture the transient hole behavior by O K-edge XAS. Although it was difficult to measure XAFS under the ambient conditions because of the large absorption of the soft X-rays by gas or liquid, recent instrumental developments have enabled even the in situ/operando measurement of O K-edge XAS [75]. In addition to soft XFELs, X-ray Raman is another candidate to observe the light element XAFS using hard X-rays [76; 77]. Multi atom resonance X-ray Raman (MARX-Raman) is a new and potential candidate to selectively observe the light elements directly bound to a photoactive metal center, even in the presence of the same elements [78]. Developments of pump-probe XAFS techniques, not only in terms of simple absorption spectra for multi-elements, but also in RIXS and Raman, will uncover the nature of photocatalysts thoroughly in the r-space through the local structures and electronic states around photocarriers, which will provide a deeper understanding of photocatalysis and accelerate the development of novel photocatalysts. [PERSON] is a main contributor of the paper. [PERSON], [PERSON] and K.A. conceived and planned the experiments and they interpret the results. T.K. : set up the SACLA experiments and Y.U. and T.K. conducted the experiment there. S.N. set up the PF-AR experiments and S.N. and Y.U. performed the experiments. All authors have read and agreed to the published version of the manuscript. This work was financially supported by the Japan Society for the Promotion of Science (JSPS) through a Grant-in- Aid for Exploratory Research (No. 26620110), a Grant-in-Aid for Scientific Research (A) (No. 15H02173), a Grant-in-Aid for JSPS Research Fellow (No. 15J07459), grants for collaborative research in the Institute for Catalysis, Hokkaido University (Nos. 15A1004 and 18A1005) and a polymer membrane fuel cell project from the New Energy and Industrial Technology Development Organization (NEDO), Japan. This work is financially supported by Innovative Catalysis of CREST--JST program. This work was partially supported by the \"X-ray Free Electron Laser Priority Strategic Program\" and the \"Photon and Quantum Basic Research Coordinated Development Program\" of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The authors appreciate all the collaborators who contributed to this work: [PERSON], [PERSON], [PERSON], [PERSON] (Hokkaido University), [PERSON] (Institute for Molecular Science), [PERSON], [PERSON], [PERSON], [PERSON] (Photon Factory), Institute for Materials Structure Science), [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] (SPring-8, JASRI and SACLA, Riken), [PERSON] (Toyama University), [PERSON], [PERSON] (Tokyo University of Science). We also appreciate [PERSON] (Utrecht University) for valuable comments and suggestions.