1）X-ray absorption fine structure spectroscopy
This method does not rely on the long-range ordered structure, is not disturbed by other elements, and highly inclusive to the physical form of samples. It is a powerful tool to study the local structure and electronic structure of materials. The absorption edge position can help estimate the valence state of the target element, the pre-edge peak contains information about the symmetry, and further fitting and calculation of XANES can obtain the three-dimensional spatial structure of the samples. As to EXAFS, we can obtain structural information about the coordination element type, coordination number and disorder factor of a matter by fitting.
2）X-ray emission spectroscopy（XES） & high-energy resolution fluorescence-detected X-ray absorption spectroscopy（HERFD-XAS）
►XES is recognized as the decay process following X-ray absorption owing to electron de-excitation from a higher energy level to the core hole. Its resolution relies on the core-hole lifetime of 2p/3p orbitals rather than that of the 1s orbital, promising narrower broadening compared to conventional XAFS. Valence-to-core XES (VtC-XES), which involves the transmission of ligand orbital contributions to the core level of the center atoms, is sensitive to fine electronic states and ligand types. So it can break the limitation that the scattering of C, N, and O atoms is unrecognizable via conventional XAFS.
►In HERFD-XAS, only the desired emission line is detected by the high-energy-resolution spectrometer, the pre-edge, shoulder, and multiple scattering features are enlarged by eliminating elastic scattering and other uninteresting fluorescence lines, making it a powerful tool for investigating elaborate atomic and electronic structures.
3）X-ray Raman Scattering Spectroscopy（XRS）
The XRS is essentially an X-ray inelastic scattering process. In this process, the energy and momentum of the photon are changed and the changes contain important electronic structure information. We can get similar information to the XAS spectrum by analysis the emitted inelastic scattered photons. Therefore, hard X-ray Raman spectroscopy is developed to investigate the K-edge absorption of 2p elements, including C, N, and O, and the L-edge XAS of 3d transition elements, consequently surpassing the limitations of the probing depth and vacuum condition from typical soft X-ray absorption measurements.
