Ultrafast and static X-ray absorption spectroscopy

X-ray absorption fine structure (XAFS) was first invented from 70’s of the 20th century. It has been one of the most powerful techniques for structural investigation is XAFS. In this method, structure is revealed based on photoelectron wave interference between the outgoing wave that is emitted from an atom when it absorbs x-ray and the scattered wave that is bounced off by neighboring atoms. XAFS is an element-specific probe of a structure about a specific atom of interest. The limitation of XAFS is that the photoelectron mean free path is relatively short and thereby only the short-range structure about the ion is probed. The more structural order the larger structure range can be probed. It can easily reach a structural accuracy of 0.01 Å.

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Illustration of interference of photoelectron wave in XAFS

Since X-ray absorption spectroscopy (XAS) is based on transition of an electron from the core-shell to continuum state of the atom, through X-ray resonances, it also allows studies of the electronic structure of the target atom environment within a molecule. A recent developed spectroscopic technique, so-called ultrafast X-ray absorption spectroscopy, allows to study transient states of atom and molecules. When a pumping laser is implemented to initialize a photochemical process and X-ray absorption spectra are measured at different time delays with respect to the pumping laser, one can achieve the evolution of molecular structure and its electronic transitions. This powerful spectroscopic tool has recently been developed and provided new insights into the ultrafast process such as light-induced spin cross-over in iron tris bypiridine or revealed the hydration structure change of iodide. It will be helpful to investigate multielectron charge accumulation process in molecules for artificial photosynthesis applications.

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Pump-probe experiment and structural study using ultrafast X-ray absorption spectroscopy