The Future of XAFS

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Abstract
The ultimate information content of XAFS is discussed. The practical limits for obtaining this information are indicated. The most general case is considered where nothing is known about the structure and there is no orientation dependence of the XAFS with respect to the sample. For close packed structures it ultimately appears possible to obtain the two and perhaps some of the three particle correlation distribution functions between the center atom and the its first neighbors; two particle correlation distribution functions between the center atom and its second nearest neighbors and possibly some of the three particle correlation distribution functions between the center atom and its first and second nearest neighbor atoms. By the fourth neighboring atoms it appears that only the two particle correlation distribution function with the center atom is feasible. Beyond that the information content in XAFS is overwhelmed by the large number of multiple scattering paths contributing and it does not appear possible to obtain even the two particle distribution functions. For open structures it appears to be possible to obtain the two particle correlation distribution function between the center atom and its first three neighboring shells of atoms and perhaps some three particle correlations. When some information about the structure is known, or in special situations, it will be possible to obtain restricted information beyond these limits. The more brilliant future x-ray sources together with computing advances will make feasible the combining of XAFS with other techniques such as microscopy and tomography to produce two or three dimensional imaging of the XAFS spectrum with submicron resolution. Time resolution of XAFS measurements in the nanosecond range appears feasible The time resolution and the imaging will require automating the analysis of data to handle the overwhelming data content of such measurements. XAFS information will be utilized in many more phenomena than presently. Hints of the possibilities are seen in the fine structure of anomalous diffraction. Other future phenomena to be exploited are x-ray Raman scattering and its combination with diffraction, fine structure in electron energy loss and its combination with diffraction. The polarization of x-rays, both plane and circular, will be more intensely exploited, especially when XAFS is combined with diffraction, etc., because richer orientation, spin, and site dependences exist. The most exciting future developments may be awaiting the creative genius of the future scientific generation and not foreseen here.

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