EXAFS for liquids

Extended x-ray absorption fine structure (EXAFS) spectroscopy exploits the quantum interference resulting from the scattering of a photoelectron, generated by the excitation of a core level, by the potential of the surrounding atoms. From the interference pattern it is possible to determine the distance and average distribution of the nearest neighbours from the photoabsorbing atomic species. This spectroscopy therefore provides a unique site-selective local structural probe in condensed matter which is ideal for investigating the average environment of specific elements in a liquid. In the last 20 years we have seen substantial developments in the experimental techniques which nowadays allow scientists to perform EXAFS experiments under extreme conditions of high pressure and temperature that were not even conceivable just a few years ago. These techniques have been applied to the investigations of metals, semiconductors, molecular fluids and solutions, as a function of pressure and temperature and through phase transitions, attracting a wide scientific community towards this spectroscopy. The complete understanding of the x-ray absorption signal is however a challenging theoretical problem involving the many-electron response of the system. Our current theoretical framework for the interpretation of the EXAFS spectra is based on the solution of an effective one-electron problem. This theory is nevertheless accurate enough to be used in a quantitative data analysis able to retrieve valuable structural information.