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Bromine-oxygen radial distribution functions [g(r)] have been calculated by means of molecular dynamics simulations for aqueous solutions of rubidium bromide, 2-bromopropane and bromoethane. X-ray absorption spectra at the bromine K edge have been recorded for these solutions. The water contribution to the extended x-ray absorption fine structure spectra has been calculated Starting from the g&O (r) distribution function. Fits of the x-ray absorption spectra have been performed directly on the raw experimental data, allowing the reliability of the g(r) distribution functions to be verified. The agreement between theoretical and experimental spectra is satisfactory. A procedure to improve model g(r) functions on the basis of the short-range structural information provided by extended x-ray absorption fine structure data is proposed.

Structural information on the pair distribution function pz(r) for single-component disordered systems is usually obtained from the experimental structure factor S(k) measured by diffraction techniques. Complementmy short-range information can be provided by the analysis of the extended x-ray absorption fine structure x(k) associated with a d n x-ray absorption edge. The intrinsic differences in the nature of the x(k) and S(k) signals are discussed and particular effort is devoted to connecting the ~( k ) signal with usual quantities familiar to the distribution function theory in disordered malter. An example of the short-range x ( k ) sensitivity is presented showing sipals associated with gz(r) functions of liquid Cu at I150 T and 1300 'C.The necessity to fit realisticgz(r) models to ULAFS spectrasatisfying bath long-distance behaviour and the compressibility sum rule is emphasized. A method to combine these constraints and previous informakion~on 8 2 0 ) with available x ( k ) data is proposed q d applied to recent BAFS data on liquid palladium.

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.