X-ray-absorption spectroscopy and n-body distribution functions in condensed matter .2. Data analysis and applications

The practical and theoretical aspects of the GNXAS method for multiple-scattering extended x-ray-absorption fine-structure (EXAFS) data analysis are treated in a comprehensive account. The model function used to fit the raw absorption coefficient is described and details on the least-squares fitting procedure and parameter definition are reported. Large emphasis is given, in EXAFS analysis, to the description of criteria for a complete statistical evaluation of the results, including error estimate and model evaluation. An extensive set of applications to prototypical molecular (Br-2, CS2), and crystalline [c-Ge and Pd (fcc)] systems is reported. Structural parameters always coincide with the known values within statistical accuracy indicating that systematic errors due to approximations in the theory are negligible. The present results also demonstrate that x-ray absorption spectroscopy can provide information on three-body atomic arrangements like average geometrical and vibrational parameters with statistical significance. In all examples, despite the inclusion of several three-body contributions, the total number of fitting parameters does not exceed the information content of the spectra; details on error evaluation and correlation plots in the parameter space are reported.