Analysis of experimental extended x-ray-absorption fine-structure (EXAFS) data using calculated curved-wave, multiple-scattering EXAFS spectra

We present an EXAFS analysis method that is accurate and generally applicable and can be implemented on a fast microcomputer within a reasonable amount of computation time. The analysis is based on the comparison between a calculated EXAFS spectrum of a parametrized model with the experimental spectrum and the refinement of the model parameters. The calculated EXAFS spectrum is evaluated taking into account the curved-wave nature of the photoelectron and multiple scattering up to and including third-order collinear scattering. Such an expansion treats accurately all multiple-scattering contributions that contribute to the Fourier transform of the EXAFS spectrum up to the fourth-neighbor distance. The comparison between theory and experiment is thus limited to this distance. In this calculation we schematically express the contributions of the various scattering configurations in the form 0=F(k)sin[kL+(k)] and expand to lowest order the amplitude F and the phase functions in terms of all structural and potential parameters. This expansion is quite accurate and significantly speeds up the computation. We discuss a set of parameters which seems to account for the limitations in the employed theory, providing a very good fit between theory and experiment. The analysis method and the parameters have been implemented successfully on a number of systems. We present here the analysis of the copper EXAFS spectrum. The experimental data contain 20 independent experimental points; yet we obtain a very good fit with only six parameters. The results also show the importance of both double and triple collinear scattering as well as the importance of noncollinear double scattering.