EXACT TREATMENT OF BEATING PHENOMENA IN EXAFS

Beating occurs when two coordination shells are closely spaced (delta R approximate to 0.1 Angstrom). Analyzing two such shells by curve fit ting is difficult because the fit parameters exhibit enhanced correlat ion due to the reduced number of degrees of freedom. Analyzing the com bined phase of the two shells in k-space, or its derivative with respe ct to k, can be performed with a reduced number of fit parameters beca use only the ratio of the two coordination numbers is required and, wi th a minor approximation, only the difference of the two EXAFS Debye-W aller factors is needed. Since the beating effect leads to structure t hat occurs localized in k-space and because this beating structure is usually at the high-k end (for very closely spaced coordination shells ) one may neglect the effects of the energy corrections Delta E(1) and Delta E(2) for the origins of the two k-scales, thus reducing the num ber of fit parameters by another two. The beating effect can be invest igated conveniently by analyzing the derivative of the combined phase of the two coordination shells in k-space. Then the occurrence of beat ing manifests itself by peaks or dips in the derivative function. This structure, however, can be modified significantly by effects resultin g from the Fourier transform. In the present paper such Fourier transf orm artifacts, the effects of window functions and k-space weighting a re taken into account.