X-ray-absorption spectra of a-Si and a-Si:H at the Si K edge are reported. By means of a nonstandard data-analysis procedure, we show that the fine-structure spectrum contains, besides the first-shell single-scattering extended x-ray-absorption fine-structure (EXAFS) signal, a higher-frequency contribution as well. Very accurate first-shell distances, coordination numbers, and mean-square relative displacements are obtained for all amorphous samples. The residual high-frequency signal is correctly reproduced only if multiple-scattering effects due to three-body correlations are taken into account. This result shows that x-ray-absorption spectroscopy has the potentiality for probing structural information beyond the pair correlation function in amorphous systems.
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Filipponi et al. (1989) studied this question.