Theoretical study of the origin of x-ray atomic background anomalies by a full-potential multiple scattering approach

Previous extended x-ray absorption fine structure (EXAFS) studies have shown evidence of anomalous contributions in the atomic background function whose origin was debated in the literature. The present study presents advanced multiple scattering calculations aimed to put to a test the existence and origin of the so-called atomic XAFS (AXAFS) contribution, discussed in details in some previous works. We show with specific examples that the muffin-tin approximation normally used in EXAFS calculations is inappropriate to study this phenomenon as spurious oscillations in the atomic background are related to the potential truncation at the border of the muffin-tin spheres. We thus employed a more sophisticated approach beyond the muffin-tin approximation based on the full-potential multiple scattering theory and self-consistent charge densities. The comparison of EXAFS calculations with previous experimental results allowed us to exclude the presence of the AXAFS as due to modulations of the charge density as previously suggested. This study confirms the importance of a proper account of the atomic background in EXAFS data analysis, which may contain effects beyond the single-electron approximation but are found not affected by AXAFS. We conclude that the contribution of AXAFS in EXAFS experiments, if existing at all, is negligible and should not be considered in EXAFS data analysis.