Including atomic vibrations in XANES calculations: polarization-dependent damping of the fine structure at the Cu K edge of (creat)$_{2}$CuCl$_{4}$

Abstract

Atomic vibrations are usually not taken into account when analyzing x-ray absorption near edge structure (XANES) spectra. One of the reasons is that including the vibrations in a formally exact way is quite complicated while the effect of vibrations is supposed to be small in the XANES region. By analyzing polarized Cu K edge x-ray absorption spectra of creatinium tetrachlorocuprate [(creat)$_{2}$CuCl$_{4}$], we demonstrate that a technically simple method, consisting in calculating the XANES via the same formula as for static systems but with a modified free-electron propagator which accounts for fluctuations of interatomic distances, may substantially help in understanding XANES of some layered systems. In particular we show that the difference in the damping of the x-ray absorption fine structure oscillations for different polarisations of the incoming x-rays cannot be reproduced by calculations which rely on a static lattice but it can be described if atomic vibrations are accounted for in such a way that individual creatinium and CuCl$_{4}$ molecular blocks are treated as semi-rigid entities while the mutual positions of these blocks are subject to large mean relative displacements.