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

摘要

Atomic vibrations are usually not taken into account when analyzing x-rayabsorption near edge structure (XANES) spectra. One of the reasons is thatincluding the vibrations in a formally exact way is quite complicated while theeffect of vibrations is supposed to be small in the XANES region. By analyzingpolarized 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 systemsbut with a modified free-electron propagator which accounts for fluctuations ofinteratomic distances, may substantially help in understanding XANES of somelayered systems. In particular we show that the difference in the damping ofthe x-ray absorption fine structure oscillations for different polarisations ofthe incoming x-rays cannot be reproduced by calculations which rely on a staticlattice but it can be described if atomic vibrations are accounted for in sucha way that individual creatinium and CuCl$_{4}$ molecular blocks are treated assemi-rigid entities while the mutual positions of these blocks are subject tolarge mean relative displacements.