International Tables for Crystallography

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X-ray linear dichroism: dependence of XAFS on the orientation of the sample with respect to the incoming radiation
Ondřej Šipr. International Tables for Crystallography (2022). Vol. I [ doi:10.1107/S1574870722001616 ]

Abstract

The probability that an X-ray photon will be absorbed by exciting a core electron depends on the orientation of the sample with respect to the incoming radiation. The probability of transitions allowed by the dipole selection rule depends on the polarization vector of the incoming radiation, whereas the probability of transitions allowed by the quadrupole selection rule additionally depends on the direction from which the radiation arrives. Angle-dependent or polarized X-ray absorption spectra contain more information than orientationally averaged spectra. The extra information obtained in angle-dependent spectra can be beneficial for investigating the local structure around the photoabsorber. Systems in which this is especially convenient include complicated geological materials, biological systems, impurities in the bulk and adsorbates on surfaces. From a purely theoretical point of view, polarized spectra present a much more stringent test of the theory than unpolarized spectra; this can be helpful in deciding between different theoretical models (for example concerning the importance of many-body effects). Furthermore, the different angular dependence of dipole and quadrupole transitions can serve as the ultimate tool to distinguish between them.


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About International Tables for Crystallography

International Tables for Crystallography is the definitive resource and reference work for crystallography. The multi-volume series comprises articles and tables of data relevant to crystallographic research and to applications of crystallographic methods in all sciences concerned with the structure and properties of materials.