Thursday, August 11, 2011, 9:30 am — Building 703, Large Conference Room
The method of x-ray standing waves (XSW) is a well established technique for the determination of adsorbate or dopant atomic positions in single crystals. It is based on the interference of incident and Bragg reflected wave,
which is calculated with the dynamical theory of x-ray diffraction. This means that highly perfect crystals are necessary. I will present an improvement of the classical XSW method, which allows application of the XSW
technique to non-perfect single crystals. This is achieved by describing the standing wave field in the frame of the kinematical approximation. Recent results of such kinematical x-ray standing waves (KXSW) investigations of
doped non-perfect crystals will be shown.
Usually the XSW and KXSW techniques are used in Bragg geometry.
However the application of both methods is also possible in Laue geometry. A particularly interesting phenomenon observed in the latter case is the Borrmann effect, which is caused by the standing wave field inside the crys-
tal. For certain reflections the low electrical field intensity at the atoms leads to a low dipole absorption (anomalous transmission). At the same time the
high field gradient results in an enhanced quadrupole absorption. Therefore the intensity of the reflected beam strongly depends on the quadrupole absorption cross section of the atoms at the nodes of the standing wave field.
This effect can be used for the identification and investigation of quadrupolar features in the x-ray absorption near edge structure (XANES). I will present
a theoretical model of this novel Borrmann Spectroscopy method and demonstrate its applicability to transition metal K edges and rare earth L edges.
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