ANISOTROPY OF ANOMALOUS SCATTERING: INVESTIGATION OF POLARIZED ABSORPTION BY FLUORESCENCE MEASUREMENTS. T. Lippmann, HASYLAB/DESY, Hamburg, A. Kirfel, University of Wurzburg, K. Fischer, University of Saarbrücken, FRG

Polarized X-ray absorption is examined both theoretically and experimentally by fluorescence measurements. Using the dipole approximation, the conventional scalar resonant scattering factors (and hence the absorption coefficient) are replaced by second-rank tensors. By application of the Jones-calculus an algorithm is developed, which includes both linear birefringence and dichroism, and provides a detailed representation of the fluorescence intensity for crystals exhibiting large faces. Moreover, neglecting birefringence and assuming not a too large relative dichroism, an adequate approximation is presented, which is applicable for most practical cases [1].

The experimental setup for a simultaneous determination of the absorption tensor elements depends on crystal system and face investigated. For the orthorhombic system, the geometrical prerequisites are discussed, and a fluorescence intensity function is developed and illustrated by model calculations.

Test experiments concerning various experimental geometries and polarization states of the primary radiation are carried out on different crystal faces of LiHSeO₃, showing results in good agreement with the theoretical considerations.

Additionally, polarized absorption spectra of (NH₄)₉[Cu(S₂O₃)₄]Br₂ as a further example of a synthetic compound and of two Amphiboles (Edenite and Hastingsite) as examples for iron bearing minerals are presented. The detection of orientational dispersion of the absorption tensor in these monoclinic crystals supports both the model and the experimental method.

[1] Lippmann, T., Kirfel, A., Fischer, K. (1996) J. Appl. Cryst. 29, 1-12