Anisotropic x-ray absorption in layered compounds

A study is presented of anisotropy effects in x-ray absorption in the layered compounds of 2H-WSe2 and 1T-TaS2. In the measurements it was essential to separate the thickness effect from the true anisotropy effect which is dependent on the angle between the x-ray polarization and the crystal axes. The Se K edge and the W and Ta L edges were measured. Anisotropy in the white line of Se was found but no anisotropy was discerned in the W and Ta white lines. It is pointed out that x-ray absorption in general, and the extended x-ray absorption fine structure (EXAFS) in particular, have the anisotropy dependence of a second-order tensor and the theoretical expression for the EXAFS anisotropy at the L2,3 edges is explicitly displayed. The anisotropy of the EXAFS in the Se and W absorption was measured and a good agreement with theory is found. The anisotropy of EXAFS at the L2,3 edges has the new feature of a cross term between the final s and d states, which permits a determination from the measurements that the average contributions of the final s state is 0.02 of that of the final d states to the total absorption of the W L3 edge. Finally, only qualitative agreement is obtained between band calculations and the near edge x-ray absorption structure, as expected theoretically.