ANISOTROPY OF ANOMALOUS DISPERSION - AN APPLICATION TO PARTIAL STRUCTURE DETERMINATION

Investigations with synchrotron radiation have shown that atoms in anisotropic chemical environment can exhibit a pronounced anisotropy of their anomalous dispersion, i.e. for absorption edge energies, the energy dependent correction terms f' and f" of the atomic form factor can depend also on the crystal orientation with respect to the polarization of the incident photons. Then, extinction rules due to screw axes and/or glide planes can be violated giving rise to the observation of 'forbidden' reflections. The effect is discussed for a special case, a structure crystallizing in the space group P2(1)2(1)2(1) and containing one 'edge' atom per asymmetric unit. The intensities of the 'forbidden' reflections are given as functions of both the azimuthal angle psi and the atom's position. The developed results are also valid for unpolarized radiation. A simple method is proposed to extract partial structure information exclusively from the relative psi-dependencies of the intensities of only 6 axial reflections, two on each lattice row. These data are, in principle, sufficient to obtain at most 16 equally possible locations of the atom in the asymmetric unit. Feasibility calculations promise satisfactory experimental results.