Secondary extinction and absolute structure factors in X-ray diffraction: determination by polarization variation

If a rocking curve is measured in symmetric Laue geometry for two different X-ray polarizations, and the incident beam meets the geometrical conditions for diffraction, i.e. the incident beam is much less divergent than the sample mosaic, then the intensity ratio R_obs, for the intensities measured with each polarization, will depend upon the reflectivity at each point on the rocking curve. From the variation of R_obs vs the observed intensity, I_obs, the absolute reflectivities and secondary extinction can be determined. If absorption is properly treated, then all data taken in this geometry should lie on a single curve of R_obs vs I_obs. Failure to fit this curve is evidence that the sample has other processes occurring such as multiple scattering or primary extinction.