Three-beam X-ray diffraction in a semi-infinite perfect crystal.

The approach to multibeam X-ray diffraction by Shen [Acta Cryst. (1986). A42, 525-533] has been carefully re-examined for three-beam Laue-Laue diffraction in a perfect non-absorbing semi-infinite crystal. A modified version of Shen's theory is shown to be equivalent to the Takagi-Taupin formalism. The perturbation of the two-beam integrated power when a third beam is being excited is thus expressed by non-diverging terms. Both Aufhellung and Umweganregung terms are recovered.