DYNAMIC CALCULATION IN A 2-BEAM APPROXIMATION FOR 3-BEAM X-RAY-DIFFRACTION

The formulation of a two-beam approximation for three-beam diffraction is derived from the dynamical theory of X-ray diffraction, with and without the assumption that the wavefield amplitudes D(G) of the diffracted waves are much smaller than the amplitude D(O) of the incident wave. Based on this formulation, calculations for the dispersion surface, intensity, excitation of mode of wave-propagation, and absorption are performed for several three-beam diffractions from centric and acentric crystals. The results are compared with those calculated without approximation. It is shown that the two-beam approximation without D(G) << D(O) gives a better agreement with the exact solution than that with the D(G) << D(O) assumption.