Forbidden X-ray wavefields of three-beam Bragg reflections from thick crystals

A detailed analysis of a three-beam diffraction dispersion surface is perfo rmed to study the forbidden wavefields of thick-crystal Bragg reflections. From the asymptotic transition between two- and three-beam diffraction, it is found that the excitation state of each wavefield can be accurately dete rmined with the two-beam criterion. Consequently, Bragg-case three-beam dif fraction from thick crystals is either a four-mode diffraction process for the Bragg-Laue geometry or a two-mode process for the Bragg-Bragg geometry, and the amplitudes of the excited wavefields can be completely determined by the entrance boundary conditions. Based on this picture, the intrinsic m echanisms underlying three-beam Bragg reflections are clearly illustrated.