Structure-Factor Phase Information from Two-Beam Electron Diffraction

The electron-induced characteristic x-ray emission from a noncentrosymmetric crystal shows a pronounced intensity difference when the Bragg condition is fulfilled for the $G$ and $-G$ reflections, respectively, along a polar direction. This two-beam effect is demonstrated fro GaAs and InP, and is a direct method of obtaining information about the phase of a structure factor and determining the sense of a polar direction.