precession electron diffraction

Precession electron diffraction is an electron diffraction method to obtain an electron diffraction pattern with a less dynamical diffraction effect by precessing the incident electron beam under a certain tilt angle with respect to the optical axis. A two-stage deflector coil in the illumination lens system is used to precess the tilted beam (up to approximately 5°) and to illuminate the beam onto a certain point on a specimen. Then, a two-stage deflector coil (below the specimen) in the image-forming lens system is used to compensate the displacement of the incident electron beam from the optical axis (de-scan), so that the incident beam subjected to the precession stays at the center of the screen. Illumination of such a precessed electron beam suppresses enhancement of the intensity of the reflections due to “double reflection” (an additional excitation via the other strongly-excited reflections). As a result, the observed diffraction intensities become close to those expected from the kinematical diffraction theory. Owing to the precession of the incident electron beam, the number of reciprocal lattice points crossed by the Ewald sphere increases and thus, high-order diffraction spots appear.
The obtained diffraction intensities are utilized for crystal structural analysis by applying the analysis techniques (direct method, etc.) developed for X-ray crystal structure analysis. The electron diffraction method is effectively used for structural analysis of inorganic crystals with complicated structures (e.g. zeolite) and organic crystals, which are often crystallized only with a size up to a micrometer order.



(a) Ray diagram of precession electron diffraction. A precession electron diffraction pattern is obtained by precessing the incident electron beam under a certain tilt angle with respect to the optical axis using a two-stage deflector coil in the illumination lens system, and then by compensating the displacement of the incident electron beam from the optical axis using a two-stage deflector coil in the image-forming lens system.
(b) An ordinary electron diffraction pattern and (c) a precession electron diffraction pattern obtained from garnet at [111] incidence. In Fig. (c), the precession of the incident electron beam enables us to obtain a diffraction pattern that has diffraction intensities being close to those expected from the kinematical diffraction theory. Furthermore, more high-order reflections appear compared with the number of reflections in the ordinary diffraction pattern (b). 

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