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In the case of thin TEM films, the relaxation of the Bragg conditions occurs at the reciprocal lattice points. Those reflection (diffraction) points are transformed into relrods elongated normal to the TEM film as shown in Figure 3908. This is also the reason why reflection spots are always shown in the electron diffraction pattern (EDP) even though the "perfect" Bragg condition sometimes is not exactly satisfied. Since this elongation is inversely proportional to the specimen thickness (t), the relrods become very elongated for very thin TEM films so that many of them can simultaneously intersect the Ewald sphere and produce diffracted beams. The number of the shown diffraction spots on the TEM screen or detector is more than that from a thick TEM film. In this case, when the specimen or the electron beam is tilted, the spot position in the diffraction pattern moves because the Ewald sphere moves and then intersects the different portions of the relrods (associated with the reciprocal lattice) to produce the diffracted
beams.
Figure 3908. Schematic illustration of relrods elongated normal to the TEM film. The points of the reciprocal lattice are transformed into elongated relrods. The inset on the top-right-hand side is a zoom-in of the gold-cycled part.
Using the diffraction information shown in the ring-like patterns below each phase contrast image that are acquired with the incident beam aligned along the optic axis. By tilting the beam off the optic axis, lens aberrations become apparent in the Fourier transforms of phase contrast images. A set of such patterns obtained at various beam tilts around the optic axis is called a "Zemlin tableaus". The distortions of the circular patterns in the Zemlin tableaus are used for characterizing the aberration. This method is well-suited for HRTEM imaging.
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