Change of Diffraction Pattern due to Change of TEM Sample Height
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The incident electron beam can be considered as made of a set of incident rays within an incident cone. In two-beam condition, the resulting pattern located in the back focal plane consists of two disks but not of two spots, namely the transmitted and hkl diffracted disks as shown in Figure 3887 (a). Given the strength of the objective lens is unchanged, when the defocus is changed from zero to Δh by adjusting the height of the TEM sample, the transmitted and diffracted beams in the diffraction pattern in the back focal plane will be changed from Figure 3887 (a) to (b). In this case, the diffraction spots in the image plane also move. Assuming the strengths of the other lenses in the TEM system are not changed, the diffraction pattern on the TEM screen or detector will be very different from the cases in Figure 3887 (a) to (b). If you are working on TEM-imaging mode, the TEM image will also be very different.

TEM optics under conditions of focused (a) and defocused (b) convergent incident electron beams

Figure 3887. TEM optics under conditions of focused (a) and defocused (b) convergent incident electron beams.

 

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