SELECTED DIFFRACTION 



As the illuminating aperture is decreased 

 from the near-critical condition the resolu- 

 tion slowly deteriorates. The optimum reso- 

 lution approaches that of bright -field opera- 

 tion. 



Secondary Effects. A small proportion 

 of the inelastically scattered electrons will 

 always be accepted by the aperture. These 

 produce faint images of parts of the object 

 which scatter electrons strongly. The diffrac- 

 tion images are usually much brighter and 

 can therefore usually be recognized. Often 

 the images of individual crystals have a faint 

 comet -like tail. This is believed to be caused 

 by electrons which have undergone both 

 elastic and then inelastic scattering. 



Applications 



Location and Sizes of Crystallites. 



The method is useful for locating the posi- 

 tions of crystallites in poly crystalline films 

 or finely divided solids. Very often individual 

 crystallites which cannot be distinguished in 

 bright-field micrographs owing to lack of 

 contrast are easily visible in the selected 

 diffraction image, enabling their positions to 

 be located and their sizes to be measured. 

 Some good examples of this are given in Re- 

 ference 1. 



Deformation of Single Crystals. O. 

 Rang and H. Schluge (2) have used the 

 method to study the deformation of single 

 crystals in the form of very thin plates or 

 foils. Alany dark bands are often to be seen 

 in the bright-field images of such crystals. 

 These occur where parts of the crystal are 

 orientated at a Bragg angle to the incident 

 electron beam. Electrons incident on these 

 parts are diffracted strongly out of the pri- 

 mary beam to form a Bragg reflection which 

 in the bright-field case is stopped out by the 

 objective aperture diaphragm. Adjusting the 

 diaphragm to accept the Bragg reflection 

 and stop out the transmitted electrons makes 

 the crystal appear dark with bright bands on 

 it. To determine the deformation of the 

 crystal the electron diffraction pattern is 



first recorded and indexed. The aperture is 

 then adjusted to accept the diffracted elec- 

 tron beams corresponding to each spot of the 

 diffraction pattern in turn and the corre- 

 sponding selected diffraction images are 

 recorded. By this means the Bragg angles 

 corresponding to each of the bands are de- 

 termined and hence the orientation of differ- 

 ent parts of the crystal can be plotted. It is 

 sometimes useful to be able to tilt the stage 

 through known angles and to follow the 

 movement of the bands as the stage is tilted. 



Distribution of Crystalline Constitu- 

 ents. The aperture system shown in Figure 

 1(b) has been used by J. H. Talbot to deter- 

 mine the distribution of certain crystalline 

 constituents, first identified by electron 

 diffraction, in samples of mine dust. These 

 samples contained numerous minute crys- 

 tals the diffraction patterns of which con- 

 sisted of concentric rings. By varying the 

 position of the aperture diaphragm along 

 the optical axis, different rings of the diffrac- 

 tion pattern could be selected and the corre- 

 sponding selected diffraction images formed. 



When using this type of aperture it must 



Fig. 2. Sample of mine dust showing the dis- 

 tribution of calcium sulfate. Selected diffraction 

 image using 60° arc of the d = 2.85A ring. 



253 



