The electron diffraction camera

Since 6 10 radian, R must be very nearly parallel to g hence. 

We see, therefore, a direct relation between R and the reciprocal lattice vector g of the operating reflection, which is normal to the reflecting planes. (XL) is called the camera constant. 

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Figure 2. The scheme of the electron diffraction camera EMR-102 (produced by SELMI Ltd. (Sumy, Ukraine). 1 - Electron gun, 2-Two condenser lenses, 3 - Specimen holder, 4 -Chamber, 5 - Optical microscope, 6- Tubule, 7 - Photo-chamber.

Although measurements with diffractometer interfaced with EDC cameras have been performed at 80-100 kv, however, this old-type system has a lot of limitations linked to the extremely long time (several hours) to scan ED patterns and the beam size (from microns to mm) of the electron diffraction cameras. Again, the problem of correcting intensities from dynamical contribution has not been addressed satisfactory, as primary extinction (dynamical) corrections have been proposed for known stmctures using the Blackman formula . ... 

Smith undertook an exhaustive study of the nature of these films. To do so, he had to build his own specialized vacuum system because he had to deposit his films on flat plates in order to examine them by electron diffraction. He also had to be able to transfer them into the electron diffraction camera without exposure to atmospheric contamination. For pressure measurements he, too, used a McLeod gauge because the ionization gauges one could build at that time were prone to outgassing, which would contaminate the films. He made films of a great many transition metals, in addition to nickel. 

Sodium bromide films were obtained by vacuum evaporation directly in the electron diffraction camera (EG—1) in a vacuum of approximately 5 10 mm amorphous celluloid films were used as the substrates. When they condensed, the NaBr crystallites tended to form grains. It was noted that the preferred orientation of the crystallites depended on the film thickness. When the film thickness was reduced, the preferred orientation disappeared. The thickness of... 

Electron diffractometry system with the combination of the precession technique can be very perspective experimental instrumentation for precise structural investigations. The technique can now be adapted in a commercial TEM (previously applied uniquely to electron diffraction cameras) taking advantage of the small beam size and can measure reflections in the ED pattern with same required precision for structure analysis. 

Figure 3.13. The essential features of a simple electron diffraction camera.

The earlier work, before the invention of the electron microscope, was done with homemade apparatus called electron diffraction cameras. Many important studies were made of the structures of metal foils, electrodeposits, films deposited by evaporation, oxide films on metals, and surface layers due to polishing. 

Fig. 1.23. The electron diffraction apparatus developed by Parks and coworkers includes an rf-ion trap, Faraday cup, and microchaimel plate detector (MCP) and is structured to maintain a cylindrical symmetry around the electron beam axis [147]. The cluster aggregation source emits an ion beam that is injected into the trap through an aperture in the ring electrode. The electron beam passes through a trapped ion cloud producing diffracted electrons indicated by the dashed hues. The primary beam enters the Faraday cup and the diffracted electrons strike the MCP producing a ring pattern on the phosphor screen. This screen is imaged by a CCD camera mounted external to the UHV chamber. The distance from the trapped ion cloud to the MCP is approximately 10.5 cm in this experiment...

In recent years, electron diffraction has been used to characterize fuel cell catalysts, as information about the crystal symmetry of active components can be obtained from electron diffraction. Most of the electron diffraction for fuel cell catalysts is performed in a Transmission Electron Microscope (TEM), where the electrons pass through a thin film of the samples being studied. The resulting diffraction pattern is then observed on a fluorescent screen and recorded on photographic film or with a CCD camera. 

Experimental Method.—The diffraction photographs were prepared with the apparatus and technique described by Brockway.3 Ten or more photographs were made for each substance, the electron wave length used being about 0.0613 A. and the camera distance 10.83 cm. The values of so = 4ir(sin 6/2)/X given in the tables are averages of the values found by visual measurement of ring diameters for ten or more films. 

Sample B provided platinum crystallites that were analyzed by both EDS and MAED. MAED of several 3 nm crystallites shows a wide variation of orientations with respect to the electron beam, however, many of the patterns match (111) and (110) orientations. An example of the MAED patterns observed Is shown In Figure 2. The diffraction pattern was made with a 25 pm objective aperture at a camera length of 2 m. 

Figure 2. Diffraction camera for single-molecule electron diffraction. A Lanthanum hexaboride electron source is used. The laser and associated optics is rotated after each data readout for a new molecular beam orientation. Organic molecules are picked up within liquid helium droplets to form a molecular beam traversing the electron beam.

Electron diffraction patterns are usually produced with transmission electron microscopes. These instruments are composed of several magnetic lenses. The main lens is the objective lens, which, in addition to forming the first magnified image of the specimen, also produces the first diffraction pattern. This original pattern is then magnified by the other lenses of the microscope so as to produce the final diffraction patterns on the screen or on a camera. 

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