Fluorescent Screen Observation

TEM offers two methods of specimen observation, diffraction mode and image mode. In diffraction mode, an electron diffraction pattern is obtained on the fluorescent screen, originating from the sample area illuminated by the electron beam. The diffraction pattern is entirely equivalent to an X-ray diffraction pattern a single crystal will produce a spot pattern on the screen, a polycrystal will produce a powder or ring pattern (assuming the illuminated area includes a sufficient quantity of crystallites), and a glassy or amorphous material will produce a series of diffuse halos. 

This experiment does have some features of a direct vision observation. First, the glowing spot is directly visible. Second, it is easy to imagine an invisible stream of particles hurtling through the triangular hole in the electrode to crash against the fluorescent screen in a burst... 

Fig. 13.8. Atomic metallic ion emission and nanotip formation, (a) At high temperature, the atoms on a W tip becomes mobile. The tip surface is macroscopically flat but microscopically rough, (b) By applying a high field (1.2-1.8 V/A,), the W atoms move to the protrusions, (c) The apex atom has the highest probability to be ionized and leave the tip. The W ions form an image of the tip on the fluorescence screen, (d) A well-defined pyramidal protrusion, often ended with a single atom, is formed. By cooling down the tip and reversing the bias, a field-emission image is observed on the fluorescence screen. The patterns are almost identical. (Reproduced from Binh and Garcia, 1992, with permission.)...

If the work functions of every portion of the crystal constituting the tip were identical, one would simply observe a uniformly bright circular patch on the fluorescent screen. Different crystal faces have slightly different work functions, however, so that one sees a pattern showing different intensities for different faces. In general, closely packed faces have higher work functions than loosely packed ones. In fee cubic crystals like Ni the (111), (100), and (110) faces are the most closely packed and appear darker than the rest of the pattern (Plate IA and B). In bee cubic metals like tungsten, the (110), (100), and (211) faces are most closely packed and appear darkest (Plate IC). 

Fig. 4. Arrangement for the electron optical reproduction of a photo cathode [according to Mahl (32)] K photocathode, S fluorescent screen, A quartz lamp, B quartz window, M magnetic coils, C window for the direct observation of the cathode.

Electrons of uniform energy (—5-500 eV) are scattered from the surface of a single crystal. Those electrons that have lost no energy are selected and accelerated to a fluorescent screen where the diffraction pattern from the surface can be observed... 

Directly across the deposition chamber from the FEM tip was a fluorescent screen and viewport with which the field emission pattern from the tip could be observed during cluster deposition. The tip was first cleaned by joule heating. The two fold symmetry characteristic of a clean W(110) field emitter was observed and the voltage necessary to observe this pattern was recorded. The voltage was then reduced to about 1/2 of the original value and the tip was exposed to clusters that had been slowed to thermal speeds in the gas cell. Clusters landing near the apex of the tip appear as bright dots on the fluorescent screen. This procedure was repeated until an individual cluster positioned near the apex of the tip was obtained. 

This is essentially a back-reflection diffraction apparatus, with observation limited to a cone of half-angle 35° about the back-reflection direction. This limiting angle could be increased if the need arose, and it would also be possible by placing the grids and fluorescent screen differently to... 

To obtain a diffraction pattern a Laue camera must be correctly oriented with respect to the x-ray tube. This alignment requires that the collimator axis point directly at the focal spot on the tube target and make an angle of about 6° with the face of the target. The camera is moved relative to the tube until the primary beam, observed on a small fluorescent screen held near the collimator exit, is of maximum intensity and circular, not elliptical, in section. 

In the apparatus of the second type, in which the diffraction pattern is observed on a fluorescent screen, the inelastically scattered electrons are excluded by a grid placed between the inner grid and the fluorescent screen (5). This method is being used in certain qualitative investigations in this laboratory but it does not furnish quantitative measurements of intensity which compare with those of the electrical method. 

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