Secondary electron microscope principle

Fig. 4 Principle of the scanning mode of a secondary electron microscope, where 1, specimen surface 2, detector 3, video processing 4, monitor screen. (From Ref 1)...

This type of electron microscope is completely different in principle and application from the conventional transmission-type electron microscope. In the scanning instrument, the surface of a solid sample is bombarded with a fine probe of electrons, generally less than 100 A in diameter. The sample emits secondary electrons that are generated by the action of the primary beam. These secondary electrons are collected and amplified by the instrument. Since the beam strikes only one point on the sample at a lime, the beam must be scanned over the sample surface in a raster pattern to generate a picture of the surface sample. The picture is displayed on a cathode ray tube from which it can be photographed. 

The principle of microscopic reversibility predicts that the reverse process must follow the same path which is indeed stereoelectronically allowed the oxygen atom in T has two secondary electronic effects (n-o ) (one electron pair of the oxygen atom is anti peri planar to the C-N bond while the other is antiperiplanar to C —Y bond) and the nitrogen has one (the nitrogen electron pair is antiperiplanar to the C —Y bond). Thus, there are three secondary electronic effects (n-o ) in ] and by the ejection of Y to form 4, two of these (due to the two electron pairs antiperiplanar to the C—Y bond) have been transformed into primary electronic effects (n- ) in the product 4. The third secondary electronic effect remains a n-o interaction in the product. The ejection of Y can therefore take place with the help of the primary and one secondary electronic effects. 

The scanning electron microscope is based on a somewhat different principle than the transmission electron microscope. In the scanning electron microscope, the viewed image is formed by the secondary electrons emitted from the sample surface when an electron beam is scanned across this surface. These secondary electrons are detected by a suitable detector and counted. An image is then formed on a cathode ray tube in which the brightness of the raster spot is proportional to the number of electrons emitted at each point on the sample surface. In order to prevent charging of the surface as the electron beam is scanned across it, the surface is coated with a conductor such as gold. 

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