Secondary electrons, information

An ion beam causes secondary electrons to be ejected from a metal surface. These secondaries can be measured as an electric current directly through a Faraday cup or indirectly after amplification, as with an electron multiplier or a scintillation device. These ion collectors are located at a fixed point in a mass spectrometer, and all ions are focused on that point — hence the name, point ion collector. In all cases, the resultant flow of an electric current is used to drive some form of recorder or is passed to an information storage device (data system). 

Edx is based on the emission of x-rays with energies characteristic of the atom from which they originate in Heu of secondary electron emission. Thus, this technique can be used to provide elemental information about the sample. In the sem, this process is stimulated by the incident primary beam of electrons. As will be discussed below, this process is also the basis of essentially the same technique but performed in an electron spectrometer. When carried out this way, the technique is known as electron microprobe analysis (ema). 

As an example of the use of AES to obtain chemical, as well as elemental, information, the depth profiling of a nitrided silicon dioxide layer on a silicon substrate is shown in Figure 6. Using the linearized secondary electron cascade background subtraction technique and peak fitting of chemical line shape standards, the chemistry in the depth profile of the nitrided silicon dioxide layer was determined and is shown in Figure 6. This profile includes information on the percentage of the Si atoms that are bound in each of the chemistries present as a function of the depth in the film. 

Backscattered electrons, however, do give some elemental information about the sample because they are more energetic than secondary electrons and escape from farther within the sample [45,46], On the molecular level, the electron beam can interact with the nucleus of an atom and be scattered with minimal loss of energy. These incident electrons may be scattered more than once and then ejected from the sample as backscattered electrons. The back-scattered electrons originate from a greater depth within the sample and are... 

The theory as presented so far is clearly incomplete. The topology of the density, while recovering the concepts of atoms, bonds and structure, gives no indication of the localized bonded and non-bonded pairs of electrons of the Lewis model of structure and reactivity, a model secondary in importance only to the atomic model. The Lewis model is concerned with the pairing of electrons, information contained in the electron pair density and not in the density itself. Remarkably enough however, the essential information about the spatial pairing of electrons is contained in the Laplacian of the electron density, the sum of the three second derivatives of the density at each point in space, the quantity V2p(r) [44]. 

In transmission electron microscopy (TEM), a beam of highly focused and highly energetic electrons is directed toward a thin sample (< 200 nm) which might be prepared from solution as thin film (often cast on water) or by cryocutting of a solid sample. The incident electrons interact with the atoms in the sample, producing characteristic radiation. Information is obtained from both deflected and nondeflected transmitted electrons, backscattered and secondary electrons, and emitted photons. 

Figure 8 shows the attenuation length of electrons in solids as a function of their kinetic energy. The few theoretical calculations available cire in good agreement with these empirical data Only unscattered electrons convey useful information, while scattered electrons contribute to a structureless background (secondary electrons). From Fig. 8, it is clear that photoelectron spectroscopy probes at most a few tens of Angstroms. 

The three pieces of information needed for a kinetic-secondary-electron yield calculation can now be related to the model of Beuhler and Friedman ... 

The electron microscope has a resolution of 10 3-10 4 p. A well-known example of an electron microscope is the TEM, the transmission electron microscope, which is used to study specimens a fraction of a micrometre or less in thickness, e g. for depicting and recognizing clay minerals. Another type of electron microscope is used to depict surfaces and is often applied for ceramics. The surface of a slide is radiated with a beam of electrons. Some electrons are bounced back and due to the collisions of fast electrons secondary electrons are liberated from the surface. In this way you can obtain more information about the surface relief and the chemical composi-tion. The SEM, the scanning electron microscope radiates a surface with a controlled electron beam. In this way a certain part of the surface can be studied. 

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