Secondary electrons, information obtained

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. 

Nearly all these techniques involve interrogation of the surface with a particle probe. The function of the probe is to excite surface atoms into states giving rise to emission of one or more of a variety of secondary particles such as electrons, photons, positive and secondary ions, and neutrals. Because the primary particles used in the probing beam can also be electrons or photons, or ions or neutrals, many separate techniques are possible, each based on a different primary-secondary particle combination. Most of these possibilities have now been established, but in fact not all the resulting techniques are of general application, some because of the restricted or specialized nature of the information obtained and others because of difficult experimental requirements. In this publication, therefore, most space is devoted to those surface analytical techniques that are widely applied and readily available commercially, whereas much briefer descriptions are given of the many others the use of which is less common but which - in appropriate circumstances, particularly in basic research - can provide vital information. 

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. 

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. 

The most essential plasma device characteristics that are needed in order to obtain impurity release rates are the fluxes of photons and of charged and neutral particles to the wall. It will be necessary to have detailed information on the energy spectra and fluxes to walls, limiters and beam dumps of thermal electrons and ions, photons, a-particles, runaway electrons, charge exchange neutrals, neutral beam and impurity neutrals and ions. The effects of sheath potentials, secondary electron emission and unipolar arcing need to be included in these calculations. 

The primary electron beam may also be inelastically scattered through interaction with electrons from surface atoms. In this case, the collision displaces core electrons from filled shells e.g, ns (K) or np (L)) the resulting atom is left as an energetic excited state, with a missing inner shell electron. Since the energies of these secondary electrons are sufficiently low, they must be released from atoms near the surface in order to be detected. Electrons ejected from further within the sample are reabsorbed by the material before they reach the surface. As we will see in the next section (re SEM), as the intensity of the electron beam increases, or the density of the sample decreases, information from underlying portions of the sample may be obtained. 

Structural data on the organization of the active clusters and the various subunits can be obtained from biochemical, biophysical and genetic studies. However, the final word is in the mouth of the crystallographer, after the biochemist hands him the crystals. Only one piece of solid information is available to date, and this is the amino acid sequences of subunits la and Ib [78,79]. These two subunits should accommodate the P-700, the primary electron acceptor (Ai), and probably the secondary electron acceptor A2 which is the nonheme iron cluster X. The amino acid sequences of subunits I and I, make it unlikely that these subunits also contain one of the bound ferredoxins A or B because subunit I, contains 4 cysteine residues and subunit I, contains only 2 cysteines [79]. These numbers are hardly sufficient for the formation of the nonheme iron cluster X which is supposed to be a nonheme iron center [74,88]. Therefore, it may be likely that sub-... 

Figure 4.16 Comparison between (a) a secondary electron image and (b) a backscattered electron image for the same area of nickel alloy. Additional compositional information is obtained from the backscattered image. (Reproduced with kind permission of Springer Science and Business Media from J.I. Goldstein et al, Scanning Electron Microscopy and X-ray Microanalysis, 2nd ed., Plenum Press, New York. 1992 Springer Science.)...

Before the chemical identity of the secondary electron acceptor and the reaction mechanism involved were known. Parson obtained some useful information indirectly from spectro-kinetic studies using a double-flash arrangement. Parson used a pair of laser flashes spaced a few microseconds apart to excite the chromatophores of Chromatium vinosum and found that while the first flash elicited photooxidation of P870, the second flash did not cause another photooxidation even though the photooxidized P870 " has been re-reduced by the endogenous, c-type cytochrome within -2 /js and presumably ready to undergo another photooxidation, provided there had been electron transfer from Qa Io Qb, i.e.,... 

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