Auger electron process

Figure 8.29 shows two of the more common processes involved in XRF. Comparison with Figure 8.23 illustrating an Auger electron process shows that the same system for labelling energy levels is used in AES and XRF. 

Two of the techniques appHed in this category are Auger electron process and X-ray fluorescence process. A schematic in Figure 7.5 shows the processes for these two techniques. These two processes compete with one another, and their relative abundance is dependent on the atomic number of the absorber. In general fluorescence is more pronounced for heavier elements, whereas the Auger process is more prominent for lighter elements. 

Figure Bl.25.6. Energy spectrum of electrons coming off a surface irradiated with a primary electron beam. Electrons have lost energy to vibrations and electronic transitions (loss electrons), to collective excitations of the electron sea (plasmons) and to all kinds of inelastic process (secondary electrons). The element-specific Auger electrons appear as small peaks on an intense background and are more visible in a derivative spectrum.

Figure 8.1 Processes occurring in (a) ultraviolet photoelectron spectroscopy (UPS), (b) X-ray photoelectron spectroscopy (XPS) and (c) Auger electron spectroscopy (AES)...

Figure 8.1(c) illustrates schematically the kind of process occurring in Auger electron spectroscopy (AES). The process occurs in two stages. In the first, a high-energy photon ejects an electron from a core orbital of an atom A ... 

Although it is conceptually useful to think of two successive processes following the initial ionization to A, the electron transfer and the generation of the Auger electron occur simultaneously. 

Figure 8.21 The competitive processes of X-ray fluorescence (XRF) and Auger electron emission...

Fig. 14. Schematic of the Auger electron emission process induced by creation of a K level electron hole.

An alternative mechanism of excess energy release when electron relaxation occurs is through x-ray fluorescence. In fact, x-ray fluorescence favorably competes with Auger electron emission for atoms with large atomic numbers. Figure 16 shows a plot of the relative yields of these two processes as a function of atomic number for atoms with initial K level holes. The cross-over point between the two processes generally occurs at an atomic number of 30. Thus, aes has much greater sensitivity to low Z elements than x-ray fluorescence. 

Auger Electrons. The fraction of the holes in an atomic shell that do not result in the emission of an x-ray produce Auger electrons. In this process a hole in the 4h shell is filled by an electron from theyth shell, and the available energy is transferred to a kth shell electron, which in turn is ejected from the atom with a kinetic energy = E — Ej —. Usually, the most intense Auger electron lines are those from holes in the K shell and involve two... 

L-subsheU electrons. For example, assume an initial hole in the K shell is filled by an electron from the subsheU and that the Auger process results in the ejection of an electron from the subsheU. The kinetic energy of the latter electron is then equal to F(K) — F(Lj) — E Ij2 ), and the electron is denoted as a KE E Auger electron. The probabUity of producing a KXY Auger electron from a hole in the K sheU is simply 1 —. ... 

The absorpdon may be monitored via a secondary decay process that is surface-sensitive, such as the emission of Auger electrons, which have a well-defined energy and a short mean ftee path. 

The complete description of the number of Auger electrons that are detected in the energy distribution of electrons coming from a surface under bombardment by a primary electron beam contains many factors. They can be separated into contributions from four basic processes, the creation of inner shell vacancies in atoms of the sample, the emission of electrons as a result of Auger processes resulting from these inner shell vacancies, the transport of those electrons out of the sample, and the detection and measurement of the energy distribution of the electrons coming from the sample. 

The inelastic collision process is characterized by an inelastic mean free path, which is the distance traveled after which only 1/e of the Auger electrons maintain their initial energy. This is very important because only the electrons that escape the sample with their characteristic Auger energy are usefrd in identifying the atoms in... 

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