The Auger Parameter

Section 2.1.3 shows that in an XPS spectrum. X-ray excited Auger peaks are often as prominent as the photoelectron peaks themselves. For many elements, the chemical shifts in Auger peaks are actually greater than the shifts in photoelectron peaks. The two shifts can be combined in a very useful quantity called the Auger parameter a, first used by Wagner [2.30] and defined in its modified form [2.31] as 

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It is well established that the Auger parameter is a very useful concept, which is not affected by the reference level used in the analysis of the data (the Fermi or the vacuum level) [75]. Moreover, the Auger parameter, being a difference between two peaks recorded on the same energy scale, does not depend on surface charging. It appears that Auger parameter measurements are very useful... 

While the Auger parameter can be expressed in several ways, one of the most commonly used definitions is that shown in Equation 1... 

Figure 19. Wagner plot for Cul-xZnxFe204 catalysts from the Cu 2p3/2 core level and Auger spectral data. Data points for standard compounds (Cu, Cu20 and CuO) are given by solid gray triangles and fresh catalyst by solid squares. Spent catalysts with x = 0.05 and 0.25 composition indicated by open diamond and x = 0.5 and 0.75 by solid circles, respectively. The Auger parameter is described by solid straight line with a slope of-1. Reprinted from Journal of Catalysis, 241, Vijayaraj M., et al., 2006, 83-95 with permission from Elsevier.

It should be pointed out that the chemical shifts observed in AES are not usually the same as in XPS for the same atoms in the same chemical state. Often, they are larger because of the two-hole nature of the final state in the Auger process. The difference between the XPS and Auger chemical shift has been termed the Auger parameter (17) and is an additional useful guide to the chemical state of the atom concerned. 

This basicity concept (further discussed in Chapter 8) may be quantified by study of the spectra of probe ions such as Pb +, leading to values of optical basicity. These optical basicities can be correlated with polarizabilities, either macroscopic optical polarizabilities or the molecular-level polarizabilities, which influence the Auger parameter. 

West, R. H., and J. E. Castle (1982). The correlation of the Auger parameter with refractive index an XPS study of silicates using Zr La radiation. Surf. Interface Anal. 4, 68-75. 

The possibility that these changes may be associated to final state effects has been ruled out by Griinert et al. [29] on the basis of a set of measurements of the Auger parameters a reported in Table 2. 

Fig. 4.27. Integrated O Is and N Is XPS areas for four different samples, a native oxide on the Si3N4 membrane, a low-power (SOW) plasma oxidation, a high-power (250 W) plasma oxidation, and a 0.4- xm thick wet-oxidized Si wafer (Si02). The inset table gives the Auger parameter for all four samples. See text for further...

Wagner has also developed a two-dimensional chemical state plot (24-25) that makes use of the Auger parameter to distinguish between compounds of an element that have the same, or nearly the same, binding energy. In cases where the XPS chemical shift alone is not enough to determine the chemical state, these plots or the Auger parameter by itself can be used to help identify the type of compounds present In the surface. 

Table III shows the results of the resolution of all carbon and oxygen species on the 13 samples studied. Values in the table are based on the overall atomic concentration of the two elements in the sample ratioed to the fractional area contribution of each component to the region. The expected error in the resolution result is anticipated to be less than 10% of the value of each species. Continued fitting of the data did not yield a significantly better result. Based on the Auger parameter listed in Table IV for all the samples, Wagner s measurement of the Auger parameter for cellulose,(13) and Gray s work on cellulose fibers,(6, 7, 8) it is possible to assign the prime constituent of all the samples as being anhydroglucose (cellulose or starch).

Tables 29 and 30 show activity and selectivity data as well as dispersion of copper on both Mn- and Ti-based perovskites. The dispersion of Cu was calculated from CO adsorption at 77 K assuming a CO Cu= 1 1 stoichiometry. A close look at the data reported in these tables indicate that neither the nature of the B cation nor the prereduction in H2 affect the catalytic behavior of the perovskites. Thus, the main conclusion from this work is that the active site is directly associated with the presence of surface copper species. To ascertain the oxidation state of Cu, Rodriguez-Ramos et al. (1991) resorted to the use of X-ray photoelectron spectroscopy. Figure 31 clearly shows that both the prereduced and used catalyst contain either Cu+ or Cu° or both types of reduced species. This technique does not allow us to discriminate between Cu° and Cu+ but a closer look at the Auger parameter corresponding to the L3M4 5M4 5 transition undoubtedly confirms that only Cu° is present in used catalysts. Furthermore, quantitative XPS measurements indicate that some migration of copper from the subsurface to the topmost layer seems to occur on prereduced catalysts and/or used catalysts.

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