Auger parameters

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... 

Figure 5 shows such an Auger parameter plot for a series of aluminum compounds. Due to crowding, several values given in Table I are omitted from this plot. Most of the compounds are grouped to the lower left, whereas aluminum metal is at the upper right. Intermediate between these are sodium zeolite and zinc aluminate (ZnAl204). 

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... 

Table II shows representative Auger parameters for several silicon and aluminum species. Both mineral and non-mineral species are shown to give some idea of the range of values. For these calculations, the fused silicon and aluminum 2P3/2 1/2 photoelectron lines are used in conjunction with the KLL Auger lines.

Table II. Representative Auger Parameters for Silicon and Aluminum3>k...

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. 

Data Analysis. Data analysis is, of course, directly related to data acquisition. However, not all good data is or can be completely analyzed. For example, McIntyre (J5) has observed that "a broad base in chemical shift data has been slow in developing" for XPS data. Until such a data base existed, it was difficult for both expert and non-expert to interpret spectra from corrosion products, particularly on complex alloys. The Handbook of X-Ray Photoelectron Spectroscopy (27) and collections of Auger parameter data (32 ) are examples of data compilations very useful to a researcher trying to interpret measurements of corrosion products. 

Rather than the afore defined a parameter, of more convenient use is the so called modified Auger parameter a, defined as the sum of a plus the photon energy. In the following, we shall use this modified Auger parameter. 

At this point of the discussion, we would like just to mention the different behaviour of the modified Auger parameter between the two growth modes the evaporated films exhibit a singularity for both Al and Si KLL transitions around Al thicknesses 10-20 A while the sputtered films show this singularity only for a j with a monotonous increase of the a., parameter. We have no present explanation for these observations, But we want to present them as an experimental evidence for differences in long range electronic properties of these interfaces. 

One feature of these plots is of additional interest. The diagonal grid represents the sum of the Auger kinetic energy and the photoelectron binding energy, which we call the modified Auger Parameter (1 ), ... 

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