Auger technique

Figure 5.29. An energy level diagram showing the physical basis of the Auger technique.

Because many environmental particles are poor electrical conductors, charging produced by the incident electron beam is a major analytical concern in EMP and SAM (11,14). Because EMP analysis is not very surface specific, the sample surface is normally coated with a thin film of a low Z conducting material such as C. The surface specificity of the Auger technique generally precludes the use of surface coatings, and electrical charging of the particles may be exceedingly difficult to overcome experimentally (11, 14). 

As with ESCA, Auger analysis can detect all elements heavier than helium. The quantitative presentation shown in Figure 25 can only be done when suitable standards are available. Since the Auger spectra depend secondarily on how the atom is bound in the thin film, it is not always possible to secure accurate standards so that absolute determination of elemental concentrations can be inaccurate. Also, the Auger technique is not very sensitive, and in this regard, is similar to ESCA. Concentrations of an element less than 0.5 atomic % cannot be predicted reliably. 

Another argument is based on mean free paths of electrons in Auger spectra of He II spectra [47]. In these spectra, the mean free path of electrons is about 3-4 A, while in the case of 4f electrons released due to Mg K< radiations is about 17-18 A. Thus the Auger technique senses the electrons in the outermost two monolayers while XPS is sensitive to about seven monolayers and in such a case, surface stabilization of the divalent state may very well be the case in accordance with the experimental findings [47]. 

Auger techniques have also proven useful in the characterization of electrochemically induced changes in thin-film electrodes. An example concerns films (500-2000 A thick) of magnesium phthalocyanine (MgPc) deposited over a gold contact layer on a glass substrate (106) ... 

If monochromatic X-rays are used as the ionizing radiation the experimental technique is very similar to that for XPS (Section 8.1.1) except that it is the kinetic energy of the Auger electrons which is to be measured. Alternatively, a monochromatic electron beam may be used to eject an electron. The energy E of an electron in such a beam is given by... 

X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (xps) and Auger electron spectroscopy (aes) are related techniques (19) that are initiated with the same fundamental event, the stimulated ejection of an electron from a surface. The fundamental aspects of these techniques will be discussed separately, but since the instmmental needs required to perform such methods are similar, xps and aes instmmentation will be discussed together. 

Auger electron spectroscopy (AES) is a technique used to identify the elemental composition, and in many cases, the chemical bonding of the atoms in the surface region of solid samples. It can be combined with ion-beam sputtering to remove material from the surface and to continue to monitor the composition and chemistry of the remaining surface as this surface moves into the sample. It uses an electron beam as a probe of the sample surface and its output is the energy distribution of the secondary electrons released by the probe beam from the sample, although only the Ai er electron component of the secondaries is used in the analysis. 

Auger electron spectroscopy is the most frequently used surface, thin-film, or interface compositional analysis technique. This is because of its very versatile combination of attributes. It has surface specificity—a sampling depth that varies... 

---