Auger measurements electron-stimulated

Some topics have been omitted from this review. This holds for the structure and function of metal sites in metalloproteins and metalloenzymes in relation to enzymatic catalysis, for which the reader is referred to Cramer and Hodgson (68) and Doniach et al. (80). Also, chemisorption studies and the structure of adsorbate-covered surfaces are not considered in this review, which deals with XAES in transmission, thus characterizing bulk material. It is noted that even in the case of chemisorbed atoms XANES data analysis requires physically the definition of clusters of considerable size. On the other hand, the analysis is simplified for adsorbed molecules. Very pronounced near-edge effects (usually obtained by electron-stimulated Auger measurements) are observed for low-Z-atom(C, N, O, F>containing chemisorbed... 

The second application is to the direct measurement of adsorption-desorption processes using the Auger peak height of the particular element as a monitor. The principal limitation is the possible influence of the electron beam on the adsorbate, which can result in beam-induced desorption, adsorption or dissociation. The basis of electron-stimulated desorption (ESD) was established some time ago independently be Menzel and Gomer [38] and Redhead [39]. Electron impact causes Franck—Condon transitions of bound electrons in the adsorbed species into excited states. There is, therefore, a probability of dissociation with subsequent desorption of the particular species involved. As an example of these effects on semiconductor surfaces, Joyce and Neave [40] have reported results on silicon, while Ranke and Jacobi [41] have discussed the electron-stimulated oxidation of GaAs. 

Quantitation is usually achieved by comparing the X-ray yields from the sample with yields obtained from standards. The ease with which measurements can be interpreted quantitatively depends on the sample. As illustrated in Fig. 7.7, the volume that is activated by the 10-100 keV electron beam has the shape of a pear with typical dimensions of a few pm. As a consequence, X-rays formed in the interior may be absorbed on their way out, and may stimulate the emission of photoelectrons, Auger electrons and again X-rays. The latter process, secondary fluorescence, can lead to an overestimate of the concentrations. For example, if the specimen is a bulk Fe-Ni alloy, Ni Ka radiation is adsorbed by iron and causes... 

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