Auger electron spectroscopy information obtained

The chemical nature and composition of catalyst surfaces are essential parameters for understanding catalytic reactivity. Electron spectroscopies, mainly Auger Electron Spectroscopy (AES), X-ray Photoelectron Spectroscopy (XPS), Secondary Ion Mass Spectroscopy (SIMS) and Ion Scattering Spectroscopy (ISS) allow such information to be obtained. AES and XPS are most likely to provide meaningful data if the surface region of the solid is homogeneous over a depth several times the inelastic mean free path of the emitted electrons. 

In the study of the surface phases of the Pt-Sn system, as well as of other binary systems, a variety of experimental methods are available. Surface spectroscopies based on ion or electron interaction with the surface provide composition information with a depth resolution that can go from a few atomic layers (X-ray photoelectron spectroscopy, XPS and Auger electron spectroscopy, AES) to single atomic layer resolution. The latter can be obtained by low energy ion scattering (LEIS) a method which has been extensively used for the study ot the Pt-Sn system. Since surface spectroscopic methods are rather well known we will not review them in detail here. 

Secondary ion mass spectrometry (SIMS) is a relatively new technique for surface chemical analysis compared with Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). SIMS examines the mass of ions, instead of energy of electrons, escaped from a solid surface to obtain information on surface chemistry. The term secondary ion is used to distinguish primary ion that is the energy source for knocking out ions from a solid surface. The advantages of SIMS over electron spectroscopy are ... 

The most valuable information about the surface of catalysts can be obtained by the different ESCA methods (electron spectroscopy for chemical analysis). Besides ultraviolet photoelectron spectroscopy (UPS) and Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) is used extensively. Details of results from these methods will be discussed in subsequent subchapters. 

Until recently, analytical investigations of surfaces were handicapped by the lack of suitable methods and instrumentation capable of supplying reliable and relevant information. Electron diffraction is an excellent way to determine the geometric arrangement of the atoms on a surface, but it does not answer the question as to the chemical composition of the upper atomic layer. The use of the electron microprobe (EMP), a powerful instrument for chemical analyses, is unfortunately limited because of its extended information depth. The first real success in the analysis of a surface layer was achieved by Auger electron spectroscopy (AES) [16,17], followed a little later by other techniques such as electron spectroscopy for chemical analysis (ESCA) and secondary-ion mass spectrometry (SIMS), etc. [18-23]. All these techniques use some type of emission (photons, electrons, atoms, molecules, ions) caused by excitation of the surface state. Each of these techniques provides a substantial amount of information. To obtain the optimum Information it is, however, often beneficial to combine several techniques. 

A number of techniques are available to interrogate material surfaces. The development of instrumentation suitable for this type of examination has occured relatively recently, in the last quarter of a century or so. Some of these methods, however, are simply not suitable for polymer analysis, (for example. Auger electron spectroscopy (AES), where the electron beam used as an excitation source is too energetic to avoid damage to organic materials) and all of them have limitations in terms of the information they can provide. By combining techniques which give complementary information one can obtain a detailed description of a polymeric interface. 

Several analytical techniques which can be used to obtain information on the chemical composition of modified surfaces are available (58,59). For example, x-ray photo electron spectroscopy (XPS) can be useful for analysis of thin layers (to depths of 20 A) on substrates. XPS can provide both qualitative and quantitative information on the elements present as well as on their oxidation state, organic structure and bonding information. Auger electron spectroscopy (AES) is a similar technique, but offers only marginal information on the chemical environment of the elements. As for XPS, AES is a highly surface-sensitive technique. It is usually the outermost 2-6 atomic layers which are analysed. These surface-sensitive techniques are very prone to interference from absorbed contaminants. Careful handling of the sample between preparation in the electrochemical cell and the characterization experiment is therefore most important. AES is quantitative only to 50% (60). Electron microprobe analysis (EPMA) provides much more accurate quantitative data. 

Robert Rye has since 1974 been at the Sandla National Laboratories, Albuquerque, and has been actively involved in the surface chemical aspects of the Fusion Power Program, and most recently in the development of the ability to obtain detailed chemical information from Auger electron spectroscopy. Before joining Sandla, he was a member of the Chemistry Department (1968-1974) at Cornell University. He received his Ph.D. degree in 1968 from Iowa State University. He has held several positions in both the Surface Science Division of the AVS and the Colloid and Surface Chemistry Division of the ACS. 

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