AES and SAM

2 AES and SAM Auger Electron Spectroscopy (AES) is a technique capable of providing the elemental composition of the outer l-5 nm from any solid, although insulators are difficult, to detection limits down to 0.5% (detection limits element dependent) with some speciation information also available. All elements from Li to U are detectable. This does so by directing a 5-30-keV electron beam at the solid of interest. This induces valence and core electron emission 

Scanning Auger Microscopy (SAM) is the scanning form of AES, i.e. the highly focused 5-30-keV electron beam is scanned across the surface as this allows for the lateral distribution of elements present on the surface to be mapped. Spatial resolution can extend to 10 nm or better. AES is similar to XPS in that both provide similar information using similar instrumentation. The primary difference lies in the fact that AES provides a superior spatial resolution, but at the cost of sensitivity (the sensitivity of XPS is slightly better). 

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Advantages of silicon x-radiation include the access of aluminum and magnesium core level (Is) lines and the corresponding (KLL) Auger transitions for chemical state identification and improved quantitation, because these lines are at least 10 times more intense than the corresponding (2p) or (2s) lines. The construction of an off-axis reactor has produced a simple, versatile and inexpensive system easily adapted to any vacuum system. The role of AES and SAM in catalyst research will also be highlighted by examples. 

AES and SAM studies probably are seen in the scientific literature more frequently than any other technique for applied surface analysis. However, SAM is, in almost all cases, so destructive to polymeric systems that meaningful analyses are particularly difficult to make. In addition, the information content is low compared to ESCA. With some exceptions, one can only determine which elements are present. SAM can be valuable for non-polymeric biomaterials, and has been used successfully in many studies, particularly where its high spatial resolution could be exploited. 

If the incident beam in an Auger spectrometer is rastcred over the specimen surface, the strength of a particular Auger transition can be used to generate an elemental map of the surface. SAM has the high surface sensitivity characteristic of AES and similar elemental sensitivity and spatial resolution (down to about 100 nm) [37]. 

Despite the prevalence of the radical SAM proteins, until recently little was understood regarding the mechanism(s) by which these reactions proceed. Early work, including label transfer studies on PFL-AE and LAM, implicated an AdoMet-derived 5 -deoxyadenosyl radical as an intermediate in catalysis. The 5 -deoxyadenosyl radical, generated by reductive cleavage of SAM (Figure 2), is now thought to be a common intermediate responsible for H-atom abstraction from substrates in the diverse reactions catalyzed by the... 

Figure 5 Coordination of SAM to the unique site of the site-differentiated radical SAM cluster, as first elucidated by ENDOR studies on PFL-AE and LAM.

Pyruvate formate-lyase activating enzyme is the member of the radical-SAM family whose cluster properties are most similar to those of aconitase. The cluster in pyruvate formate-lyase activating enzyme is quite labile, and in fact until 1997 it was not known that the enzyme contained an iron-sulfur cluster, as all preparations to that time had been done aerobically, under which conditions the cluster falls apart. It was initially reported that PFL-AE contained a mixture of [2Fe-2S] and [4Fe-4S] clusters, and subsequent reconstitution studies of the apo enzyme provided evidence for a [4Fe-4S] cluster. Further studies showed that anaerobic isolation resulted in purification of a form of PFL-AE that contained primarily [3Fe-4S] clusters, which upon reduction converted to [4Fe-4S] clusters.This reductive cluster conversion from [3Fe S] to [4Fe-4S] clusters even in the absence of added iron was remarkably reminiscent of aconitase (see Section 8.27.2.2), and suggested a labile cluster site. Adding to the similarity to aconitase, Mossbauer spectroscopy provided evidence for a linear [3Fe-4S] cluster in PFL-AE isolated under appropriate conditions.Therefore all of the cluster forms previously identified in aconitase were also found in PFL-AE, and like aconitase it appeared to be relatively simple to interconvert between these cluster forms. 

Laser ionization mass spectrometry or laser microprobing (LIMS) is a microanalyt-ical technique used to rapidly characterize the elemental and, sometimes, molecular composition of materials. It is based on the ability of short high-power laser pulses (-10 ns) to produce ions from solids. The ions formed in these brief pulses are analyzed using a time-of-flight mass spectrometer. The quasi-simultaneous collection of all ion masses allows the survey analysis of unknown materials. The main applications of LIMS are in failure analysis, where chemical differences between a contaminated sample and a control need to be rapidly assessed. The ability to focus the laser beam to a diameter of approximately 1 mm permits the application of this technique to the characterization of small features, for example, in integrated circuits. The LIMS detection limits for many elements are close to 10 at/cm, which makes this technique considerably more sensitive than other survey microan-alytical techniques, such as Auger Electron Spectroscopy (AES) or Electron Probe Microanalysis (EPMA). Additionally, LIMS can be used to analyze insulating sam-... 

A system has been constructed which allows combined studies of reaction kinetics and catalyst surface properties. Key elements of the system are a computer-controlled pilot plant with a plug flow reactor coupled In series to a minireactor which Is connected, via a high vacuum sample transfer system, to a surface analysis Instrument equipped with XFS, AES, SAM, and SIMS. When Interesting kinetic data are observed, the reaction Is stopped and the test sample Is transferred from the mlnlreactor to the surface analysis chamber. Unique features and problem areas of this new approach will be discussed. The power of the system will be Illustrated with a study of surface chemical changes of a Cu0/Zn0/Al203 catalyst during activation and methanol synthesis. Metallic Cu was Identified by XFS as the only Cu surface site during methanol synthesis. 

The application of surface analytical techniques, most notably X-ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES), or its spatially resolved counterpart. Scanning Auger Microanalysis (SAM), is of great value in understanding the performance of a catalyst. However, the results obtained from any of these techniques are often difficult to interpret, especially when only one technique is used by itself. 

XPS spectra were obtained using a Perkin-Elmer Physical Electronics (PHI) 555 electron spectrometer equipped with a double pass cylindrical mirror analyzer (CMA) and 04-500 dual anode x-ray source. The x-ray source used a combination magnesium-silicon anode, with collimation by a shotgun-type collimator (1.). AES/SAM spectra and photomicrographs were obtained with a Perkin-Elmer PHI 610 Scanning Auger Microprobe, which uses a single pass CMA with coaxial lanthanum hexaboride (LaBe) electron gun. 

SAM is a combination of the techniques of SEM and AES an electron beam of high energy (3-10 keV) is scanned over the surface and the electrons excited from the... 

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