Electron spectroscopy of chemical analysis

Surface Chemical Analysis. Electron spectroscopy of chemical analysis (ESCA) has been the most useful technique for the identification of chemical compounds present on the surface of a composite sample of atmospheric particles. The most prominent examples Include the determination of the surface chemical states of S and N in aerosols, and the investigation of the catalytic role of soot in heterogeneous reactions involving gaseous SO2, NO, or NH3 (15, 39-41). It is apparent from these and other studies that most aerosol sulfur is in the form of sulfate, while most nitrogen is present as the ammonium ion. A substantial quantity of amine nitrogen also has been observed using ESCA (15, 39, 41). 

During a considerable long period, the gaseous chemisorption method is the sole one to probe the surface species of solid catalysts. Some of the modern techniques for surface measuring are Measurements of effusion works, Auger electron spectroscopy (AES), Electron spectroscopy of chemical analysis (ESC A), X-ray photoelectron spectroscopy (XPS), Electron probe microanalysis (EPMA), Ion probe microanalysis (IPM), Ion scattering spectroscopy (ISS), Second ion mass spectroscopy (SIMS), Low energy electron diffraction (LEED), Vibration spectrum and Mossbauer spectroscopy etc. All these techniques provide favorable conditions for the surface research indepth. 

Fig. 9.1 The schematic of first liquid electron spectroscopy of chemical analysis (ESCA) designed by the Siegbahn group. Reproduced from refs. [18-20]...

Madey and co-workers followed the reduction of titanium with XPS during the deposition of metal overlayers on TiOi [87]. This shows the reduction of surface TiOj molecules on adsorption of reactive metals. Film growth is readily monitored by the disappearance of the XPS signal from the underlying surface [88, 89]. This approach can be applied to polymer surfaces [90] and to determine the thickness of polymer layers on metals [91]. Because it is often used for chemical analysis, the method is sometimes referred to as electron spectroscopy for chemical analysis (ESCA). Since x-rays are very penetrating, a grazing incidence angle is often used to emphasize the contribution from the surface atoms. 

X-ray photoelectron spectroscopy (XPS), also called electron spectroscopy for chemical analysis (ESCA), is described in section Bl.25,2.1. The most connnonly employed x-rays are the Mg Ka (1253.6 eV) and the A1 Ka (1486.6 eV) lines, which are produced from a standard x-ray tube. Peaks are seen in XPS spectra that correspond to the bound core-level electrons in the material. The intensity of each peak is proportional to the abundance of the emitting atoms in the near-surface region, while the precise binding energy of each peak depends on the chemical oxidation state and local enviromnent of the emitting atoms. The Perkin-Elmer XPS handbook contains sample spectra of each element and bindmg energies for certain compounds [58]. 

Siegbahn, K., Nordling, C., Fahlman, A., Nordberg, R., Hamerin, K., Hedman, J., Johansson, G., Bergmark, T., Karlsson, S.-E., Lindgren, I. and Lindberg, B. (1967) Electron Spectroscopy for Chemical Analysis Atomic, Molecular, and Solid State Structure Studies by Means of Electron Spectroscopy, Almqvist and Wiksells, Uppsala. 

Other techniques in which incident photons excite the surface to produce detected electrons are also Hsted in Table 1. X-ray photoelectron Spectroscopy (xps), which is also known as electron spectroscopy for chemical analysis (esca), is based on the use of x-rays which stimulate atomic core level electron ejection for elemental composition information. Ultraviolet photoelectron spectroscopy (ups) is similar but uses ultraviolet photons instead of x-rays to probe atomic valence level electrons. Photons are used to stimulate desorption of ions in photon stimulated ion angular distribution (psd). Inverse photoemission (ip) occurs when electrons incident on a surface result in photon emission which is then detected. 

One other very important attribute of photoemitted electrons is the dependence of their kinetic energy on chemical environment of the atom from which they originate. This feature of the photoemission process is called the chemical shift of and is the basis for chemical information about the sample. In fact, this feature of the xps experiment, first observed by Siegbahn in 1958 for a copper oxide ovedayer on a copper surface, led to his original nomenclature for this technique of electron spectroscopy for chemical analysis or esca. 

X-rays provide an important suite of methods for nondestmctive quantitative spectrochemical analysis for elements of atomic number Z > 12. Spectroscopy iavolving x-ray absorption and emission (269—273) is discussed hereia. X-ray diffraction and electron spectroscopies such as Auger and electron spectroscopy for chemical analysis (esca) or x-ray photoelectron spectroscopy are discussed elsewhere (see X-raytechnology). 

Each type of mass spectrometer has its associated advantages and disadvantages. Quadrupole-based systems offer a fairly simple ion optics design that provides a certain degree of flexibility with respect to instrument configuration. For example, quadrupole mass filters are often found in hybrid systems, that is, coupled with another surface analytical method, such as electron spectroscopy for chemical analysis or scanning Auger spectroscopy. 

X-ray photoelectron spectroscopy (XPS) is currently the most widely used surface-analytical technique, and is therefore described here in more detail than any of the other techniques. At its inception hy Sieghahn and coworkers [2.1] it was called ESCA (electron spectroscopy for chemical analysis), hut the name ESCA is now considered too general, because many surface-electron spectroscopies exist, and the name given to each one must be precise. The name ESCA is, nevertheless, still used in many places, particularly in industrial laboratories and their publications. Briefly, the reasons for the popularity of XPS are the exceptional combination of compositional and chemical information that it provides, its ease of operation, and the ready availability of commercial equipment. 

Lithium foil is commercially available. Its surface is covered with a "native film" consisting of various lithium compounds [Li0H,Li20,Li3N, (Li20-C02) adduct, or Li2C03], These compounds are produced by the reaction of lithium with 02, H20, C02, or N2. These compounds can be detected by electron spectroscopy for chemical analysis (ESCA) [2], As mentioned below, the surface film is closely related to the cycling efficiency. 

Other techniques utilize various types of radiation for the investigation of polymer surfaces (Fig. 2). X-ray photoelectron spectroscopy (XPS) has been known in surface analysis for approximately 23 years and is widely applied for the analysis of the chemical composition of polymer surfaces. It is more commonly referred to as electron spectroscopy for chemical analysis (ESCA) [22]. It is a very widespread technique for surface analysis since a wide range of information can be obtained. The surface is exposed to monochromatic X-rays from e.g. a rotating anode generator or a synchrotron source and the energy spectrum of electrons emitted... 

The information contained in ESCA (Electron Spectroscopy for Chemical Analysis) spectra [331] makes the method particularly suitable for determinations of surface compositions, chemical bonding of surface atoms and changes which occur at solid surfaces during reaction [312], Applications of this technique to the study of reactions of and between solids are awaited with interest. 

X-ray scattering studies at a renewed pc-Ag/electrolyte interface366,823 provide evidence for assuming that fast relaxation and diffu-sional processes are probable at a renewed Sn + Pb alloy surface. Investigations by secondary-ion mass spectroscopy (SIMS) of the Pb concentration profile in a thin Sn + Pb alloy surface layer show that the concentration penetration depth in the solid phase is on the order of 0.2 pm, which leads to an estimate of a surface diffusion coefficient for Pb atoms in the Sn + Pb alloy surface layer on the order of 10"13 to lCT12 cm2 s i 820 ( p,emicai analysis by electron spectroscopy for chemical analysis (ESCA) and Auger ofjust-renewed Sn + Pb alloy surfaces in a vacuum confirms that enrichment with Pb of the surface layer is probable.810... 

X-ray photoelectron spectroscopy (XPS), which is synonymous with ESCA (Electron Spectroscopy for Chemical Analysis), is one of the most powerful surface science techniques as it allows not only for qualitative and quantitative analysis of surfaces (more precisely of the top 3-5 monolayers at a surface) but also provides additional information on the chemical environment of species via the observed core level electron shifts. The basic principle is shown schematically in Fig. 5.34. 

A relatively new arrangement for the study of the interfacial region is achieved by so-called emersed electrodes. This experimental technique developed by Hansen et al. consists of fully or partially removing the electrode from the solution at a constant electrical potential. This ex situ experiment (Fig. 9), usually called an emersion process, makes possible an analysis of an electrode in an ambient atmosphere or an ultrahigh vacuum (UHV). Research using modem surface analysis such as electron spectroscopy for chemical analysis (ESCA), electroreflectance, as well as surface resistance, electrical current, and in particular Volta potential measurements, have shown that the essential features (e.g., the charge on... 

The effects of tin/palladium ratio, temperatnre, pressnre, and recycling were studied and correlated with catalyst characterization. The catalysts were characterized by chemisorption titrations, in situ X-Ray Diffraction (XRD), and Electron Spectroscopy for Chemical Analysis (ESCA). Chemisorption studies with hydrogen sulfide show lack of adsorption at higher Sn/Pd ratios. Carbon monoxide chemisorption indicates an increase in adsorption with increasing palladium concentration. One form of palladium is transformed to a new phase at 140°C by measurement of in situ variable temperature XRD. ESCA studies of the catalysts show that the presence of tin concentration increases the surface palladium concentration. ESCA data also indicates that recycled catalysts show no palladium sulfide formation at the surface but palladium cyanide is present. 

We shall concern ourselves here with the use of an X-ray probe as a surface analysis technique in X-ray photoelectron spectroscopy (XPS) also known as Electron Spectroscopy for Chemical Analysis (ESCA). High energy photons constitute the XPS probe, which are less damaging than an electron probe, therefore XPS is the favoured technique for the analysis of the surface chemistry of radiation sensitive materials. The X-ray probe has the disadvantage that, unlike an electron beam, it cannot be focussed to permit high spatial resolution imaging of the surface. 

XPS or ESCA (electron spectroscopy for chemical analysis) is a surface sensitive technique that only probes the outer atomic layers of a sample. It is very useful tool to study polymer surfaces [91]. An XPS spectrum is created by focusing a monochromatic beam of soft (low-energy) X-rays onto a surface. The X-rays cause electrons (photoelectrons) with characteristic energies to be ejected from an electronic core level. XPS, which may have a lateral resolution of ca. 1-10 pm, probes about the top 50 A of a surface. 

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