ESCA-300

In principle, ESCA can be a most effective technique applied to differentiate between nonclassical carbocations and equilibrating classical species. Hie time scale of the measured ionization process is of the order of 10 s so that definite species are characterized regardless of inherently slower intra- and intermolecular exchange reactions, for example, hydride shifts, Wagner-Meerwein rearrangements, proton exchange, and so on. Technical difficulties associated with such studies of carbocations have made the technique not so versatile. 

This result is contrary to what has been deduced from other studies of the norbornyl ion in strong acid media. Accordingly, we now turn to a reappraisal of the available ESCA, H-nmr, C-nmr, kinetic, Raman, quenching and isotope studies that have been brought to bear on the structural problem. 

The C-ls x-ray electron spectroscopic spectra of the norbornyl ion has recently been reported (Olah et al., 1972). In principle, ESCA appears to provide an ideal method to decide upon the ion s structure, as one would expect a classical ion to exhibit two peaks with an area ratio of 6 1 and the non-classical ion to show two peaks with a 5 2 ratio. 

In preliminary experiments, it was reported that the t-butyl and cyclopentyl cations exhibited ESCA spectra wherein the peak representing the cationic centre was shifted about 4 eV to the high 

The norbomyl spectrum shows a much smaller shift, estimated as 1 7 eV, and this, together with the fact that the area ratio was reported as 5 2, suggested that the non-classical ion was being observed. 

This decision, however, appears to be based on a miscalculation. A closer inspection of the published spectra leads to the conclusion that the area ratio is much closer to 6 1 than 5 2 and therefore it cannot represent the non-classical ion. Thus, if the spectrum is that of a norbomyl cation it must be that of the classical ion and would therefore be consistent with the implications of the exchange study just described. 

Ion gun is used lor sample cleaning or for depth-composition analysis. 

Gate valve isolates introduction chamber from UHV test chamber. 

Turbopump evacuates introduction chamber and differentially pumps the ion gun. 

Oil-free Ion pump provides clean, ultrahigh vacuum pumping. 

Hemispherical analyzer equipped with single channel or position sensitive detector offers excellent sensitivity and energy resolution. 

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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. 

Fig. Vni-11. ESCA spectrum of A1 surface showing peaks for the metal, A1(0), and for surface oxidized aluminum, Al(III) (a) freshly abraided sample (b) sample after five days of ambient temperature air exposure showing increased A1(III)/A1(0) ratio due to surface oxidation. (From Instrument Products Division, E. I. du Pont de Nemours, Co., Inc.)...

ESCA Electron spectroscopy for chemical analysis [106, 138-142] Same as XPS Same as XPS... 

Protein adsorption has been studied with a variety of techniques such as ellipsome-try [107,108], ESCA [109], surface forces measurements [102], total internal reflection fluorescence (TIRE) [103,110], electron microscopy [111], and electrokinetic measurement of latex particles [112,113] and capillaries [114], The TIRE technique has recently been adapted to observe surface diffusion [106] and orientation [IIS] in adsorbed layers. These experiments point toward the significant influence of the protein-surface interaction on the adsorption characteristics [105,108,110]. A very important interaction is due to the hydrophobic interaction between parts of the protein and polymeric surfaces [18], although often electrostatic interactions are also influential [ 116]. Protein desorption can be affected by altering the pH [117] or by the introduction of a complexing agent [118]. 

The composition and chemical state of the surface atoms or molecules are very important, especially in the field of heterogeneous catalysis, where mixed-surface compositions are common. This aspect is discussed in more detail in Chapter XVIII (but again see Refs. 55, 56). Since transition metals are widely used in catalysis, the determination of the valence state of surface atoms is important, such as by ESCA, EXAFS, or XPS (see Chapter VIII and note Refs. 59, 60). 

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]. 

Kolb D M, Rath D L, Wille R and Flansen W N 1983 An ESCA study on the electrochemical double layer of emersed electrodes Ber. Bunsenges. Phys. Chem. 87 1108-11 131... 

It has already been said that the merits of a method for charge calculation can be assessed mainly by its usefulness in modeling experimental data. Charges from the PEOE procedure have been correlated with Cls-ESCA shifts [28], dipole moments [33], and NMR shifts [34], to name but a few. 

Protonated methanes and their homologues and derivatives are experimentally indicated in superacidic chemistry by hydrogen-deuterium exchange experiments, as well as by core electron (ESCA) spectroscopy of their frozen matrixes. Some of their derivatives could even be isolated as crystalline compounds. In recent years, Schmidbaur has pre-... 

Acronyms abound in phofoelecfron and relafed specfroscopies buf we shall use only XPS, UPS and, in Sections 8.2 and 8.3, AES (Auger elecfron specfroscopy), XRF (X-ray fluorescence) and EXAFS (exfended X-ray absorption fine sfmcfure). In addition, ESCA is worth mentioning, briefly. If sfands for elecfron specfroscopy for chemical analysis in which elecfron specfroscopy refers fo fhe various branches of specfroscopy which involve fhe ejection of an elecfron from an atom or molecule. Flowever, because ESCA was an acronym infroduced by workers in fhe field of XPS if is mosf often used to refer to XPS rather than to electron spectroscopy in general. 

Barr, T. L. (1994) Modern ESCA The Principles and Practice of X-ray Photoelectron Spectroscopy, CRC Press, Boca Raton, FL. 

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). 

EID = electron impact desorption ESCA = electron spectroscopy for chemical analysis ESD = electron-stimulated desorption ... 

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