Species, electron spectroscopy

Most techniques in chemistry are limited to determining properties of the electronic ground state of a species. Electronic spectroscopy is one of the few techniques that directly probes excited states. 

Electronic spectra of surfaces can give information about what species are present and their valence states. X-ray photoelectron spectroscopy (XPS) and its variant, ESC A, are commonly used. Figure VIII-11 shows the application to an A1 surface and Fig. XVIII-6, to the more complicated case of Mo supported on TiOi [37] Fig. XVIII-7 shows the detection of photochemically produced Br atoms on Pt(lll) [38]. Other spectroscopies that bear on the chemical state of adsorbed species include (see Table VIII-1) photoelectron spectroscopy (PES) [39-41], angle resolved PES or ARPES [42], and Auger electron spectroscopy (AES) [43-47]. Spectroscopic detection of adsorbed hydrogen is difficult, and... 

Transient species, existing for periods of time of the order of a microsecond (lO s) or a nanosecond (10 s), may be produced by photolysis using far-ultraviolet radiation. Electronic spectroscopy is one of the most sensitive methods for detecting such species, whether they are produced in the solid, liquid or gas phase, but a special technique, that of flash photolysis devised by Norrish and Porter in 1949, is necessary. 

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. 

Transmission electron microscopy (TEM) The dispersion of cobalt oxide species on the titania supports were determined using a JEOL-TEM 200CX transmission electron spectroscopy operated at 100 kV with 100k magnification. 

A nickel-promoted C—S bond cleavage has been reported,860 which occurs when solutions of the Ni1 complex of (330) are electrogenerated. The product was identified by cyclic voltammetry and spectroscopy as [Ni(C6H4S2)2]2. EPR and NMR evidence suggests a one-electron mechanism, involving reduction to a 19-electron Ni1 species, electron transfer, and concomitant C—S bond cleavage, extrusion of ethylene followed by a further one-electron reduction and extrusion of ethylene sulfide. 

Throughout this section on pressure transients we have emphasized electron spectroscopy as a procedure for directly detecting surface species, and, with difficult calibration, their concentration. It is important to keep in mind that the detection limit for these is about 0.01 of a monolayer. Using flash desorption as a complementary technique this limit can be extended to 0.001 monolayer in certain cases. The fact remains that extremely labile chemisorbed species may be present in kinetically important but undetectable concentrations. Since residence times as short as 2 x 10 - seconds can be determined, molecular beam techniques, as described below, afford an alternative but indirect method of measuring the properties of these very reactive species. 

B.M. If all the Ti ions in the sample had formed superoxo species upon interaction with H2O2, the effective magnetic moment should have been 1.73-1.78 B.M. The concentration of superoxo-Ti species is, thus, about 45% of the total Ti, comparable to the values found by EPR (55%) and electronic spectroscopies. The remaining fraction is, presumably, the diamagnetic hydroperoxo-/peroxo-Ti species. 

Polymer films were produced by surface catalysis on clean Ni(100) and Ni(lll) single crystals in a standard UHV vacuum system H2.131. The surfaces were atomically clean as determined from low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Monomer was adsorbed on the nickel surfaces circa 150 K and reaction was induced by raising the temperature. Surface species were characterized by temperature programmed reaction (TPR), reflection infrared spectroscopy, and AES. Molecular orientations were inferred from the surface dipole selection rule of reflection infrared spectroscopy. The selection rule indicates that only molecular vibrations with a dynamic dipole normal to the surface will be infrared active [14.], thus for aromatic molecules the absence of a C=C stretch or a ring vibration mode indicates the ring must be parallel the surface. 

The elemental composition, oxidation state, and coordination environment of species on surfaces can be determined by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) techniques. Both techniques have a penetration depth of 5-20 atomic layers. Especially XPS is commonly used in characterization of electrocatalysts. One common example is the identification and quantification of surface functional groups such as nitrogen species found on carbon-based catalysts.26-29 Secondary Ion Mass spectrometry (SIMS) and Ion Scattering Spectroscopy are alternatives which are more surface sensitive. They can provide information about the surface composition as well as the chemical bonding information from molecular clusters and have been used in characterization of cathode electrodes.30,31 They can also be used for depth profiling purposes. The quantification of the information, however, is rather difficult.32... 

Electronic spectroscopy has been employed to study substitution reactions of sulfoxide complexes. An interesting example (104) is the reaction of [Fe(0-Me2S0) P+ with chloride ion. Addition of one equivalent of chloride ion to a Me2SO solution of [Fe(0-Me2S0)6P+ causes a change in spectrum, but further additions have no effect. Comparisons with known compounds indicate that [Fe(0-Me2S0)5Cip+ is the major species in solution. 

X-ray photoelectron spectroscopy (also called electron spectroscopy for chemical analysis, or ESCA) is a surface technique that can be used to detect elements qualitatively (and quantitatively, in some cases) in the surface layers of solids, as well as the chemical states (species) of the elements. The basic experimental apparatus for performing XPS studies includes an x-ray source (most commonly,... 

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