Electronic state oxide materials

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]. 

The classical geochemical material balance (12) assumes that the balance and the electron balance (oxidation state) ki our environment have been estabhshed globally by the kiteraction of primary (igneous) rocks with volatile substances (Table 7). 

In the case of polyaniline (see Fig. 26.4), the situation is more complicated, since polymerization occurs across the basic nitrogen atom, the electron state of which depends on pH. Thus, doping is possible not only by oxidation but also by a pH change conduction of the material rises by 9 to 10 orders of magnitude between pH 5 and 1 ( 1M aqueous HCl). 

Oxide materials in the colored state are usually mixed-valence compounds with a variable range of composition. The color usually arises from low-energy intraband electronic transitions. 

Spectroelectrochemical analysis of charge-insertion nanostructured materials already offers important insight into these systems. These methods were recently exploited to characterize the electrochemical processes of nanostructured manganese oxide ambi-gel and xerogel films. " 6-229 Spectroelectrochemical measurements were used to corroborate electronic state changes with the observed electrochemical response for the insertion of small cations (Li+, Mg2+) and the unexpected insertion of a bulky organic cation (tetrabutylammonium). Vanadium pentoxide exhibits two distinct electrochromic features that can be assigned to the transition at either sto-... 

In the chapter, we have illustrated some results of photoelectron spectroscopy on two classes of actinide materials, elemental metals and oxides, which we thought particularly relevant as they represent metallic and almost completely ionic bonding. Our interest having been focused on the localization vs. itineracy problem of the 5 f states, as well as on their hybridization with other electron states, we have particularly concentrated on those results which could throw light on these two aspects. 

Although direct excited-state electron transfer from 2PA dyes to monomer is successful for polymerizing acrylates and depositing silver, few other materials can be patterned in the same way for effective initiation, the reduction potential for the monomer, V2(M/M- ), needs to be greater, i.e. less negative, than the excited-state oxidation potential for the initiator, E /2(M+/M ), which can be estimated from... 

According to International Union of Pure and Applied Chemistry (IUPAC), the terms speciation and chemical species should be reserved for the forms of an element defined as to isotopic composition, electronic or oxidation state and/or complex or molecular structure (Templeton el al, 2000). This classical definition, appropriate to speciation in solution samples, would exclude most speciation studies on solid materials, such as soils and sediments, more properly defined as fractionation studies. The terminology used in this chapter is based on the broader definition of speciation given by Ure and Davidson (2002), which encompass the IUPAC s narrow definition and includes the selective extraction and fractionation techniques of solid samples. 

The electronic transitions probed by x-ray absorption spectroscopy involve the excitation of a core electron into either unoccupied bound electron states near the Fermi level of the material or at higher energies into the continuum of states producing a photoelectron. These electronic excitations must obey spectroscopic selection rules and thus can provide information about the symmetry of an atom s environment, its oxidation state, and sometimes, with the assistance of comprehensive theoretical calculations, details about the geometry of ligands and other nearby atoms. This information is derived from excitations into bound states and low lying resonances above the Fermi level and is referred to as the x-ray absorption near edge structure (XANES). 

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