Surface Color Centers

Iron is associated with silica sand, usually as a light surface stain on the grains. Amber glass develops ionic color centers or complexes of Fe-S-C added to the batch as iron sulfide and powdered anthracite. Although the Fe content be four or five times that shown in the example in Table I, it appears to be bound in the complex so that no greater extraction occurs with the S and C. Titanium is associated with sand as... 

The concept of color centers has been extended to surfaces to explain a number of puzzling aspects of surface reactivity. For example, in oxides such as MgO an anion vacancy carries two effective charges, V(2. These vacancies can trap two electrons to form an F center or one electron to form an F+ center. When the vacancy is located at a surface, the centers are given a subscript s, that is, Fs+ represents a single electron trapped at an anion vacancy on an MgO surface. As the trapping energy for the electrons in such centers is weak, they are available to enhance surface reactions. 

The Fs+ color center in the surface of an MgO crystal is an oxygen vacancy plus ... 

Since its formulation, solid state theory has been concerned also with non-strictly-periodic systems, due principally to the theoretical and technological importance of defects (point impurities, color centers, dislocations, surfaces, etc.). However, most of these theoretical studies and approaches exploit the results of the ideal periodic crystal as the basic ingredient on which to include impurity effects. 

An interesting case of reactivity of color centers is that of the interaction with CO, O2 and N2 molecules. EPR studies have clearly shown that exposure of a MgO surface with an high concentration of F centers to these molecules results in the formation of stable or metastable radical anions [151-153]. The general reaction can be written as ... 

The second mechanism to generate O radicals on the MgO surface implies first the creation of color centers and then their bleaching with N2O ... 

It should be noted that oxygen vacancies at oxide surfaces are of major importance for the physical and chemical behavior of these systems, apart from consequences for catalytic reactivity. As examples we mention point defects at silicon dioxide [130] or the existence of color centers at MgO surfaces which are due to localized electron states at oxygen vacancy sites and have been discussed extensively in the literature [131]. 

Imaging plate A surface that behaves like photographic film and can be used to store X-ray intensities as latent images in the form of color centers. The stored image is scanned by laser light. The plate is erasable and can be used many times. It is more sensitive than photographic film and useful because intensities can be retrieved electronically. 

F center (FC) Color center defect sites (oxygen vacancies) existing on oxide surfaces. 

As explained above, the pottant should contain little or no plasticizer since It can generate electrical problems. The last chemical requirement for the pottant Is that Its melt equilibrium contact angle with all the surfaces to which It bonds be as low as possible below 90°C. This speeds processing as well as maximizing adhesion and minimizing the collection of water and oxygen at the Interfaces to reduce metal corrosion and metal oxide catalyzed polymer changes to form color centers. 

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