Color centers electronic spectra

When electrons or holes are trapped at defects in a solid, both the electronic and the optical properties are modified. Such composite defects are called color centers. Large numbers of these defects have been characterized, and the term includes defects that have absorption maxima outside the visible spectrum. The expression color center is thus used... 

Color centers in alkali halide crystals are based on a halide ion vacancy in the crystal lattice of rock-salt structure (Fig. 5.76). If a single electron is trapped at such a vacancy, its energy levels result in new absorption lines in the visible spectrum, broadened to bands by the interaction with phonons. Since these visible absorption bands, which are caused by the trapped electrons and which are absent in the spectrum of the ideal crystal lattice, make the crystal appear colored, these imperfections in the lattice are called F-centers (from the German word Farbe for color) [5.138]. These F-centers have very small oscillator strengths for electronic transitions, therefore they are not suited as active laser materials. 

Color centers result from electrons trapped in anion vacancies. Those involving single anion vacancies are called F-centers (from Fabre meaning color in German). Two adjacent anion vacancies produce what is known as an M-center, and three such adjacent vacancies is an R-center. The trapped electron has different quantum states that can be excited by photons in the visible spectrum resulting in an intense coloration in an otherwise transparent colorless crystal. The absorption frequency, hence the color, is a characteristic of the material. 

Since most of the carbenes 1 have triplet ground states, ESR spectroscopy allows to see the unpaired electrons and determine the local symmetry at the carbene center and the amount of spin delocalization.13-18 Most of the ESR spectra of carbenes reported in the literature have been recorded in organic glasses or powder samples at temperatures between 4 and 77 K. Many carbenes are slightly colored and exhibit characteristic absorptions extending to the visible region of the spectrum. UV/vis spectroscopy not only provides information on the excited states of carbenes, which in many cases are the reactive species during precursor photolyses, but also links low temperature spectroscopy to LFP in solution at room temperature. 

As has been noted above, [Fe(HB(pz)3)2] undergoes a color change from deep violet to white upon heating, a change that is clearly revealed in its electronic absorption spectrum, see Fig. 4. The 297 K spectrum is dominated by a very intense charge-transfer band centered in the ultraviolet region and a less intense band centered at 19,000 cm-1. These absorptions account for... 

All varieties of color are mainly connected with two main absorption bands in the violet and yellow parts of the spectrum. The secondary bands are also present - in blue and green diapasons. The main absorption bands are connected with F and M-centers. The first one is anion vacancy, which traps electrons and the second is two neighboring anion vacancies with two trapped electrons. The short-wave band in fluorite is generated by mutual absorption of F and M-centers, while the long-wave band is connected with M-center absorption only. In the green varieties the REE (Sm ", Yb and Dy ) are also appreciable. Besides that, the centers O2, O3 and (Y, TR)02 sometimes have influence with resulting yellow and pink colors (Platonov 1979 Krasilschikova et al. 1986). 

The 2Fe2S (S, acid-labile sulfur) ferredoxins have a redox active binuclear center, with each of the two iron atoms attached to the protein by two cysteinyl sulfur ligands and connected by two inorganic acid-labile sulfur ligands. At cty-ogenic temperatures these clusters are EPR detectable, with characteristic features in the vicinity of g = 1.94. Spinach ferredoxin has principal g values of 2.03, 1.96, and 1.88 and a broad absorbance spectrum with a weak maximum around 420 nm, giving these proteins a reddish brown color which bleaches on reduction. Ferredoxins are low potential electron carriers chloroplast ferredoxins function in photosynthetic electron transfer, but related proteins such as adrenal ferredoxin are involved in steroidogenic electron transfer in mitochondria in tissues which produce steroid hormones. 

There is very little optical data on the 0 ion on the surface, mainly because many of the supports are colored and the ion is often unstable. However, MgO provides a substrate with good reflectance properties (see Section VI,B), and the reflectance spectrum of the surface, after N20 has been reacted with electrons trapped at surface centers, shows a broad band centered at 2.0 eV (50). This band is destroyed by the adsorption of oxygen to give a new band attributed to O J. The formation of O and its subsequent reaction with oxygen to give O3 were confirmed by parallel EPR experiments (50). The optical band at 2.0 eV is at energy lower than that observed in the bulk. 

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