Color center: also electron centers

Color from Color Centers. This mechanism is best approached from band theory, although ligand field theory can also be used. Consider a vacancy, for example a missing CF ion in a KCl crystal produced by irradiation, designated an F-center. An electron can become trapped at the vacancy and this forms a trapped energy level system inside the band gap just as in Figure 18. The electron can produce color by being excited into an absorption band such as the E transition, which is 2.2 eV in KCl and leads to a violet color. In the alkaU haUdes E, = 0.257/where E is in and dis the... 

Vo) in the crystal. (Vo) can catch electrons to form F and centers. (Pb) is also able to attract electrons while (Vb)" can trap holes to give rise to color centers. They vdll make a contribution to the X-ray irradiation-induced absorption. Of course, the charge balance of the crystal is kept by charge compensation among these defects. Regretfully, the detailed characterization of these defects is too difficult to cover here and further experiments need to be performed. 

Chapter 6 is devoted to discussing the main optical properties of transition metal ions (3d" outer electronic configuration), trivalent rare earth ions (4f 5s 5p outer electronic configuration), and color centers, based on the concepts introduced in Chapter 5. These are the usual centers in solid state lasers and in various phosphors. In addition, these centers are very interesting from a didactic viewpoint. We introduce the Tanabe-Sugano and Dieke diagrams and their application to the interpretation of the main spectral features of transition metal ion and trivalent rare earth ion spectra, respectively. Color centers are also introduced in this chapter, special attention being devoted to the spectra of the simplest F centers in alkali halides. 

Luminescence of Lattice Defects. Many defect centers are known in the case of the alkali-metal halides, which are derived from electrons in anion vacancies (F-centers, or color centers). Association of two or more F-centers gives new defect centers, which can each also take up an electron. These lattice defects act as luminescence centers, the emission spectra of which sometimes exhibit a large number of lines. 

A color center (marked e ) in a sodium halide crystal. Note that the electronic position is an anionic site. Also, for simplicity, anions are not shown here. 

Fig. 3. (A) Arrangement of pigment molecules and electron-transfer cofactors in the PS-1 reaction center, viewed along the membrane plane. Numerical values are distances in A. (B) stereo view ofthe same pigment and cofactor molecules as in (A). Both figures adapted from Schubert, Klukas, KrauB, Saenger, Fromme and Witt (A) (1995) Present state of the crystal structure analysis of photosystem I at 4.5 A resolution. In P Mathis (ed) Photosynthesis From Light to Biosphere, II 5. Kluwer (B) (1997) Photosystem I of Synechococcus elongatus at 4 A resolution comprehensive structure analysis. J Mol Biol 272 p 756. Also see Color Plate 10 for a color rendition ofthe electron-density map of (A).

Fig. 2. Spectroscopy of electrons in fluids. Ensemble shows absorption maxima and coefficients in n-alcohols at picosecond (A ) and nanosecond (A) times, diols (B), amines (C), ethers (D), alkanes ( ), and color center (F). The absorption (F ) and stimulated (Fg) emission in KBr color center laser are also shown.

Purple Cua centers display an intense purple color with strong electronic absorption bands around 480 (e ax--5,000 M cm ) and 530 nm (e ax--4,000 M cm l (Table 3).25-28,98-ioi Another moderately strong absorption around 800 nm (e ax" F600M cm ) has also been observed. They are found in cytochrome c oxidase a terminal oxidase in the... 

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