Color centers, in alkali halides

Color centers in alkali halide crystals II. Rev. mod. Physics 26, 7 (1954). 

In general, two types of tunable solid state lasers have been developed those based on color centers in alkali halide crystals, and those based on transition metal ions (3d) in a crystalhne host. In both cases, the tunabihty rehes on the large spectral gain profile provided by the active center. 

Figure 6.12 The structures of some typical color centers in alkali halide crystals (such as NaCl). The defects are represented on a plane of the alkali halide crystal. The circles represent the lattice ions and a is the anion-cation distance.

Electron donors and acceptors for reversible redox systems must invariably exhibit at least two stable oxidation states, or the net result will be an irreversible chemical reaction. The donor or acceptor components of the redox system need not be confined to independent atoms, ions, or molecules but could even be imperfections in crystal lattices capable of functioning as electron traps. The well-known color centers in alkali halides are just such acceptor systems. 

Color Centers. Lattice defects in alkali halide crystals provide ideal trapping sites for electrons which in turn cause marked color changes in the system. Symons and Doyle (112) have reviewed the research on color centers in alkali halide crystals to about 1960. In... 

FIGURE 11.7 Schematics of color centers in alkali halides. 

Defects In crystals (e.g., color centers In alkali halides)... 

FIGURE 1 A representative sample of color centers in alkali halide crystals. The large and small circles represent negative and positive ions, respectively. Colored circles represent alkali impurities. 

GouiaryBS, Adrian FJ (1960) Wave functions for electron-excess color centers in alkali halide crystals. Solid State Physics 10 127-247... 

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. 

Fig.5.76a-e. Color centers in alkali halides (a) F-center (b) Fa-center (c) Fb-center (d) F2-center and (e) fJ-center... 

Figure 5.83 Color centers in alkali halides a F-center b pA-center c pB-center d p2-center and e F -center...

Brandt W, Waung HP, Levy PW (1968) Proc Int Symp Color Centers in Alkali Halides, Rome, Italy, 23-27 Sep 1968, p 48... 

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. 

F centers in alkali halides, which result from heating in alkali vapor. An F center in metal halides consists of a halide ion vacancy that has trapped an electron. This process creates additional energy levels between the valence band and the conduction band. A similar situation is found in MgO heated in Mg vapor the F center consists of two electrons trapped at an oxygen vacancy. F centers are one example of so-called color centers. There are other types of possible color centers in ionic materials involving electrons and holes. (See Section 11.9 F is Farben.)... 

F. Liity Fa-Centers in Alkali Halide Crystals . In Physics of Color Centers, ed. by W.B. Fowler (Academic, New York 1968)... 

In solid-state inorganic chemistry. MCD is an important technique for the determination of color centers in alkali-metal halide crystals, rare earths, and highly symmetric compounds such as hexaha-lides of platinum or ruthenium. In organic chemistry, MCD is observed with, e,g.. metal poiphy-rins, pyrimidine ba.ses, and nucleoside derivatives. 

In alkali halide crystals containing color-centers (F-centers) illumination with light of appropriate energy causes transient changes of hardness (Nadeau, 1964). This effect apparently results from changes in the effective sizes of the F-centers when they become excited. 

Recoilless Optical Absorption in Alkali Halides. Recently Fitchen et al (JO) have observed zero phonon transitions of color centers in the alkali halides using optical absorption techniques. They have measured the temperature dependence of the intensity of the zero phonon line, and from this have determined the characteristic temperatures for the process. In contrast to the Mossbauer results, they have found characteristic temperatures not too different from the alkali halide Debye temperatures. 

Very important are associations between ionic and electronic carriers. Examples are color centers formed in alkali halides.18 74 If Na... 

The formation of color centers in the alkali halides, especially silver, has been studied extensively and in great detail, in an attempt to understand the photographic process. At least half a dozen color centers have been identified in these materials, of which the most widely studied is probably the F center, defined as an electron trapped at an anion vacancy. The name comes from the German word for color Farbe. In the case of KBr, the F center (Fig. 16.7) is believed to be an electron trapped at a bromine vacancy. The F center can be modeled by assuming the electron is trapped in a box of side d, which scales with the lattice parameter of the alkali halide. The F center transition is believed to be between the ground and first excited state of this particle in a box. This model, while crude, qualitatively explains the data for some of the alkali halide F center spectra. 

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