Alkali halides, color centers

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. 

Several different types of color center have been identified and some of the common ones are illustrated in Figure 11.7 for the alkali halides. V centers are is cation... 

The alkali halides cire noted for their propensity to form color-centers. It has been found that the peak of the band changes as the size of the cation in the alkali halides increases. There appears to be an inverse relation between the size of the cation (actually, the polarizability of the cation) and the peak energy of the absorption band. These are the two types of electronic defects that are found in ciystcds, namely positive "holes" and negative "electrons", and their presence in the structure is related to the fact that the lattice tends to become charge-compensated, depending upon the type of defect present. 

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. 

When a crystal of an alkali halide has the vapor of the alkali metal passed over it, the alkali halide crystal becomes colored. The reason for this is that a type of defect that leads to absorption of light is created in the crystal. Such a defect is known as an F-center because the German word for "color" is Farbe. It has been shown that such a defect results when an electron occupies a site normally occupied by an anion (an anion "hole"). This arises as a result of the reaction... 

The first experiments that connected color with defects were carried out in the 1920s and 1930s by Pohl, who studied synthetic alkali halide crystals. A number of ways were discovered by which the colorless starting materials could be made to display intense colors. These included irradiation by X rays, electrolysis (with color moving into the crystal from the cathode), or heating the crystals at high temperatures in the vapor of an alkali metal. Pohl was a strict empiricist who did not openly speculate upon the mechanics of color formation, which he simply attributed to the presence of Farbzentren (lit. color centers), later abbreviated to F centers. 

The origin of the color is as follows. The electron trapped at an anion vacancy in an alkali halide crystal is an analog of a hydrogen atom. The electron can occupy one of a number of orbitals, and transitions between some of these levels absorb light and hence endow the solid with a characteristic color. F centers and related defects are discussed further in Chapter 9. 

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

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. 

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. 

So far, we have dealt with optically active centers based on dopant ions, which are generally introduced during crystal growth. Other typical optically active centers are associated with inhinsic lattice defects. These defects may be electrons or holes associated with vacancies or interstitials in ionic crystals, such as the alkali halide matrices. These centers are nsually called color centers, as they prodnce coloration in the perfect colorless crystals. 

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.

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. 

If the levels D and CD in Fig. 29 lie far enough beneath the conduction band (i.e., vd and vcd are sufficiently large), as is the case, for example, with the alkali halide crystals, then the transitions 3 and 4 may be neglected (in the absence of any other ionizing agents besides the temperature), and adsorption processes on the neutral and on the ionized F centers will be independent of each other. In this case the adsorptivity (at small pressures) of a colored crystal (i.e., a crystal containing neutral F centers) will be... 

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. 

COLOR CENTERS. Certain crystals, such as the alkali halides, can be colored by the introduction of excess alkali metal into the lattice, or by irradiation with x-rays, energetic electrons, etc. Thus sodium chloride acquires a yellow color and potassium chloride a blue-violet color. The absorption spectra of such crystals have definite absorption bands throughout the ultraviolet, visible and near-infrared regions. The term color center is applied to special electronic configurations in the solid. The simplest and best understood of these color centers is the F center. Color centers are basically lattice defects that absorb light. 

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... 

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

Quadricyclane in Csl or KBr matrices, prepared by deposition in the salt under conditions that yield single-molecule isolation, is rapidly converted into norbornadiene under conditions that induce color center formation in the alkali halide rapid-growth vapor deposition, or UV or X-ray irradiation. The reaction proceeds only at temperatures at which color centers of the missing electron type (H center) are mobile. At lower temperatures (T < 90 K), UV irradiation of norbornadiene converts it into quadricyclane in the usual fashion (Kirkor et al., 1990). 

---