Electron color centers

Color Centers. Characteristics of a color center (1,3,7) include production by irradiation and destmction by heating. Exposure to light or even merely time in the dark may be sufficient to destroy these centers. Color arises from light absorption either from an electron missing from a normally occupied position, ie, a hole color center, or from an extra electron, ie, an electron color center. If the electron is a valence electron of a transition element, the term color center is not usually used. 

Any material which can form a color center contains two types of precursors as shown in Figure 2a. The hole center precursor is an atom, ion, molecule, impurity, or other defect which contains two paired electrons, one of which can be ejected by irradiation, leaving behind a hole center (Fig. 2b). The electron center precursor is an atom, ion, etc, which can produce an electron center by trapping the electron ejected from the hole center precursor. A hole and an electron center are thus formed simultaneously. Either or both can be the color center. Almost all materials have hole center precursors. If there is no electron center precursor, however, the displaced electron returns to its original place and the material remains unchanged. 

Fig. 2. (a) Irradiation of a material containing A, a hole precursor having an electron pair, and B, an electron precursor, to form (b) a color center having... 

The electron can be trapped, for example by an interstitial which is converted to an H atom. The AlO is the hole color center which absorbs light and gives the color to smoky quart2. Bleaching is the result of thermal energy releasing the trapped electron, which then produces the reverse of reaction 1. The amethyst color center in quart2 is exactly like the smoky, except that Fe " replaces. ... 

The Fe(III)0 4 hole color center gives the purple color. On being heated, the trapped electron is released and the reverse of reaction 2 occurs producing Fe(III)0 4, which provides the pale yellow color of citrine. 

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... 

An example of a hole color center is smoky quart2 [14808-60-7]. Here itradiation (either produced by nature or in the laboratory) of Si02 containing a trace of A1 ejects an electron from an oxygen adjacent to the A1 or, in customary nomenclature [AlO ] — [AlO ]" + e the ejected electrons are... 

In a more general application, thermoluminescence is used to study mechanisms of defect annealing in crystals. Electron holes and traps, crystal defects, and color-centers are generated in crystals by isotope or X-ray irradiation at low temperatures. Thermoluminescent emission during the warmup can be interpreted in terms of the microenvironments around the various radiation induced defects and the dynamics of the annealing process (117-118). ... 

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. 

A second kind of electronic defect involves the electron. Let us suppose that the second plane of the cubic lattice has a vacancy instead of a substitutional impurity of differing valency. This makes it possible for the lattice to capture and localize an extraneous electron at the vacancy site. This is shown in the following diagram. The captured electron then endows the solid structure with special optical properties since it ean absorb photon energy. The strueture thus becomes optically active. That is, it absorbs light within a well-defined band and is called a "color-center" since it imparts a specific color to the crystal. 

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. 

Fig. 9 EPR spectra of color center in MgCl2 films for two different preparation conditions a film grown at lower temperature b initially well-ordered film after bombardment with electrons and argon ions...

What is the nature of the defects seen in the EPR spectra For alkali and alkali earth halogenides it is well known that irradiation with X-ray, neutrons, gamma-radiation, or electrons produce paramagnetic color centers (F-center) [109-111]. If these centers are created in large amounts, they can be stabilized by the formation of metal clusters as observed for MgCl2 films after prolonged electron radiation [106]. From the temperature dependence... 

Related ion-radicals have been prepared photochemically in rigid media by Lewis.146 When tetramethylbenzidine is exposed to ultraviolet light in a rigid medium, an electron is ejected as in the production of color centers in glass. 

Since the original studies of F centers many other color centers have been characterized that may be associated with either trapped electrons or trapped holes. These are called electron excess centers when electrons are trapped and hole excess centers when holes are trapped. 

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. 

Within its orbit, which has some of the characteristics of a molecular orbital because it is shared with electrons on the surrounding atoms, the electron has two possible spin multiplicity states. These have different energies, and because of the spin-multiplicity rule, when an (N-V) center emits a photon, the transition is allowed from one of these and forbidden from the other. Moreover, the electron can be flipped from one state to another by using low-energy radio-frequency irradiation. Irradiation with an appropriate laser wavelength will excite the electron and as it returns to the ground state will emit fluorescent radiation. The intensity of the emitted photon beam will depend upon the spin state, which can be changed at will by radio-frequency input. These color centers are under active exploration for use as components for the realization of quantum computers. 

The F center absorption maximum for KC1 is at 565 nm and that for KF is 460 nm (Table 9.1). (a) What is the composition of a natural crystal with color centers showing an absorption peak at 500 nm (b) If the absorption peak for KF corresponds to the promotion of an electron from the F center to the conduction band, determine the energy of the color center with respect to the conduction band. (The band gap in KF is 10.7 eV.) If the relative position of the color center energy level remains the same throughout the KF-KC1 solid solution range, estimate (c) the band gap of KC1 and (d) the band gap for the natural crystal. 

The E center is an electron trapped at a negative Cl vacant site. These centers can be created in NaCl by irradiation or by additive coloration, as shown in the next chapter (Section 6.5). The band at 443 nm corresponds to a certain concentration, iV, of E centers that have been introduced by the irradiation process. The other band peaking at about 280 nm is related to other types of color center (which are formed by F center aggregation), beyond the scope of this example. 

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. 

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