Color centers electronic structure

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. 

Color signaling of pH-induced pendant arm relocation is less eye-catching for Cu(II) than for Ni(II), where axial binding induces a drastic change of the electronic structure of the d8 cation (from low-spin to high-spin), a unique feature, which cannot be experienced by a d9 center like Cu(II). On the other hand, fluorescence signaling for the Cu(II)-3 system is as powerful and showy as observed with Ni(II)-3. 

Band structure details of insulators can be determined from their UV/VIS spectra. Defects in the crystal produce electronic levels within the gap between the conduction and the valence bands. Spectroscopic measurements at low temperature allow the investigation of the phonon structure of a crystal. Absorptions due to lattice or point defects can be used to describe the optical and electronic properties of the insulator. For example, Cr in AI2O3 crystals leads to an intense color change of the crystal. Many so-caUed color centers are based on lattice defects caused by intercalation of atoms in the crystal lattice. 

Scintillators are required to have high stabiUly and reproducibility of light output for many applications. Radiation damage refers to the degradation of scintillation efficiency due to the formation of defects directly caused by the radiation. Because these defects are usually color centers, their electronic structures could impart the... 

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. 

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