Color center
Color centers are lattice defects that absorb visible light. F centers are the simplest color centers and consist of vacancies in ion crystals that are missing anions . The charge of these missing anions is compensated by occupying the vacancy with one or more electrons .
The electrons of a color center can absorb electromagnetic radiation in the wavelength range of visible light , which leads to a discoloration of the crystal . The absorption spectrum of such a crystal has a sharp line ( F-band ), the position of which depends on the size and shape of the anion vacancy available to the electron. The spectrum depends exclusively on the structure of the host system and is independent of where the electron comes from. The F centers in ion crystals of the sodium chloride structure type have been studied best . In sodium chloride , the F band has a wavelength of 465 nm . The crystal then has a deep yellow color.
In terms of quantum mechanics , a color center resembles the particle in a box , one of the simplest model systems in quantum mechanics. The ESR spectrum provides proof that the electrons are really locked in the vacancies and do not interact with other color centers . Every unpaired electron has an unpaired spin and thus a paramagnetic moment .
Ionic compounds, in which the anions are systematically replaced by electrons, are called “electrides”. Here one no longer speaks of color centers, but of solvated electrons .
Color centers played an important role in the development of semiconductor physics in the school of Robert Wichard Pohl in Göttingen in the 1920s. Normally the band gap in alkali halides is too large to be overcome by visible light and the crystals are therefore colorless. However, they can be colored by defects in the form of color centers (and from there electrons are excited with light into the conduction band). In the case of semiconductors with a smaller band gap, such as silicon or germanium, excitation by light is possible; as a result, they are opaque.
There are other types of color centers, such as M, R, N centers and associated bands, in which several color centers are adjacent.
Generation and use
Color centers can arise when a salt crystal is heated in the vapor of the corresponding metal , for example sodium chloride in the metal vapor of sodium . In the crystal, positive sodium ions are replaced by neutral sodium atoms. The electrons released in the process then occupy chloride vacancies and form color centers. The chloride vacancies do not have to be present at the beginning of the experiment, but can also only arise during heating. The voids then only appear on the surface and slowly diffuse into the interior of the crystal, which can be clearly seen in the discoloration.
Another way to create color centers is to use X-rays . After a lengthy experiment in X-ray diffraction , sodium chloride has a greenish-yellowish color, probably due to the discharge of some chloride ions.
Color center lasers are widely tunable lasers
literature
- Charles Kittel: Introduction to Solid State Physics , Oldenbourg, 11th edition 1996, ISBN 3-486-23596-6
Individual evidence
- ↑ for example Udo Scherz, Fundamentals of Solid State Physics, in Rainer Kassing (Hrsg.): Bergmann-Schaefer, Textbook of Experimental Physics. Solid, De Gruyter 2005, p. 104
- ↑ http://www.spektrum.de/lexikon/physik/farbzentrenlaser/4779 Lexicon of Physics: color center laser on Spektrum.de