F-Center

Color centers are lattice defects which absorb visible light. F- centers are the easiest color centers and consist of vacancies in ionic crystals, to which anions are missing. The charge of this lack of anions is balanced by the empty space is filled with one or more electrons.

The electron of the color center can absorb electromagnetic radiation in the wavelength range of visible light, resulting in coloring of the crystals. The absorption spectrum of such a crystal has a sharp curve ( R band), wherein the position is dependent on the size and shape of anion vacancy, which is the electron available. The spectrum only depends on the structure of the host system and is independent of where the electron originates. The best studied the F- centers in ionic crystals of sodium chloride structure type. In sodium chloride, the F- band has a wavelength of 465 nm, the crystal then has a deep yellow color.

A color center is similar to the quantum mechanical particles in the box, one of the simplest model systems of quantum mechanics. A proof that the electrons are really locked in the blanks and do not interact with other color centers provides the ESR spectrum. Each unpaired electron has an unpaired spin and thus a paramagnetic moment.

Ionic compounds in which the anions are systematically replaced by the electrons, referred to as " electride ". Here we no longer speak of color centers, but of solvated electrons.

Generation

Color centers can arise when heating a salt crystal in the vapor of the corresponding metal, so for example, sodium chloride in the metal vapor of sodium. This positive sodium ions are replaced by neutral sodium atoms in the crystal. The released electrons are then occupy chloride vacancies and form color centers. The chloride vacancy need not be present at the beginning of the experiment, but can also occur only during heating. The vacancies arise then only at the surface and slowly diffuse into the interior of the crystal, which is good to see the discoloration.

Another way to produce color centers, the X-ray irradiation. Thus sodium chloride after an extended X-ray diffraction experiment has a greenish-yellow color, probably due to discharge of some chloride ions.