F-center

Electrons in such a vacancy in a crystal lattice tend to absorb light in the visible spectrum such that a material that is usually transparent becomes colored.

[1] Thirty years later similar results were achieved by melting crystals together with a specific metal.

One set of these tests measured a photoelectric conductivity 40,000 times larger, after the salt was radiated with x-rays.

The photoelectric effect mainly happened around specific wavelengths, which was later found to be non-colloidal in nature.

[2] F centers can occur naturally in compounds (particularly metallic oxides) because when heated to high temperature the ions become excited and are displaced from their normal crystallographic positions, leaving behind some electrons in the vacated spaces.

Alkali metal halides are normally transparent; they do not show absorption from the far ultraviolet into the far infrared.

In other alkali chlorides the location of the F center absorption band ranges from violet to yellow light.

[7] There are different types of electron centers, depending on the material and radiation energy.

An F center is usually a position in a lattice where an anion, a negatively charged ion, is replaced by an electron.

With F centers being less bound than electrons at regular lattice sites, they work as a catalyst for adsorption.

The ESR spectrum of Fs center is temperature dependent in the hyperfine structure in oxides.

This must arise from an increasing overlap of the unpaired electron wave function at the Nucleus of the positive ion.

crystals start to slowly discolour at 200 K. For oxides temperatures to destroy these defects is substantially higher, 570 K for CaO.

The energy gained (typically 7 or 8 eV) will partly be lost again through the emission of a luminescent photon.

However, it turns out that his energy is too low to move ions and therefore not capable of generating F centers.

This corresponds to separating an electron from a halide ion; the energy required is about 2 eV more than exciton formation.

One can imagine that the halide ion which lost an electron, is not properly bound on its lattice site any more.

The ion remaining is very unstable and will quickly move to another position, leaving a vacancy which can trap an electron to become an F center.

The effective positive charge of the Cl− vacancy traps the electron released by the Na atom.

[16] It is possible to create stable Fs centers on alkali halide crystals using vapour depositions at low temperatures, below -200 °C.

[15] Certain F centers have optical absorption and emission bands that makes them useful as laser gain media.

They provide a wavelength range from 0.8 to 4.0 μm, the near infrared region of light, thus picking up where dye lasers fail to operate.

These crystals have been found to be good materials for color center lasers with emission lines of wavelengths between 2.45 and 3.45 μm.

[6]: 432–438 F centers usually have an absorption band in visible range, and the emission is Stokes shifted to longer wavelengths.