Single vacancy F center

Sometimes the F center might acquire an additional electron, making the F center negatively charged, such that it is called an F⁻ center. Similarly, when the F center misses an electron, when it is ionised, it will be an F⁺ center.^[8] It is also possible to have a -2e charged anion, needing 2 electrons to form an F center. Adding or taking away an electron will make it an F⁻ or F⁺ center respectively according to the convention.

Another type of a single vacancy F center is the F_A center which consists of an F center with one neighbouring positive ion replaced by a positive ion of a different kind. These F_A centers are divided into two groups, F_A(I) and F_A(II) depending on the type of replacement ion. F_A(I) centers have similar properties as regular F centers, whereas F_A(II) centers cause two potential wells to form in the excited state due to the repositioning of a halide ion. Similar to the F_A is the F_B center, which consists of an F center with two neighbouring positive ions replaced by a positive ion of a different kind. The F_B centers are also divided into two groups, F_B(I) and F_B(II), with similar behaviour to the F_A(I) and F_A(II) centers. Due to the statistical nature of the distribution of impurity ions, F_B centers are much more rare than F_A centers.^[9]

Complex F center

Combinations of neighbouring F centers due to neighbouring anion vacancies will be called, for two and three neighbours respectively, F₂ and F₃ centers. Larger aggregates of F centers is certainly possible, but the details of its behaviour are yet unknown.^[10] An F₂ center can also be ionised, and form an F₂⁺ center. When this type is found next to a cation impurity, this is an (F₂⁺)_A center.^[9]

F_s centers

F centers can appear anywhere in the crystal but have substantially different properties if formed on the surface of an oxide crystal. Electrons bound in F_s centers have smaller transition energies compared to bulk F centers. Surface F centers in alkali halide crystals behave as a slightly perturbed bulk center, with a shift of below -0.1eV. ^[11] They tend to protrude from the surface compared to regular lattice points as well. With F centers being less bound than electrons at regular lattice sites, they work as a catalyst for adsorption.^[12] However this means that these defects quickly deteriorate in open air by absorbing oxygen, but are reversible by removing the oxygen from the environment. The ESR spectrum of F_s 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. F_s center can be changed or destroyed by heating. The defects in alkali halide crystals are destroyed at low temperatures. crystals start to slowly discolour at 200 K. For oxides temperatures to destroy these defects is substantially higher, 570 K for CaO. In oxides it is possible to create complex F_s centers by annealing.^{[13][14][15][16]}

Irradiation

The first F centers created were in alkali halide crystals. These halides were exposed to high-energy radiation, such as X-rays, gamma radiation or a tesla coil.^[17]

There are three mechanisms of energy absorption by radiation:^{[4]: 209–216}

a) Exciton formation. This amounts to an excitation of a valence electron in a halide ion. The energy gained (typically 7 or 8 eV) will partly be lost again through the emission of a luminescent photon. The rest of the energy is available for displacing ions. This energy radiates through the lattice as heat. However, it turns out that his energy is too low to move ions and therefore not capable of generating F centers.

b) Single ionization. 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. It is possible that it will move through the lattice. The created vacancy can now trap the electron, creating the F center. If the halide ion recaptures the electron first, it can release more thermal energy than by exciton formation (2 eV more) and it could cause other ions to move also.

c)  Multiple ionization. This process requires the most energy. A photon interacts with a halide ion, ionizing it twice, leaving it positively charged. 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. To free two electrons, about 18 eV is required (in the case of KCl or NaCl). Research suggests about one double ionization occurs in ten single ionizations. However, the created positive halide ion will easily and quickly adopt an electron; making it unable to create the F center.

The most likely mechanism of F center creation is not yet determined. Both are possible and likely, but which once occurs the most is unknown.

The formation of an F₂ center is very similar. An F center is ionized and becomes a vacancy; the electron moves through the material to bind to another F center, which becomes an F⁻ center. The electron vacancy moves through the material and ends up next to the F⁻ center, which gives its electron back to the vacancy, forming two neighbouring F centers, i.e. an F₂ center.

Additive coloring

A different way of creating color centers is by additive coloring. A crystal with F centers is chemically equivalent as a perfect crystal plus stoichiometric excess of the alkali metal. ^[18] This is done by heating the crystal to a high temperature in the vapour of the corresponding metal. The temperature is bounded by its melting point and the temperature at which colloids form, e.g. for KCl between ~400 and 768°C. Metal atoms are captured on the surface of the crystal, where they are ionized, and the valence electron is shunted to the crystal lattice. Since this process happens at high temperatures, the mobility of ions is also high. A negative ion will move towards the newly formed ion. This leaves behind an anionic vacancy which can trap the electron to form an F center. Afterwards the crystal is quenched to prevent the F centers moving through the crystal to form colloids. An example of this process is heating NaCl in a metallic sodium atmosphere.

Na⁰ → Na⁺ + e⁻
Na⁺ is incorporated into the NaCl crystal after giving up an electron.
A Cl⁻ vacancy is generated to balance the excess Na⁺. The effective positive charge of the Cl⁻ vacancy traps the electron released by the Na atom.

In oxides it is possible to additively color a crystal with a different metal than the cation. The resulting absorption spectra are substantially the same as if the component metal was used. ^[16]

Low temperature vapour deposition

It is possible to create stable F_s centers on alkali halide crystals using vapour depositions at low temperatures,below -200 °C ^[15]