Luminescent Center

a lattice imperfection that is responsible for the luminescence of a phosphor (seeLUMINESCENCE).

In crystal phosphors, luminescent centers may result from crystal defects—such as positive-and negative-ion vacancies or interstitial atoms and ions—or from activators, which are specially introduced atoms or ions. Luminescent centers that result from crystal defects are called host-crystal centers; those that result from activators are known as activator centers. A distinction is made between simple and complex luminescent centers. Simple luminescent centers are point defects or single activator atoms or ions; complex centers are pairs or clusters of defects or activator atoms, often atoms of different species.

In luminescent glasses, the luminescent centers are activator centers. The centers are produced during the manufacture of the glasses by adding an activator material to the batch.

The main characteristics of luminescent centers are their emission and absorption spectra. As a rule, the absorption spectrum lies in a spectral region where the crystal does not absorb light. Hence, luminescent centers are often color centers as well, although not all color centers are luminescent centers. On the other hand, if a luminescent center absorbs light in the same spectral region as does the crystal, the center will luminesce and, thus, would not be a color center.

The absorption and emission spectra of simple activator centers are related to the activator atoms. For example, when a phosphor is activated by ions of rare earth elements, the spectra of the luminescent centers turn out to be line spectra produced by quantum transitions in the inner electron shells of the ions. The effect of the lattice is manifested in the shifting and splitting of the spectral lines by the crystal field—for example, the Stark effect—and in the superposition of additional frequencies corresponding to lattice vibrations (seeSPECTRUM, CRYSTAL). When a phosphor is activated by atoms of elements whose spectra are produced by transitions in an outer electron shell, the lattice causes the spectral lines to be broadened into bands.

In a regular lattice, the activator ions usually replace the cations. However, under certain conditions of phosphor preparation, the activator ions also may be localized in planes within the crystal that contain defects or in the vicinity of some crystal defect, thus also constituting a luminescent center.

A single phosphor often contains two or more types of luminescent centers. The centers may interact with one another by exchanging electrons and holes or directly by means of excitation energy. The first type of interaction is called a recombination interaction; the second type is known as a resonance interaction.