Editing Exciton (section)

=== Frenkel exciton ===
In materials with a relatively small [[dielectric constant]], the Coulomb interaction between an electron and a hole may be strong and the excitons thus tend to be small, of the same order as the size of the unit cell. Molecular excitons may even be entirely located on the same molecule, as in [[fullerene]]s. This ''Frenkel exciton'', named after [[Yakov Frenkel]], has a typical binding energy on the order of 0.1 to 1 [[electron volt|eV]]. Frenkel excitons are typically found in alkali halide crystals and in organic molecular crystals composed of aromatic molecules, such as [[anthracene]] and [[tetracene]]. Another example of Frenkel exciton includes on-site ''d''-''d'' excitations in transition metal compounds with partially filled ''d''-shells. While ''d''-''d'' transitions are in principle forbidden by symmetry, they become weakly-allowed in a crystal when the symmetry is broken by structural relaxations or other effects. Absorption of a photon resonant with a ''d''-''d'' transition leads to the creation of an electron-hole pair on a single atomic site, which can be treated as a Frenkel exciton.