The self-trapped exciton

In cases of strong coupling of electrons or holes to the crystal lattice, a carrier may be self-trapped as a small polaron in its own lattice distortion field. A bound electron-hole pair involving such a carrier is generally described as a self-trapped exciton, and it may dramatically influence luminescence, energy transport, and lattice defect formation in the crystal. The phenomenon of exciton self-trapping is particularly common in metal halide and rare-gas crystals, where the strong exciton-lattice coupling can usually be ascribed to the possibility of covalent bond formation in the excited state of a crystal which does not admit such bonding in its ground state. Alkali halides are the most thoroughly studied of these materials. This paper will review experimental data, primarily spectroscopic, and theoretical treatments, primarily by methods developed for defects in solids, on the nature and consequences of self-trapped excitons.