Three Band Theory for Three Types Self-Trapped Excitons in Alkali Halides

Abstract

Self-trapped excitons (STE) in alkali halides are theoretically studied by the following three band model. A hole moves about p -orbitals centered at anions. While an electron moves over vacancy orbitals centered at regular anion sites as well as s -orbitals of cations. Thus, the electron, initially in a s -orbital, can fall into the vacancy orbital, when an anion is removed from its lattice site. Only the iodine series is studied, and we have found three types of minima, A, B and C in the adiabatic potential energy surface of the STE. The type A is a one-center STE with even parity, while B is a deformed V K + e , and C is a charge-separated STE. The type A is always the highest state, independently of the alkali atom. While, relative stability between the B and the C depends on the size of alkali ion. When it is small the B is the lowest, and the C is absent. As the size increases, the C appears as well as the B and becomes the lowest.