Simple Organic Molecular Crystals

Abstract

We saw in Chap. 5 that an STE in alkali halides can be described as an excited dimer (excimer), X²⁻ ₂, on the halide sublattice, generally accompanied by off-center relaxation of the excimer ion (X⁻ ₂) relative to the excited electron wave function. Partly because of the prominent nature of STE phenomena in alkali halides and related ionic crystals, one sometimes sees self-trapped excitons characterized as a phenomenon of ionic insulators. However, the experimental study of exeimers in pure organic crystals actually predated their discovery in alkali halides by about 6yr [8.1]. The organic excimers were not commonly called self-trapped excitons until the late 1970’s, when Matsui and Nishimura showed that excimer formation in a pure organic crystal is quite naturally described within the framework of exciton self-trapping [8.2]. Some of the experimental observations on excimers to be discussed bear an interesting resemblance to phenomena in alkali halides.