Electron-phonon and exciton-phonon bound states

A review is given of the theoretical and experimental work which has shown the possibility of forming bound states of an electron or an exciton with an optical phonon. Most attention is devoted to wide-band systems, in which the width of the electron (or exciton) band is larger than the phonon frequency; this is the normal situation for semiconductors and ionic crystals. A discussion is also given of excitons in molecular crystals, which are treated as narrow-band systems; here the existence of bound states is not related to the presence of a threshold in the exciton spectrum. The existence of bound and hybrid state must have a marked effect on a number of physical phenomena, in particular the optical properties, as has already been demonstrated in a series of experiments.