In this paper we discuss the self-trapping of a carrier or exciton in an insulator.
The qualitative differences between small self-trapped molecular polarons and dielectric
polarons are stressed. We point out that, for the formation of a molecular polaron or selftrapped
exciton, a potential barrier must be penetrated or surmounted by the configuration
coordinate, leading to a delay in the self-trapping process. This does not exist for dielectric
polarons. The observable consequence of the delay time before self-trapping is discussed,
and applications are made to alkali halides and to SOz
