2-SITE SELF-TRAPPING OF BULK AND SURFACE WIDE-BAND EXCITONS

The self-trapping of bulk and surface excitons is considered. Both the initial free exciton state and the final state (diatomic quasi-molecu le) are formed via resonant binding. Under self-trapping, the binding is redistributed to concentrate mainly inside the molecule, forming a strong intra-molecule bond. But some of the weak resonant bonds persis t in the vicinity of the molecule, which results in a lowering of the self-trapping barrier. These phenomena manifest themselves differently in the three- and two-dimensional cases, being better pronounced in t he latter case.