Intermediate-radius surface excitons in ideal crystals

The theory of intermediate bound surface excitons is presented. A rather accurate approximate analytical expression for the surface exciton energy is obtained for simple models of ideal crystals. It is shown that in the absence of a surface potential, surface exciton levels may exist and, if they do, they are situated in the gap between the top of the bulk exciton band and the bottom of the electron-hole continuum.