Spectroscopy of Localized States

Any disturbance of the periodic potential of a perfect solid, like that caused by impurities, a missing ion, the surface or other defects, introduces a set of allowed levels into the forbidden gap of the crystal. When such levels are below the Fermi level of an insulator or a semiconductor, electrons are trapped at the defect and can be excited by absorption of light. The study of the optical transitions among such levels is the subject of the present paper. (author)