Optical Transitions and Surface Structure

To obtain information on the structure of a solid surface probes should be employed having a wavelength as short as the interatomic distance, which is typically of the order of an angstrom. In the case of electromagnetic radiation, this corresponds to X-rays. On the other hand, optical transitions involving electronic surface states are excited by near IR, visible and near UV light, whose wavelength is too large to probe the atomic structure directly. In some cases, however, valuable structural information can be obtained from optical spectroscopy indirectly, that is via the electronic structure. This is accomplished by exploiting symmetry arguments and making use of models. The present article exemplifies the above concepts by showing their application to the cleavage surface of the covalent semiconductors Si and Ge.