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The advantages of the surface Green function (SGF) method which gives a unified description of electrons at solid surfaces are summarized. A direct parametrization of the SGF is introduced, not requiring an explicit knowledge of the effective potential. This procedure is useful for approximate solution of both the direct and the inversion problem of LEED. Its use is demonstrated on a two-band model: general nonuniformity of erosion of Bragg peaks when inelastic processes are considered and a characteristic influence of resonant levels on the intensity profiles in LEED are observed.

The surface Green function for systems with two interfaces is reduced to the characteristics of the three component media. A simple application to the electron structure of a model thin film is given.

Extension of the full zone kp -method to calculations of surface states of electrons is formulated and its approximations are critically discussed on (100)-surfaces of Ge and Si.

Exchange and correlation contributions to the vacuum part of the effective electron potential of a semiinfinite crystal are modelled by a simple shifted image potential. Modifications, introduced by this potential, are studied for the cases of electron surface states and of the diffraction of low-energy electrons on a simple one-dimensional model.

The existence of bands of surface states in the energy gap of an infinite crystal gives rise to additional absorption of light. The characteristic features of behaviour of the absorption coefficient are discussed. Die Existenz von Energiebandern der Oberflachenzustande in der verbotenen Zone eines unendlichen Kristalls verursacht zusatzliche Lichtabsorption. Die charakteristischen Eigenschaften der Absorptionskonstante werden diskutiert.

Local densities of electron states at and near the surface of a semi-infinite solid are discussed and compared with the corresponding ones in the infinite solid. The reflectivity coefficients for electrons, incident from both sides on the surface, are shown to be closely related to these densities. Illustrating results are given for the one-dimensional model crystal. The surface Green function (SGF) method is used.

The complex band structure of Si and Ge with (100)-surface has been computed in six points of the basic region of the two-dimensional Brillouin zone by means of the full-zone kp approach. Related symmetry considerations are discussed.