Theory of isomer shifts. Chapter 2

The authors consider mainly the electronic isomer shifts observed in Moessbauer spectroscopy. The main emphasis is on the description of the charge distribution of atomic electrons inside the nuclear volume. Some of the approximations discussed are no longer considered valid, but they have to be mentioned in order to acquaint the reader with the different stages of development, and also to guide him through the often confusing formalism found in the literature. At present, a relativistic free-ion self-consistent procedure including finite nuclear size, is the only method for calculating electron densities that can be sanctioned for general applicability, despite the fact that even such calculations are still crude approximations of the actual electronic structure of an atom bound in a solid. Strictly speaking all results derived here are only applicable to free atoms. (Auth.)