ABSTRACT
This chapter provides the simplified models for the analysis of the electronic and optical properties of impurity ions is solids. It begins with the necessary explanations of why certain simplifications are strongly needed and will be continued with their subsequent introduction and description. The most important effects of interaction of the impurity centers with the crystal lattice vibrations and their manifestation in the optical spectra will be explained as well. Adiabatic approximation can be also applied to the description of the impurity center properties in crystals. In this case, it is also possible to divide the whole crystal into two sub-systems: the fast-moving electrons and the “frozen” nuclei. The chapter discusses the main ideas needed to describe the vibrational motion of the atoms in a molecule or the ions in an impurity center in a crystal.
