ABSTRACT
In this chapter several coupling schemes that are relevant for subsequent discussions on hyperfine structure are discussed. The choice of a coupling scheme amounts to a choice of a particular basis in which the description of a structure of particular atomic system is simple and efficient. Different coupling schemes are often appropriate for different regions of the periodic table, since their choice is based on the physical reality of each system. Thus for light elements, where the Coulomb interactions are dominant over the spin-orbit interaction, the L S−coupling scheme (or Russell-Saunders coupling) is favored. In such a scheme the Coulomb interactions are diagonal, whereas the spin-orbit interactions can couple different L S terms. For heavy elements the spin-orbit interaction becomes comparable to, or is even greater than, the Coulomb interaction, and j j−coupling may be relevant. In other situations the relevant strengths of various interactions may favor other coupling schemes which we shall shortly explore.
