ABSTRACT
This chapter discusses the transitions between atomic levels in a general sense, postulating the operators responsible for the decay. It concentrates on the most important few multipoles or terms in the expansion of the electromagnetic field. It also introduces a number of different properties which describes the transition, eventually amve at the measurable transition rate and, directly links to it, the lifetime of an excited state. An electromagnetic transition between two states is characterized by the angular momentum and parity of the corresponding photon. The author emphasizes that the transition codes handles all two-electron systems with completely independent orbitals for valence correlation but a common core, and most three-electron systems with a common core. CSF is classified based on different angular momenta as active or passive, depending on whether they ‘participate’ in the transition. The chapter concludes that the relative importance of forbidden lines, especially the ones between fine sffucture levels, increases rapidly along the isoelectronic sequence.
