Helium and Other Two Electron Systems

After a general introduction and a survey of the experimental observations (Sect. 7.1) the quantum mechanical basis for treating multi-electron systems is reviewed in Sect. 7.2. The reader should know about these tools or get used to them by reading this section. Section 7.3 expands this theme by introducing electron exchange and the characteristic excited state configurations. Fine structure interaction in He and He like ions are addressed in Sect. 7.4 – a consolidation of our knowledge acquired in Chap. 6 which will turn out to be essential also for later chapters. In Sect. 7.5 the most important selection rules for E1 transitions in multi-electron system are treated. This directly leads to “double excitation” in Sect. 7.6. It is of importance far beyond atomic physics: resonances of states imbedded into a continuum are found in all fields of physics. The ensuing interference structures, known as Fano resonances and their origin are described in a practical, easy to access approach. Finally, in Sect. 7.7 the alkaline earth atoms and the Hg atom are treated: they are related to He in an analogous manner as alkali atoms are related to the H atom.