More Details on Atomic Spectra
Spectroscopic measurements normally reach a very high level of accuracy, which means that the non-relativistic approximation introduced in the previous chapter is not sufficient, in the vast majority of cases, to give a quantitatively adequate description of the atomic spectra. In particular, on the basis of this approximation, we cannot explain the presence in the atomic spectra of an important phenomenon such as the fine structure. This phenomenon can be adequately described considering the contribution to the energy of the atom due to the intrinsic angular momentum of the electron. The result is the removal of the degeneracy in the non-relativistic Hamiltonian and the consequent separation of the terms (identified with the quantum numbers L and S) in atomic levels characterized by the quantum number J. In this chapter we describe in detail this phenomenon, together with other phenomena which similarly produce the removal of degeneracy of the atomic states, either due to external agents (such as a magnetic field) or internal ones (nuclear spin). This will lead to the description of other characteristic effects of atomic spectra such as the Zeeman effect, the Paschen-Back effect and the hyperfine structure.