Split in energy levels due to interaction between electron up or down spin and the electron orbitals.
Numerically explained by the Dirac equation when solving it for the hydrogen atom, and it is one of the main triumphs of the theory.
Small splits present in all levels due to interaction between the electron spin and the nuclear spin if it is present, i.e. the nucleus has an even number of nucleons.
As the name suggests, this energy split is very small, since the influence of the nucleus spin on the electron spin is relatively small compared to other fine structure.
The most important examples:
- hydrogen line useful in astronomy, and also the simplest possible case between 1s
- caesium standard, which is used to define the second in the International System of Units since 1967.
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This is a section about Fine structure!
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