Hyperfine structure

In atomic physics, hyperfine structure is the different effects leading to small shifts and splittings in the energy levels of atoms, molecules and ions. The name is a reference to the fine structure which results from the interaction between the magnetic moments associated with electron spin and the electrons' orbital angular momentum. Hyperfine structure, with energy shifts is typically orders of magnitudes smaller than those of a fine structure shift, which results from the interactions of the nucleus (or nuclei, in molecules) with internally generated electric and magnetic fields.