Definition of Hyperfine structure
Many nuclei have spins, which are accompanied by magnetic moments. The interaction between the nuclear magnetic moment and the electron causes the energy level to split. The nuclear magnetic moment is small, and the splitting of the energy level is also small. The ultrafine structure energy level is marked by the total angular momentum quantum number J of electrons, nuclear spin I and the total angular momentum quantum number F including nuclear spins. The energy level interval follows the similar Rounde interval rule. Many nuclei also have electrical quadrupole moments. The interaction of nuclear quadrupole moments with the electric field gradient generated by the electrons at the nucleus causes small changes in energy, superimposed on the ultrafine structure caused by magnetic moments, which causes the split to deviate from the Landes interval rule. The ultra-fine structure of the energy level causes the ultra-fine structure of the spectral lines.