Hyperfine structure

Hyperfine structure is the small splitting of atomic or molecular energy levels caused chiefly by interactions between nuclear magnetic or electric moments and surrounding electrons.

Nuclear magnetic dipole moment: A measure of a nucleus’s magnetic strength and orientation, arising from its internal spin and charge distribution. Its interaction with the electrons’ magnetic field produces the dominant hyperfine splitting in many atoms.
Nuclear spin: The intrinsic angular momentum of an atomic nucleus, measured in units of the reduced Planck constant. It combines with electronic angular momentum to determine the possible hyperfine quantum numbers.
Atomic clock: A clock that measures time by referencing a stable atomic transition frequency. The cesium standard defines the second using a microwave transition between hyperfine levels.
Hydrogen hyperfine splitting: The energy separation between hydrogen’s ground-state levels with total angular momentum zero and one. Its 21-centimeter transition is the classic magnetic-dipole example of hyperfine structure.
Electric quadrupole moment: A measure of a nucleus’s nonspherical electric charge distribution. It couples to the electric-field gradient at the nucleus and shifts hyperfine levels when nuclear spin is at least one.
Electron spin: The intrinsic angular momentum carried by an electron. Its magnetic moment contributes to the electronic field that couples to the nucleus.
Cesium standard: The realization of the SI second using the 9,192,631,770 Hz transition between two ground-state hyperfine levels of cesium-133. It is the defining practical use of a hyperfine transition in modern metrology.
Cesium-133: A stable cesium isotope with nuclear spin one-half and atomic number 55. Its ground-state hyperfine interval supplies the transition used to define the second.
Total angular momentum: The vector sum of a system’s angular momenta, including orbital and spin contributions. Atomic hyperfine states are classified by combining electronic angular momentum with nuclear spin.
Angular momentum coupling: The quantum-mechanical combination of angular momenta into total angular-momentum states. Hyperfine labels follow this general rule, coupling nuclear spin to total electronic angular momentum.

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