Hyperfine Structure

Abstract

Hyperfine structure in atomic and molecular spectra is a result of the interaction between electronic degrees of freedom and nuclear properties other than the dominant one, the nuclear Coulomb field. It includes splittings of energy levels (and thus of spectral lines) from magnetic dipole and electric quadrupole interactions (and higher multipoles, on occasion). Isotope shifts are experimentally entangled with hyperfine structure, and the so-called field effect in the isotope shift can be naturally included as part of hyperfine structure. Studies of hyperfine structure can be used to probe nuclear properties, but they are an equally important probe of the structure of atomic systems, providing especially good tests of atomic wave functions near the nucleus. There are also isotope shifts owing to the mass differences between different nuclear species, and the study of these shifts provides useful atomic information, especially about correlations between electrons.

Hyperfine effects are usually small and often, but not always, it is sufficient to consider only diagonal matrix elements for the atomic or molecular system and for the nuclear system. In some cases, however, matrix elements off-diagonal in the atomic space, even though small, can be of importance; one possible result is to cause admixtures sufficient to make normally forbidden transitions possible.