hyperfine structure

When the nucleus of an atom has a nonzero magnetic moment, some of the energy levels that electrons can occupy in the atom are very finely spaced. The arrangement of these finely spaced levels is called "hyperfine structure." The difference in energy between hyperfine levels typically corresponds to a microwave photon frequency or light with a wavelength on the order of centimeters. The energy levels in the cesium atom used to define the second are hyperfine levels.