Laser spectroscopy is an experimental technique probing the minute splitting of atomic energy levels, or hyperfine structure, induced by the nuclear electromagnetism. To achieve such precision, narrow-band lasers are used, and the atom-laser interaction is performed in-flight on a fast atomic beam.

At the ALTO laboratory, singly-charged ions were first accelerated to an energy of 30 keV, then passed through a volume of vaporized sodium for neutralization and overlapped with a co-propagating laser beam. Before neutralization the velocity of the ions was varied in order to scan for the transitions of the hyperfine structure via the Doppler effect. At each transition the atomic beam resonantly absorbed photons from the laser and began to fluoresce. The fluorescence light was detected with a pair of photomultiplier tubes designed for single-photon counting.

The spectrum shows the atomic hyperfine structure of the only naturally occurring isotope of sodium composed of 11 protons and 12 neutrons. The spacing between the resonances is determined by the magnetic moment of this nucleus, which is thus measured. The hyperfine splitting caused by the nuclear electromagnetism is extremely small, about a millionth of the transition energy. To achieve such precision, narrow-band lasers are used, and the atom-laser interaction is performed on a fast beam to remove the Doppler broadening associated with the thermal motion in the ion source. Such measurements can be performed also on radioactive isotopes. Apart from magnetic moments, nuclear charge radii, spins, and quadruple moments can be measured as well.