Sub-Doppler laser spectroscopy

Ideal primary data in atomic and molecular physics would include the energy levels of single isolated atoms and molecules, and an approximation to these is obtained from spectra of gases at low pressures. In classical spectroscopy a severe barrier to spectral resolution and measurement precision is often encountered because of the Doppler effect. The absorption frequency of an atom or molecule is modified by its velocity, and so a spectral line will be broadened because of the thermal distribution of velocities in a gas. The barrier can, however, be overcome by using lasers in a variety of different ways. This article introduces the two primary techniques now employed, namely Lamb dip and two-photon spectroscopy, and a variety of examples are discussed. Alternative strategies employing molecular beams, laser cooling and time resolution are also outlined. Limitations on extreme resolution are considered, and the article concludes with a section on laser frequency stabilization and frequency standards which often rely on sub-Doppler spectra.