This is a brief account of recent results obtained in Paris by the development of optical-pumping techniques, and especially on the thesis of Cagnac and the thesis of Cohen-Tannoudji. Cagnac has used the optical-pumping technique to achieve nuclear orientation in the ground state of the odd mercury isotopes and has made an extensive study of the longitudinal relaxation time. Barrat and Cohen-Tannoudji have developed the quantum-mechanical theory of the optical-pumping cycle of an atom and Cohen-Tannoudji in his thesis has confirmed the theoretical predictions experimentally by applying Dehmelt’s cross-beam technique to Hg<sup>199</sup>: If a collection of atoms is irradiated permanently by light which can be absorbed and re-emitted, this pumping cycle has different effects on the ground-state properties of the atoms; these effects are experimentally detectable by observing the magnetic resonance of the atomic ground state. A broadening of the magnetic-resonance line proportional to the light intensity occurs. It is due to the shortening of the lifetime of the ground-state Zeeman levels by light absorptions. Other effects are displacements of the Zeeman levels of the ground state caused by irradiation resulting in a change of the magnetic-resonance frequency. Theory predicts displacements of 2 different kinds: (1) <i>Displacenments caused by real transitions</i>—During an up-and-down transition of an atom, coherence is partly conserved. As a result, a certain amount of the g factor of the excited state is mixed with the g factor of the ground state. (2) <i>Displacements caused by virtual transitions</i>—These displacements are related to the dispersion of light. All these effects, predicted by the theory, have been qualitatively and quantitatively confirmed by Cohen’s experiments.
ALFRED KASTLER, "Displacement of Energy Levels of Atoms by Light," J. Opt. Soc. Am. 53, 902-906 (1963)