Méthodes optiques d'étude de la résonance magnétique

Abstract

The methods used for the investigations of space quantization and the determination of atomic quantum numbers and nuclear spin ( j, i, f) can be divided into magnetic methods, optical methods (including radiofrequency spectroscopy) and magneto-optical methods. The last method is illustrated by the studies of Zeeman effect. A purely magnetic method is applied in the well-known Stern-Gerlach experiment andinits refinements to test nuclear spin. The magnetic resonance radiofrequency methods belong to the magneto-optical class. Among the purely optical methods we shall mention the analysis of multiplet structure of optical spectra which led to the concept of the spinning electron and to the vector model of Russel-Saunders and the analysis of hyperfine structure of spectral lines leading to the discovery of nuclear spin. The degree of polarization of resonance radiation and fluorescence radiation of atoms is directly connected to the Zeeman structure of the spectral lines involved and is very sensitive to the alteration of this structure caused by nuclear spin. This polarization is the result of a selective excitation of m-sublevels of the excited state by the exciting radiation. As magnetic resonance between these sublevels tends to equalize their populations it will cause a depolarization of the emitted radiation, and this effect permits optical detection of magnetic resonance of excited states. Magnetic resonance of the 6 3 P 1 state of mercury has been detected in this manner. Optical excitation of atoms by non isotropic radiation, especially by a parallel beam of circularly polarized light, gives a possibility of changing the populations of m-sublevels of the ground state or of metastable states, and by optical re-excitation of this state this dissymetry of population can be tested. As this dissymetry is destroyed by magnetic resonance this resonance can be detected optically. This optical method seems to be particularly suitable for the analysis of metastable states and the study of hyperfine structure Zeeman patterns in weak fields.