Alfred Kastler

Optical Methods of Atomic Orientation and of Magnetic Resonance

In the optical excitation of atoms with polarized light, producing excited atoms, only some of the Zeeman sublevels of the excited state are actually reached, so that large differences of population can be built up between Zeeman sublevels or between hyperfine structure (hfs) levels. This property can be used to detect radio-frequency resonance in optically excited atomic states. These resonances produce a characteristic change in intensity or in the degree of polarization of the light re-emitted. Zeeman intervals, Stark effects, and hfs intervals can be measured in this manner. (The Stark constant of the 63P1 level of Hg and the electric quadrupole moments of the alkali atoms have been obtained in this way.)The technique of “optical pumping” gives a way to concentrate atoms in some of the Zeeman sublevels of one of the hfs levels of the ground state.Atomic orientation has been obtained with the Na atom, in an atomic beam and in the vapor in equilibrium with the metal. The orientation effects have been studied by detection of radio-frequency resonance signals in the ground state. Orientation can be increased many times by adding a variable pressure of a foreign gas to the pure Na vapor. Because of the coupling between nuclear spin and electron spin, nuclear orientation is produced at the same time as atomic orientation.

Displacement of Energy Levels of Atoms by Light

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 Hg199: 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) Displacements caused by real transitions—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) Displacements caused by virtual transitions—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.

Quelques réflexions à propos des phénomènes de résonance magnétique dans le domaine des radiofréquences

Guided by the analogies and connexions between optical spectroscopy and radiofrequency spectroscopy, the author suggests some experiments on radiofrequency resonance phenomena. In a first part, he shows that in certain cases irradiation with light will modify the intensity of radiofrequency resonance signals, and that such an effect is particularly to be expected at low temperature and with circularly polarized light. In a second part, connexions between ultrasonic waves and magnetic resonance are examined and attention is drawn to the fact that magnetic resonance in crystals at very low temperature acts as a generator of ultrasonic waves of high frequency, those waves being detectable by study of the angular distribution of light scattered by the crystal. Finally it is emphasized that investigation of nuclear quadrupole resonance in monocrystals must reveal striking anisotropy and coherence effects and that the study of destruction of coherence by small magnetic fields provides a new method for measuring spin-spin relaxation time.

I Optical Pumping

Publisher Summary This chapter focuses on optical pumping, which is a method for producing important changes in the population distribution of atoms and ions among their energy states by optical irradiation. These population changes can be monitored by the change of intensity of the light transmitted by the sample in which optical pumping is produced or by the change of intensity or of polarization of the scattered resonance light. The methods of optical pumping and of optical detection can also be used, either together or separately to investigate excited states of atoms. The change of population produced by optical pumping is associated not only with a change of energy of the atomic assembly, but also with a peculiar change of angular momentum. By optical pumping, polarization of the spin vectors is produced, and macroscopic magnetization of the medium is obtained. The chapter states the principles of optical pumping and discusses the characteristics of the optical pumping methods. Optical pumping leads to a simple device to study radiofrequency resonance, to investigate Zeeman intervals and hyperfine intervals. It also leads to a systematic study of the interactions of atoms with electromagnetic fields in the optical range and in the radiofrequency range.

The Hanle effect and its use for the measurements of very small magnetic fields

The Hanle effect is briefly reviewed. It is shown that optical pumping can be used to create alignment of states. The combined use of Hanle effect and optical pumping then permits one to measure magnetic fields as small as 10−9 G.

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.

La polarimétrie dans le domaine des ondes hertziennes

ConclusionsNous avons vouln montrer, dans cc bref aperçu, que Ia polariniśtrie hertzienne n’est qn’a ses debuts, mais quo los perspectives d’avenir sont vastes. Nous voudrions attirer l’attention des specialistes des ondes courtes snr ces techniques qui ne tarderont pas a devenir fecondes pour l’exploration des propriétés de la matiére et pour la technique même des microondes.