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Journal of Experimental and Theoretical Physics Letters

Features of the Formation of the Spin Polarization of an Alkali Metal at the Resolution of Hyperfine Sublevels in the 2S1/2 State

Е. Н. Попов, V. A. Bobrikova, S. P. Voskoboinikov, К. А. Баранцев, S. M. Ustinov, A. N. Litvinov, Anton K. Vershovskii, Sergei P. Dmitriev, Victor A. Kartoshkin, Anatoly S. Pazgalev, Mikhail V. Petrenko

Published Oct 1, 2018• 25 citations
Publisher

Abstract

The optical orientation of the angular momenta of alkali atoms in the presence of a buffer gas (molecular nitrogen) has been studied experimentally. It has been shown that, even at a low concentration of molecular nitrogen in the cell, the excitation of 133 Cs atoms from the lower hyperfine level with F = 3, which belongs to the ground 2 S 1/2 state, results in a larger amplitude of the magnetic resonance than the excitation from the hyperfine level with F = 4. This result has been theoretically explained under the assumption that the spin state of the alkali atomic nucleus does not change at collision with a nitrogen molecule, which is accompanied by a nonradiative transition of the alkali atom from the excited 2 P 1/2 state to the ground 2 S 1/2 state.

DOI
10.1134/s0021364018200122

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•Dec 16, 2020•0 citations
The optical orientation of the angular momenta of alkali atoms in the presence of a buffer gas (molecular nitrogen) has been studied experimentally. It has been shown that, even at a low concentration of molecular nitrogen in the cell, the excitation of 133 Cs atoms from the lower hyperfine level with F = 3, which belongs to the ground 2 S 1/2 state, results in a larger amplitude of the magnetic resonance than the excitation from the hyperfine level with F = 4. This result has been theoretically explained under the assumption that the spin state of the alkali atomic nucleus does not change at collision with a nitrogen molecule, which is accompanied by a nonradiative transition of the alkali atom from the excited 2 P 1/2 state to the ground 2 S 1/2 state.
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.
The Landé factors of hyperfine sublevels belonging to the same spectral term differ from each other. This leads to the appearance of an effective gyromagnetic ratio for an atomic ensemble, which can be defined as the ratio of the average magnetic moment to the average angular momentum. Characteristically, this quantity is not a constant, but rather depends on the populations of the hyperfine sublevels. States of an ensemble of atoms occupying their ground level in which the effective gyromagnetic ratio is zero are of much interest. Here, one of such states is considered by the example of alkali metal 87 Rb. The main idea is to suggest a method for preparing states with zero effective gyromagnetic ratio by means of two-frequency pumping of a saturated alkali-metal vapor in a cell containing a buffer gas. It is noteworthy that, according to calculations, correlation between the two fields is not required for attaining this goal.
Abstract We analyze the polarization of the total angular momentum of alkali atoms in a gas cell with a buffer gas under pumping by monofrequency circularly polarized laser radiation. The mathematical model takes into account the motion of atoms and collisions preserving the nuclear spin under the chaotization of the angular momentum of the outer electron shell, total hyperfine structure, and the Zeeman structure. We analyze the stationary solution to equations for the density matrix of alkali atoms and the effect produced on them by the fraction of inelastic collision in which the quenching of atomic excitation takes place. We consider the contribution of various hyperfine states to the polarization of alkali atoms. The results are compared with the results of the model of instantaneous mixing of populations in the excited state.
It has been demonstrated that optical pumping of alkali atoms (rubidium and cesium) results to a highly efficient indirect optical orientation of metastable He (2 3 S 1 ) atoms. However, in an attempt to perform the inverse experiment (i.e. to receive spin-polarization of alkali atoms upon optical pumping of metastable helium atoms) the result was quite unexpected: the magnetic resonance signal of Cs or Rb atoms was very weak (several hundred times weaker compared to conventional optical pumping of helium in the presence of cesium (rubidium)). We propose an explanation for the unusually small ratio of the magnetic resonance signals from Cs (Rb) and metastable He atoms under condition of optical orientation of He atoms. Except of Cs atoms we are the first to have observed magnetic resonance signals from atoms of 85 Rb and 87 Rb isotopes when using the indirect optical orientation in conditions of helium-rubidium gas discharge plasma. It is shown that the anomaly in the amplitude ratios of the observed MR signals can be explained by the unidirectional action of three different factors (the presence of a large nuclear spin of alkali atoms, depolarization of alkali atoms between collisions, and spin exchange between atoms of rubidium isotopes (for the He-Rb system).
We have experimentally detected a signal of magnetic resonance (MR) from 85Rb atoms in the 52 S 1/2 state by measuring the absorption of light from a helium lamp producing optical orientation of metastable 23 S 1 4He atoms in a cell containing alkali metal vapor and gaseous helium at a pressure of 1 Torr. The ratio of MR signal amplitudes in Rb and He atoms did not exceed 10–2. Peculiarities of the formation and observation of the collisional polarization of Rb atoms in alkali–helium plasma are discussed.
In this study we explored the angular momentum alignment-to-orientation conversion occurring in various alkali metals -- K, Rb, Cs. We used a theoretical model that is based on Optical Bloch equations and uses the density matrix formalism. Our model includes the interaction of all neighboring hyperfine levels, the mixing of magnetic sublevels in an external magnetic field, the coherence properties of the exciting laser radiation, and the Doppler effect. Additionally we simulated signals where the ground- or the excited-state coherent processes were switched off allowing us to determine the origins of obtained signals. We also performed experiments on Cs atoms with two laser beams: linearly polarised Cs D1 pump and circularly polarized Cs D2 probe. We used the pump beam to create angular momentum alignment in the ground state and observed the transmission signal of the probe beam as we changed the magnetic field. Full analysis of the experimentally obtained transmission signal from a single circularly polarized probe laser component is provided.