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Physical Review Letters

Optical Pumping between Levels of a Bistable State of Alkali Atoms Trapped in Rare-Gas Matrices

L. C. Balling, John F. Dawson, Mark D. Havey, J. J. Wright

Published Aug 6, 1979• 25 citations
Publisher

Abstract

The results of pseudopotential calculations of the interaction of an alkali atom with its rare-gas neighbors in a solid matrix are presented. They can explain an interesting effect which appears to be due to optical pumping between two equilibrium positions in a bistable trapping site.
DOI
10.1103/physrevlett.43.435

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In real atoms, levels connected by a dipole transition contain a number of degenerate Zeeman states. Thus in practice, the phenomena of optical pumping and collisional transfer amongst states severely restrict the application of a simple two-state model. There are a number of cases, however, where the macroscopic dipole induced in an atomic gas has the same general form as predicted by the two-state model, and these are presented here. The effects of optical pumping and collisional transfer, and the implications for the description of optical bistability, are discussed. In particular, ground-state optical pumping can, under certain conditions of low perturber density, considerably reduce the power necessary to achieve bistability.

Journal of Physics B Atomic and Molecular Physics•Oct 28, 1981•10 citations
Relaxation experiments performed on alkali atoms polarized in their ground state by optical pumping in the presence of a rare gas have been previously reported and interpreted. It was shown that the relaxation governed by the spin-orbit interaction is strongly affected by the formation of chemically unstable Rb-Kr molecules bound by van der Waals forces. The previous theoretical analysis, restricted to the longitudinal relaxation, is extended here to relaxation processes involving the transverse components of the electronic polarization and hyperfine "coherences" (elements of the density matrix between different hyperfine states). The effects resulting from the action of a rf field are also examined. The main results of this work, among others, are (a) the prediction of a Zeeman transition-line pressure shift, in some circumstances larger by several orders of magnitude than the one calculated by Herman, and having a very peculiar variation with the rare-gas pressure; (b) a quantitative explanation of the anomalous pressure dependence of the hyperfine transition linewidth discovered by Bender and Cohen; (c) a proposal of a method of detection of rf transitions between states of the alkali---rare-gas molecules by their effect on the relaxation process.
Using a three-level model and semiclassical density matrix formalism, we show that optical pumping influences both line positions and amplitudes in Doppler-broadened alkali-atom D1 and D2 spectra for intensities much lower than the two-level atom saturation intensity. The influence on the D2 spectrum is particularly interesting due to the presence of closed hyperfine transitions unaffected by optical pumping. This effect is of importance for example when lasers are frequency stabilized using linear absorption or when the alkali-atom vapour pressure is determined using absorption measurements.
The effect of excited-state mixing on the optical pumping of alkali-metal vapors is considered for the case where the energy separation between the 2P12^{2}P_{\frac{1}{2}}2P21​​ and 2P32^{2}P_{\frac{3}{2}}2P23​​ first excited states is relatively large, allowing the isolation of mixing effects within a particular JJJ level. Two models for the mixing process are proposed, one a random reorientation of the total electronic angular momentum J, and the other a reorientation of J subject to the selection rule $\ensuremath{\Delta}{m}_{J}=0, \ifmmode\pm\else\textpm\fi{}2$. The probabilities for mixing transitions among JJJ-state sublevels have been calculated for an alkali atom of nuclear spin 32\frac{3}{2}23​, and can be used to calculate the optical-pumping transition probabilities for a vapor subject to any degree of excited-state mixing. Several possible experiments suggested by these calculations are discussed.
The distribution of population in the ground-state sublevels of an optically pumped alkali-metal vapor has been found to be strongly dependent on the mechanism assumed responsible for relaxation. Two modes of optical pumping zero mixing and complete mixing in the excited state, and three modes of alkali relaxation, uniform, Zeeman, and electron randomization, are considered. Calculations of the sublevel relative populations are made in terms of experimentally measurable parameters for all six combinations of pumping and relaxation modes. Optical-pumping transient signals are then used to demonstrate that alkali relaxation due to wall collisions is of the low-frequency "Zeeman" variety, while relaxation due to alkali-rare-gas collisions takes place through randomization of the spin of the alkali valence electron. Equations for the rate of change of the electronic spin polarization are derived for all cases, and are found to be nonexponential, in contrast with the case of an alkali of zero nuclear spin.
In real atoms, levels connected by a dipole transition contain a number of degenerate Zeeman states. Thus in practice, the phenomena of optical pumping and collisional transfer amongst states severely restrict the application of a simple two-state model. There are a number of cases, however, where the macroscopic dipole induced in an atomic gas has the same general form as predicted by the two-state model, and these are presented here. The effects of optical pumping and collisional transfer, and the implications for the description of optical bistability, are discussed. In particular, ground-state optical pumping can, under certain conditions of low perturber density, considerably reduce the power necessary to achieve bistability.