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Physica Scripta

Properties of Alkali Atoms Confined in a Cold Helium Gas

A. Hatakeyama, Katsunari Enomoto, T. Yabuzaki

Published Jan 1, 2004• 7 citations
Publisher

Abstract

Properties of alkali atoms (mainly rubidium and cesium) in a cold helium gas have been extensively studied. Experiments are performed at temperatures down to 1.2 K using alkali-metal vapor cells containing a helium buffer gas, with alkali atoms driven from the cell walls by one of three loading methods: laser ablation, light-induced atom desorption, and spark discharge. For alkali atoms loaded into the cold helium gas, we have found that the relaxation time of the electronic spin of optically polarized rubidium atoms in the ground state is about a minute, much longer than that achieved in conventional vapor cells at room temperature and above. We have also spectroscopically investigated the interactions of excited alkali atoms with helium atoms, especially the formation of alkali–He exciplexes.

DOI
10.1238/physica.topical.110a00294

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Properties of alkali atoms (mainly rubidium and cesium) in a cold helium gas have been extensively studied. Experiments are performed at temperatures down to 1.2 K using alkali-metal vapor cells containing a helium buffer gas, with alkali atoms driven from the cell walls by one of three loading methods: laser ablation, light-induced atom desorption, and spark discharge. For alkali atoms loaded into the cold helium gas, we have found that the relaxation time of the electronic spin of optically polarized rubidium atoms in the ground state is about a minute, much longer than that achieved in conventional vapor cells at room temperature and above. We have also spectroscopically investigated the interactions of excited alkali atoms with helium atoms, especially the formation of alkali–He exciplexes.
We have studied the loading of gaseous alkali-metal atoms into glass cells held at about 2 K by light-induced atom desorption (LIAD). In LIAD loading, the cell containing dense helium gas and a small amount of alkali metal is irradiated with cw laser light. This technique has two unique features to be investigated: alkali-metal atoms are desorbed, even by weak laser irradiation, from the cell walls not covered visibly with alkali metal, and loading efficiency is much higher below the superfluid transition temperature for the helium inside the cell than above this temperature. On the basis of extensive experimental studies for Rb presented in this paper, we give a consistent picture of the loading mechanism: alkali-metal atoms are optically desorbed from alkali-metal particles on the walls, then transported effectively into the cell volume by helium-gas flow produced by the continuous evaporation of the helium film, which is supplied by the superfluid-film flow on the walls from the bulk liquid helium at the bottom of the cell.
Observation of nonthermal, light-induced desorption of Cs and Rb atoms from paraffin coating is reported. Buffer-gas-free paraffin-coated cells containing Cs and Rb were exposed to laser light of various intensities and frequencies, and the change of the vapor densities as a function of time was investigated. Because atomic polarization relaxes very slowly in paraffin-coated cells (spin-relaxation times can be $\ensuremath{\gtrsim}1\mathrm{s}),$ they are applied in a variety of precision measurements (e.g., magnetometry, discrete symmetry tests, etc.). Light-induced desorption may be useful in such measurements as a method to control atomic density without changing the temperature of the cell.
The interaction of spin polarized cesium atoms on coated surfaces is examined. Coated surfaces find extensive applications in atomic devices-hydrogen masers and advanced rubidium frequency standards utilize wall coatings to obtain very narrow hyperfine resonances. A Cs atomic beam is hyperfine or spin polarized by optical pumping techniques using a single mode AlGaAs diode laser. The ground state distribution is probed before and after scattering from coated surfaces using laser induced fluorescence spectroscopy. The coatings under investigations are paraffin and organosilanes and these are known to be very weakly relaxing surfaces for polarized alkali atoms. The spin relaxation properties of paraffin and organosilane coatings have been studied in glass cell-vapor environment. In our experiments using polarized atomic beams, our goal is to examine in detail the hyperfine and spin relaxation properties of these coatings in a carefully controlled environment which is not possible in the gas cell environment. We measure the spin relaxation probability per wall collision as a function of temperature of the surface. Results of our studies will be presented.
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).
NMR signals from diatomic alkali molecules have been observed in optically pumped cesium and rubidium vapors. The nuclear relaxation times were found to be strongly dependent on the temperature and buffer-gas pressure. The relaxation time seems to be determined by at least two and possibly three major factors: motional narrowing of the molecular hyperfine structure, breakup of the diatomic molecules by collisions with buffer-gas atoms, and possibly chemical exchange of free atoms with atoms bound in the molecules.
Atomic vapor of four different paramagnetic species: gold, silver, lithium, and rubidium, is produced and studied inside several buffer gases: helium, nitrogen, neon, and argon. The paramagnetic atoms are injected into the buffer gas using laser ablation. Wires with diameters 25, 50, and $100\text{ }\ensuremath{\mu}\text{m}$ are used as ablation targets for gold and silver; bulk targets are used for lithium and rubidium. The buffer gas cools and confines the ablated atoms, slowing down their transport to the cell walls. Buffer gas temperatures between 20 and 295 K and densities between ${10}^{16}\text{ }{\text{cm}}^{\ensuremath{-}3}$ and $2\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }{\text{cm}}^{\ensuremath{-}3}$ are explored. Peak paramagnetic atom densities of ${10}^{11}\text{ }{\text{cm}}^{\ensuremath{-}3}$ are routinely achieved. The longest observed paramagnetic vapor density decay times are 110 ms for silver at 20 K and 4 ms for lithium at 120 K. The candidates for the principal paramagnetic-atom loss mechanism are impurities in the buffer gas, dimer formation, and atom loss on sputtered clusters.