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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

What do we know about atomic bubbles in helium crystals?

T. Eichler, Klaus Winkler, S. Lang, S. I. Kanorsky, Antoine Weis

Published Feb 25, 1999• 0 citations

Abstract

We present new results on the properties of Celsium and Rubidium atoms trapped in crystalline helium matrices. We have extended the measurements of optical excitation spectra and excited state lifetimes, previously performed in superfluid helium, to solid helium. Cs atoms in the hcp phase of 4He exhibit several distinctive features, such as dipole-forbidden transitions and zero-field magnetic resonance lines. We interpret these features as being due to static quadrupolar deformations of the local trapping sites (atomic bubbles). The degree of deformation of the bubbles can be obtained by comparing hyperfine coupling constants in the cubic and hexagonal phases. We have investigated in detail optical pumping of Cs in solid helium, and have gained new insights into the pumping mechanism. We have also succeeded, for the first time, in detecting Rb and Rb in solid helium, by applying an absorption technique, based on driven spin precession.

DOI: 10.1117/12.340109

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We present new results on the properties of Celsium and Rubidium atoms trapped in crystalline helium matrices. We have extended the measurements of optical excitation spectra and excited state lifetimes, previously performed in superfluid helium, to solid helium. Cs atoms in the hcp phase of 4He exhibit several distinctive features, such as dipole-forbidden transitions and zero-field magnetic resonance lines. We interpret these features as being due to static quadrupolar deformations of the local trapping sites (atomic bubbles). The degree of deformation of the bubbles can be obtained by comparing hyperfine coupling constants in the cubic and hexagonal phases. We have investigated in detail optical pumping of Cs in solid helium, and have gained new insights into the pumping mechanism. We have also succeeded, for the first time, in detecting Rb and Rb in solid helium, by applying an absorption technique, based on driven spin precession.

We analyze the effect of the host crystal symmetry on the optical spectra of the 6P(1/2)-6S(1/2) and 6P(3/2)-6S(1/2) transitions of atomic Cs in solid (4)He matrices. In particular, we address the deformation of the bubble structures formed by Cs in such quantum crystals. We show that the anisotropy of the stiffness tensor leads to static quadrupolar bubble shape deformations in hexagonally close-packed (hcp) crystals, while the corresponding deformations in the body-centered cubic (bcc) phase of the matrix have a hexadecupolar symmetry. A comparison of the measured excitation spectra with our model calculations allow us to infer quantitative values of the deformation parameters.

We have studied both theoretically and experimentally the optical pumping of Cs atoms trapped in (body-centered-cubic and hexagonal-close-packed) crystalline

{}^{4}\mathrm{He}

matrices. The theoretical approach is based on rate equations for which time-dependent and asymptotic solutions are obtained in the case of depopulation and repopulation pumping. Comparison with experiments show that repopulation pumping, i.e., a process in which spin polarization in the excited state is not destroyed, is the dominant pumping mechanism in both crystalline phases.

We have detected by optical means nonfluorescing

{}^{85}\mathrm{Rb}

and

{}^{87}\mathrm{Rb}

atoms implanted in a body centered cubic

{}^{4}\mathrm{He}

crystal. In contrast to cesium the resonance fluorescence of rubidium is strongly quenched by the helium matrix, and the weak resonance absorption of the two Rb isotopes was detected using a double resonance technique. From a comparative study of the (optically detected) magnetic resonance spectra of

{}^{85}\mathrm{Rb}

,

{}^{87}\mathrm{Rb}

, and

{}^{133}\mathrm{Cs}

we infer their effective

{g}_{F}

factors and conclude that they are not perturbed by the He matrix at a level of $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. We show further that optical pumping of Rb proceeds via depopulation, whereas for Cs it proceeds via repopulation.

We present a systematic experimental study of the absorption and fluorescence spectra of the $6{S}_{1∕2}\text{\ensuremath{-}}6{P}_{1∕2}$ transition in Cs atoms isolated in solid

^{4}\mathrm{He}

matrices. The bubble model developed earlier for alkali-metal atoms in liquid He is revised and applied to the present system. The analysis of the dependencies of absorption and fluorescence wavelengths on He pressure in liquid and solid He (cubic and hexagonal) environments leads us to modify the bubble model by taking the elastic deformation of solid He by the atomic bubble into account.

By

{\mathit{D}}_{1}

-line optical pumping of alkali-metal atoms (Cs and Rb) in superfluid helium (He II), we could achieve large spin polarization of the atoms in the ground states, more than 50% for both electron and nuclear spins, especially for the Cs atoms. The alkali-metal atoms were implanted into He II by laser sputtering. We detected optically the magnetic resonances between the ground-state Zeeman sublevels, and found that the g values and hyperfine constants in He II were the same as in vacuum within the experimental error. We discuss possible applications of these spin-polarized atoms in He II.

The excitation and emission spectra of alkali-metal atoms (Cs and Rb) in liquid helium are studied. The helium pressure is changed from the saturated vapor pressure to about 25 atm at 1.6 K, under which conditions the liquid helium is in a superfluid state. The pressure shift and broadening of the observed spectra corresponding to the D lines are explained qualitatively by a theoretical calculation based on the spherical atomic bubble model. The Cs

{\mathit{D}}_{2}

excitation spectra are found to have double peaks, indicating the existence of anisotropic oscillations of the bubble surface. The disappearance of the

{\mathit{D}}_{2}

emission lines and the pressure-dependent quenching of the Rb

{\mathit{D}}_{1}

emission lines are also discussed.