The Journal of Physical Chemistry A

Spectroscopy of Nonspherical Atomic Bubbles in Solid Helium

Victor Lebedev, Peter Moroshkin, Antoine Weis

Published Apr 4, 2011• 5 citations

Abstract

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.

DOI: 10.1021/jp200159s

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The Journal of Physical Chemistry A

Spectroscopy of Nonspherical Atomic Bubbles in Solid Helium

Victor Lebedev, Peter Moroshkin, Antoine Weis

Published Apr 4, 2011• 5 citations

Publisher

Abstract

DOI: 10.1021/jp200159s

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Physical Review LettersMar 7, 200219 citations

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.

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 measured the pressure broadening and shift of the excitation and emission lines of the 6

{\mathit{s}}^{2}

^{1}

{\mathit{S}}_{0}

\ensuremath{\rightarrow}6s6p

^{1}

{\mathit{P}}_{1}

transition of Ba atoms implanted in liquid and solid helium at 1.5 K. The observed spectra are quantitatively explained within the adiabatic line-broadening theory by assuming a bubble structure of the trapping sites for the Ba atoms. The evolution of the bubble shape and size in the optical excitation-deexcitation cycle is modeled theoretically.

{\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.

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.