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arXiv:cond-mat/0601414 · Physical Review A

Homonuclear ionizing collisions of laser-cooled metastable helium atoms

R. J. W. Stas, J. M. McNamara, W. Hogervorst, W. Vassen

Published Mar 14, 2006• 23 citations
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Abstract

We present a theoretical and experimental investigation of homonuclear ionizing collisions of laser-cooled metastable $(2\phantom{\rule{0.2em}{0ex}}^{3}S_{1})$ helium atoms, considering both the fermionic $^{3}\mathrm{He}$ and bosonic $^{4}\mathrm{He}$ isotopes. The theoretical description combines quantum threshold behavior, Wigner's spin-conservation rule, and quantum-statistical symmetry requirements in a single-channel model, complementing a more complete close-coupling theory that has been reported for collisions of metastable $^{4}\mathrm{He}$ atoms. The model is supported with measurements (in the absence of light fields) of ionization rates in magneto-optically trapped samples that contain about $3\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms of a single isotope. The ionization rates are determined from measurements of trap loss due to light-assisted collisions combined with comparative measurements of the ion production rate in the absence and presence of trapping light. Theory and experiment show good agreement.

DOI
10.1103/physreva.73.032713

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We present a theoretical and experimental investigation of homonuclear ionizing collisions of laser-cooled metastable (23S1)(2\phantom{\rule{0.2em}{0ex}}^{3}S_{1})(23S1​) helium atoms, considering both the fermionic 3He^{3}\mathrm{He}3He and bosonic 4He^{4}\mathrm{He}4He isotopes. The theoretical description combines quantum threshold behavior, Wigner's spin-conservation rule, and quantum-statistical symmetry requirements in a single-channel model, complementing a more complete close-coupling theory that has been reported for collisions of metastable 4He^{4}\mathrm{He}4He atoms. The model is supported with measurements (in the absence of light fields) of ionization rates in magneto-optically trapped samples that contain about 3\ifmmode×\else\texttimes\fi1083\ifmmode\times\else\texttimes\fi{}{10}^{8}3\ifmmode×\else\texttimes\fi108 atoms of a single isotope. The ionization rates are determined from measurements of trap loss due to light-assisted collisions combined with comparative measurements of the ion production rate in the absence and presence of trapping light. Theory and experiment show good agreement.
We have investigated cold ionizing heteronuclear collisions in dilute mixtures of metastable (23S1)(2\phantom{\rule{0.2em}{0ex}}^{3}S_{1})(23S1​) 3He^{3}\mathrm{He}3He and 4He^{4}\mathrm{He}4He atoms, extending our previous work on the analogous homonuclear collisions [R. J. W. Stas et al., Phys. Rev. A 73, 032713 (2006)]. A simple theoretical model of such collisions enables us to calculate the heteronuclear ionization rate coefficient, for our quasiunpolarized gas, in the absence of resonant light (T=1.2mK)(T=1.2\phantom{\rule{0.2em}{0ex}}\mathrm{mK})(T=1.2mK): ${K}_{34}^{(\mathrm{th})}=2.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{3}∕\mathrm{s}$. This calculation is supported by a measurement of K34{K}_{34}K34​ using magneto-optically trapped mixtures containing about $1\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms of each species, ${K}_{34}^{(\mathrm{exp})}=2.5(8)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{3}∕\mathrm{s}$. Theory and experiment show good agreement.
Scattering and ionizing cross sections and rates are calculated for ultracold collisions between metastable helium atoms using a fully quantum-mechanical close-coupled formalism. Homonuclear collisions of the bosonic 4He∗+4He∗^{4}\mathrm{He}^{*}+^{4}\mathrm{He}^{*}4He∗+4He∗ and fermionic 3He∗+3He∗^{3}\mathrm{He}^{*}+^{3}\mathrm{He}^{*}3He∗+3He∗ systems, and heteronuclear collisions of the mixed 3He∗+4He∗^{3}\mathrm{He}^{*}+^{4}\mathrm{He}^{*}3He∗+4He∗ system, are investigated over the temperature range 1 $\ensuremath{\mu}\mathrm{K}$ to 1 K. Carefully constructed Born-Oppenheimer molecular potentials are used to describe the electrostatic interaction between the colliding atoms, and complex optical potentials to represent loss through ionization from the ${}^{1,3}\mathrm{\ensuremath{\Sigma}}$ states. Magnetic spin-dipole-mediated transitions from the ${}^{5}\mathrm{\ensuremath{\Sigma}}$ state are included and results reported for spin-polarized and unpolarized systems. Comparisons are made with experimental results, previous semiclassical models, and a perturbed single-channel model.
This thesis presents experimental and theoretical work performed at the Laser Centre of the Vrije Universiteit in Amsterdam to study laser-cooled metastable triplet helium atoms. Samples containing about 3x10^8 helium atoms-either fermionic helium-3 atoms, bosonic helium-4 atoms or mixtures thereof-are cooled to a temperature around 1 mK and form the starting point of the presented studies. The studies include an investigation of cold ionizing collisions in the absence of resonant light, an investigation of magneto-optical trapping with ultraviolet light, a feasibility study of an atomic fountain clock based on metastable helium-3 atoms and the first demonstration of magneto-optical trapping of an isotopic mixture of metastable helium atoms.
By confining 3He{}^{3}\mathrm{He}3He and 4He{}^{4}\mathrm{He}4He in their respective magneto-optical traps, ionization rate coefficients for Penning collisions between two cold He(2s3S1)\mathrm{He}(2s^{3}S_{1})He(2s3S1​) atoms have been measured. At 0.5 mK, the rate coefficients obtained are $(3.8\ifmmode\pm\else\textpm\fi{}1.1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}{\mathrm{cm}}^{3}/\mathrm{s}$ for 4He{}^{4}\mathrm{He}4He- 4He{}^{4}\mathrm{He}4He collisions and $(1.1\ifmmode\pm\else\textpm\fi{}0.4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}{\mathrm{cm}}^{3}/\mathrm{s}$ for 3He{}^{3}\mathrm{He}3He- 3He{}^{3}\mathrm{He}3He, a difference of a factor of 3. Theoretical calculations have shown that such a large difference arises only when the lowest order partial wave alone contributes to the ionization, and that it is mainly due to the difference in the quantum statistical symmetry between 3He{}^{3}\mathrm{He}3He and 4He{}^{4}\mathrm{He}4He.
We give an overview of the experiments at the Laser Centre of the Vrije Universiteit in Amsterdam on ultracold gases of metastable helium-4 and helium-3 as well as mixtures of both isotopes. We describe our experimental setup and discuss our experiments on Bose-Einstein condensation of 4He* (more than 10 million atoms in a BEC) and sympathetic cooling of 3He* towards Fermi degeneracy (more than 1 million atoms below the Fermi temperature as well as realization of a degenerate boson-fermion mixture). We also present our results on Penning Ionization in ~1 mK clouds containing a single isotope (homonuclear losses) or a mixture of both isotopes (heteronuclear losses).