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Trapping fermionic and bosonic helium atoms

R. J. W. Stas

Published Jun 1, 2005• 1 citation
OpenAlex

Abstract

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.

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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.
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).
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 $3\ifmmode\times\else\texttimes\fi{}{10}^{8}$ 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.
About 10 4 triplet metastable helium atoms have been trapped in a magneto-optical trap, based on the 2 3 S 1 –2 3 P 2 transition at 1.083 μm (volume 0.1 mm 3 , temperature 1 mK). The losses of the trap are analysed by monitoring the ion current due to Penning collisions either with background gases, or between trapped atoms. The rate coefficient for these collisions between trapped metastable helium atoms has been found to be unusually large, and the crucial role of the quasi-resonant laser light has been experimentally demonstrated. The rate coefficient of the dominant process (He*(2 3 S 1 ) + He*(2 3 P 2 ) Penning collisions) has been measured by two different methods, and found to be K sp ≃ 10 -7 cm 3 s -1 within an estimated factor of 4. The corresponding cross-section is 6 · 10 6 Å 2 .
During the last decades the study of cold atoms has grown in a great measure. Research in this field has been made possible due to the development of laser cooling and trapping techniques. We use laser cooling to cool helium atoms down to a temperature of 1 mK and we are able to trap 106 atoms in our magneto-optical trap. The helium atoms are in a metastable state, the He 23S1 state, which has a lifetime of more than 2 hours. With cold atoms phenomena can be studied that are not accessible using thermal atoms, such as high resolution spectroscopy. Furthermore cold atoms are the first step towards Bose-Einstein condensation. Cold alkali-metal atoms have been extensively studied, using photoassociation spectroscopy. In this thesis the first photoassociation results for rare gas atoms, namely metastable helium atoms will be discussed. Various experiments are described and an analysis of the results is given.
We have studied the optical collisions of cold, metastable helium atoms in a magneto-optical trap. We have detected the rate of Penning and associative ionization of two metastable helium atoms at a temperature of 1 mK with and without nearly resonant light. We find that the associative ionization rate is increased with more than a factor 20 due to the presence of the light field. We present a simple, semiclassical model, which accounts for all the observed features, and which agrees, also on an absolute scale, with our experimental results.
We simultaneously confine fermionic metastable 3He^{3}\mathrm{H}\mathrm{e}3He atoms and bosonic metastable 4He^{4}\mathrm{H}\mathrm{e}4He atoms in a magneto-optical trap. The trapped clouds, containing up to $1.5\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms of each isotope, are characterized by measuring ions and metastable helium atoms escaping from the trap. Optical pumping of 3He^{3}\mathrm{H}\mathrm{e}3He atoms to a nontrapped hyperfine state is investigated and it is shown that large atom numbers can be confined without additional repumping lasers. Unique possibilities for quantum degeneracy experiments with mixtures of spin-polarized metastable 3He^{3}\mathrm{H}\mathrm{e}3He and 4He^{4}\mathrm{H}\mathrm{e}4He atoms are indicated.