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Physical Review Letters

Optical Collisions of Cold, Metastable Helium Atoms

Hennie C. Mastwijk, Jan W. Thomsen, P. van der Straten, Arend Niehaus

Published Jun 22, 1998• 49 citations
Publisher

Abstract

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.

DOI
10.1103/physrevlett.80.5516

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Physical Review A•Dec 1, 1999•45 citations
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.
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 .
We have studied a cloud of cold metastable helium (He*) atoms interacting with near-resonant light at 1083 nm and 389 nm. The 1083 nm light allows for efficient loading of a large magneto-optical trap (MOT) and the 389 nm light is subsequently used to increase the density and reduce the temperature of the He* cloud during a brief compression stage. Cold collisions in the cloud yield ions and fast metastables which are monitored separately using calibrated microchannel plate (MCP) detectors. We thus measure absolute production rates of ions and fast metastables escaping from the MOT during the various stages of the experiment. We observe that 389 nm optical collisions, apart from Penning ionization, produce a relatively large flux of fast metastables, which we relate to the short-range behaviour of the molecular potentials involved. Furthermore, by rapidly switching between 389 nm and 1083 nm the ratio between the respective two-body loss rate constants is determined. Using these values, together with the observed time dependence of the cloud size, the temporal behaviour of the absolute ion production rate during the compression stage is well reproduced.
This thesis presents investigations into the collision dynamics of cold metastable neon atoms in a magneto-optical trap (MOT). At very cold temperatures (approx. 1 mK) the atomic interaction occurs at long range and cannot be treated classically as the interaction between two nuclear point particles. In addition, the collision dynamics can be considerably altered in the presence of an optical eld. In this thesis results are presented for collisions that are altered by the presence of an optical eld and also collisions where the optical eld can be neglected. Controlled optical collisions of cold, metastable Ne atoms are performed by using a control laser tuned close to the (3s)3P2 to (3p)3D3 cooling transition. The control laser modies the rate of ionizing collisions by promoting a pair of colliding atoms onto an excited state potential. In order to induce collisions between pairs of trapped metastables, a control laser is tuned to the red of the transition where we measure a factor of 4 increase in the rate for ionizing collisions. We also investigate the eect of optical shielding by tuning the laser to the blue of the transition and observe a factor of 5 suppression in the ionization rate. However, the measured ionization spectrum does not contain resonances due to the formation of photoassociated molecules as predicted by Doery et al [1]. Instead, we observe a broad unresolvable ionization spectrum that is well described by established theory [2]. Collision cross sections are measured for neon in the (3s)3P2 metastable state with ground state, thermal atoms and molecules. The technique is based on the measurement of the loss rate due to collisions with a background gas. Generally, these type of collisions are considered undesirable, however, it is possible to infer information about the collision cross section if the rate of collisions is accurately known. This novel technique is described and collision cross section measurements are presented for metastable neon in the (3s)3P2 state with He, Ne, Ar, H2, O2, and N2. The average thermal energy of the collision ranges from 11 meV to 27 meV and the measurements have relatively small errors, of the order of 10%.
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.
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.
A quantum-mechanical close-coupled calculation for collisions of spin-polarized triplet helium atoms in a magnetostatic trap is presented in which the Penning and associative ionization processes are represented by a complex optical potential. The rate coefficients for the spin relaxation and relaxation-induced Penning ionization processes for a range of magnetic fields and temperatures, spanning the cold and ultracold temperature regimes, have been calculated and the sensitivity of these results to uncertainties in the molecular potentials and autoionization width investigated. The rate constants for both elastic and inelastic processes were found to be particularly sensitive to the short-range ${}^{5}{\ensuremath{\Sigma}}_{g}^{+}$ molecular potential and, to a lesser extent, the rate constants for relaxation-induced Penning ionization displayed sensitivity to the short-range form of the ${}^{1}{\ensuremath{\Sigma}}_{g}^{+}$ molecular potential and the autoionization width.