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New Journal of Physics

Magnetic trapping of NH molecules with 20 s lifetimes

Edem Tsikata, Wesley C. Campbell, Matthew T. Hummon, H-I Lu, John M. Doyle

Published Jun 28, 2010• 40 citations
Publisher

Abstract

Buffer gas cooling is used to trap NH molecules with 1/ e lifetimes exceeding 20 s. Helium vapor generated by laser desorption of a helium film is employed to thermalize 10 5 molecules at a temperature of 500 mK in a 3.9 T magnetic trap. Long molecule trapping times are attained through rapid pumpout of residual buffer gas. Molecules experience a helium background gas density below 1 × 10 12 cm −3 .

DOI
10.1088/1367-2630/12/6/065028

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Buffer gas cooling is used to trap NH molecules with 1/e lifetimes exceeding 20 s. Helium vapor generated by laser desorption of a helium film is employed to thermalize 105molecules at a temperature of 500mK in a 3.9 T magnetic trap. Long molecule trapping times are attained through rapid pumpout of residual buffer gas. Molecules experience a helium background gas density below 1 x 1012cm-3. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

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Physical Review A•Sep 17, 2008•38 citations
Buffer gas cooling is used to trap NH molecules with 1/ e lifetimes exceeding 20 s. Helium vapor generated by laser desorption of a helium film is employed to thermalize 10 5 molecules at a temperature of 500 mK in a 3.9 T magnetic trap. Long molecule trapping times are attained through rapid pumpout of residual buffer gas. Molecules experience a helium background gas density below 1 × 10 12 cm −3 .
A new technique for buffer gas loading is described. This technique greatly extends the range of atoms and molecules that may be magnetically trapped at low temperature. The advance is made possible by the rapid removal of the buffer gas on a time scale one hundred times greater than in previous buffer gas loading experiments. A new cryogenic valve is developed and employed. The following benchmarks were attained in our first experimental run; approximately 10 atoms with effective magnetic moments μeff & 3μB were trapped and thermally isolated with near unit efficiency, ~ 10 μeff = 2μB atoms were trapped and thermally isolated, and ~ 10 μeff = 1μB atoms were trapped, but without thermal isolation. For comparison, all previous buffer gas loading experiments that achieved thermal isolation after trapping were done with atoms having a magnetic moment of at least 6μB. In our second run of the experiment, better temperature management allowed us to increase the number of trapped and thermally isolated μeff = 2μB atoms to ~10 and trapped μeff = 1μB atoms to ~ 10. The performance of the present apparatus is limited by the presence of a desorbing helium film that compromises the vacuum in the cell after the bulk of the buffer gas is removed. Future improvements to address this problem are suggested that will likely allow for efficient trapping and thermal isolation of atoms and molecules with magnetic moments as low as 1μB. Analysis of the trapping and pumping dynamics is presented.
We describe a method for loading paramagnetic atoms or molecules into a magnetic trap. A 3He^{3}\mathrm{He}3He buffer gas is employed to thermalize atoms or molecules to a temperature of approximately 240 mK, lower than the depth of the trap. A model is described that indicates an initial loading density of approximately 1013{10}^{13}1013 ${\mathrm{cm}}^{\ensuremath{-}3}$. Once loading has taken place the buffer gas is removed by cryopumping. Evaporative cooling can then be applied to further lower the temperature and increase the density of the trapped sample.
Achieving precise control over an array of ultracold molecules would provide a unique tool-set for carrying out quantum simulations and quantum computations, as a result of the molecules' rich internal structure. To realise this aim, the molecules have to be cooled and trapped. This is much more difficult for molecules than for atoms due to their complex internal structure. This thesis presents preliminary work towards realising a versatile, permanent magnet trap for buffer gas cooled molecules. Atoms are used throughout to test the feasibility of the trap, as they are easier to produce and detect. Two novel methods for trapping buffer gas cooled atoms in a permanent magnet trap are investigated. The first of these involves trapping the atoms directly from a cryogenic buffer gas cooled ablation plume. Dy atoms, with a magnetic moment of 10µB, are trapped with a lifetime of 800±30 μs, thought to be limited by collisions with a high density of background buffer gas atoms remaining in the trap region. Information gained from the direct trapping experiments motivated the design of a second trapping set-up. Here, a beam of Dy atoms is first extracted from a cryogenic buffer gas source, and when this beam reaches the trapping region, a fraction of the atoms are stopped through collisions with cold helium gas present in the trapping region. This second method reduces the density of buffer gas required in the trap region. The trap lifetime achieved in this arrangement of 810±40 μs is no longer than in the direct trapping experiments, but this arrangement is much more stable and repeatable. The lifetime here is also thought to be limited by collisions with background buffer gas atoms.
We observe magnetic trapping of atomic nitrogen (14N)(^{14}\mathrm{N})(14N) and cotrapping of ground-state imidogen (14NH^{14}\mathrm{N}\mathrm{H}14NH, $X\phantom{\rule{0.2em}{0ex}}^{3}\ensuremath{\Sigma}^{\ensuremath{-}}$). Both are loaded directly from a room-temperature beam via buffer gas cooling. We trap approximately $1\ifmmode\times\else\texttimes\fi{}{10}^{11}$ 14N^{14}\mathrm{N}14N atoms at a peak density of $5\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ at 550mK550\phantom{\rule{0.3em}{0ex}}\mathrm{mK}550mK. The $12\ifmmode\pm\else\textpm\fi{}4\phantom{\rule{0.3em}{0ex}}\mathrm{s}$ 1∕e1∕e1∕e lifetime of atomic nitrogen in the trap is consistent with a model for loss of atoms over the edge of the trap in the presence of helium buffer gas. Cotrapping of 14N^{14}\mathrm{N}14N and 14NH^{14}\mathrm{N}\mathrm{H}14NH is accomplished, with 108{10}^{8}108 NH trapped molecules at a peak density of ${10}^{8}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$.
We report on the buffer-gas cooling and trapping of CrH and MnH molecules in a magnetic quadrupole trap with densities on the order of ${10}^{6}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ at a temperature of 650mK650\phantom{\rule{0.3em}{0ex}}\mathrm{mK}650mK. Storage times of up to 180ms180\phantom{\rule{0.3em}{0ex}}\mathrm{ms}180ms have been observed, corresponding to a 20-fold lifetime enhancement with respect to the field-free diffusion through the 3He^{3}\mathrm{He}3He buffer-gas. Using Monte Carlo trajectory simulations, inelastic molecule-3He^{3}\mathrm{He}3He collision cross sections of $1.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}$ and $3.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}$ are extracted for CrH and MnH, respectively. Furthermore, elastic molecule-3He^{3}\mathrm{He}3He collision cross sections of $1.4(\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}$ are determined for both species. We conclude that the confinement time of these molecules in a magnetic trapping field is limited by inelastic collisions with the helium atoms leading to Zeeman relaxation.