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Physical Review A

Buffer-gas loading of atoms and molecules into a magnetic trap

John M. Doyle, Břetislav Friedrich, Jinha Kim, David Patterson

Published Oct 1, 1995• 257 citations
Publisher

Abstract

We describe a method for loading paramagnetic atoms or molecules into a magnetic trap. A $^{3}\mathrm{He}$ 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 ${10}^{13}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. 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.

DOI
10.1103/physreva.52.r2515

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We have magnetically trapped and evaporatively cooled atomic molybdenum. Using a cryogenic helium buffer gas, $2\ifmmode\times\else\texttimes\fi{}{10}^{10}$ molybdenum atoms are loaded into a magnetic trap at an initial temperature of 500 mK. The molybdenum atoms undergo two-body decay with a measured inelastic rate constant of ${g}_{\mathrm{in}}=(1.2\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{3}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ for the initial conditions after loading. The molybdenum atoms are cooled by adiabatic expansion and forced evaporation to 200 mK.

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arXiv:0802.1662•Physical Review A•Nov 12, 2008•43 citations
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 cm\ensuremath−3{\mathrm{cm}}^{\ensuremath{-}3}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.
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 report the successful buffer-gas cooling and magnetic trapping of chromium atoms with densities exceeding 10 12 atoms per cm 3 at a temperature of 350 mK for the trapped sample. The possibilities of extending the method to buffer-gas cool and magnetically trap molecules are discussed. To minimize the most important loss mechanism in magnetic trapping, molecules with a small spin–spin interaction and a large rotational constant are preferred. Both the CrH ( 6 Σ + ground state) and MnH ( 7 Σ + ) radicals appear to be suitable systems for future experiments.
We have magnetically trapped and evaporatively cooled atomic molybdenum. Using a cryogenic helium buffer gas, $2\ifmmode\times\else\texttimes\fi{}{10}^{10}$ molybdenum atoms are loaded into a magnetic trap at an initial temperature of 500 mK. The molybdenum atoms undergo two-body decay with a measured inelastic rate constant of ${g}_{\mathrm{in}}=(1.2\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{3}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ for the initial conditions after loading. The molybdenum atoms are cooled by adiabatic expansion and forced evaporation to 200 mK.
Summary form only given. The very low temperatures (/spl les/1 /spl mu/K) and simultaneously high densities (/spl ges/10/sup 14/ cm/sup -3/) attainable with magnetic trapping and evaporative cooling techniques have enabled a number of experiments with atoms whose de Broglie wavelengths are on the order of, or larger than, the mean distance between the atoms. We first demonstrated the technique by buffer-gas loading atomic europium (a 7 Bohr magneton species) into a magnetic trap. This work corroborated the feasibility of the scheme and produced the first benefits of magnetic trapping of a complex atom: a hyperfine resolved Zeeman spectrum whose variation with time provided dues about the relaxation dynamics of the trapped states.
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 ${10}^{8}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$.