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Quantum Electronics and Laser Science Conference

Laser spectroscopy of atoms and molecules in buffer-gas loaded magnetic traps

John M. Doyle

Published Jan 20, 2003• 1 citation
Publisher

Abstract

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.

DOI
10.1109/qels.1999.807464

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•Mar 1, 2015•0 citations
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.
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.
Atomic europium has been magnetically trapped using buffer-gas loading. Laser ablated Eu(8S7/2)\mathrm{Eu}(^{8}S_{7/2})Eu(8S7/2​) atoms are thermalized to 800 mK in a 4He{}^{4}\mathrm{He}4He buffer gas (to 250 mK in a 3He{}^{3}\mathrm{He}3He buffer gas). Anti-Helmholtz superconducting coils produce a quadrupole magnetic field to trap the MJ=7/2{M}_{J}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}7/2MJ​=7/2 state of Eu. Detection is via absorption spectroscopy at 462.7 nm. Up to $1\ifmmode\times\else\texttimes\fi{}{10}^{12}\mathrm{Eu}$ atoms are loaded at a central density of $5\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0ex}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. Atoms can be held for longer than 100 s.
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.
We present a simulation of the laser absorption spectra of Eu atoms in the $a{}^{8}{S}_{7/2}\ensuremath{-}y{}^{8}{P}_{7/2}$ band at 462.7nm462.7 \mathrm{nm}462.7nm measured previously in a buffer-gas-loaded magnetic trap [J. Kim et al., Phys. Rev. Lett. 78, 3665 (1997)]. The simulations of the hyperfine-resolved Zeeman spectra are based on exact magnetic eigenproperties of the states involved, and yield a complete assignment of all the features observed. This reveals that apart from the MJ=7/2{M}_{J}=7/2MJ​=7/2 state and its hyperfine substates, the MJ=5/2{M}_{J}=5/2MJ​=5/2 and 3/23/23/2 states are also trapped at Bmax=0.52T{B}_{\mathrm{max}}=0.52 \mathrm{T}Bmax​=0.52T and a temperature of about 250mK.250 \mathrm{mK}.250mK.
Cold gases of atoms and molecules provide a system for the exploration of a diverse set of physical phenomena. For example, cold gasses of magnetically and electrically polar atoms and molecules are ideal systems for quantum simulation and quantum computation experiments, and cold gasses of large polar molecules allow for novel spectroscopic techniques. Buffer-gas cooling is a robust and widely applicable method for cooling atoms and molecules to temperatures of approximately 1 Kelvin. In this thesis, I present novel applications of buffer-gas cooling to obtaining gases of trapped, ultracold atoms and diatomic molecules, as well as the study of the cooling of large organic molecules. In the first experiment of this thesis, a buffer-gas beam source of atoms is used to directly load a magneto-optical trap. Due to the versatility of the buffer-gas beam source, we obtain trapped, sub-milliKelvin gases of four different lanthanide species using the same experimental apparatus. In the second experiment of this thesis, a buffer-gas beam is used as the initial stage of an experiment to directly laser cool and magneto-optically trap the diatomic molecule CaF. In the third experiment of this thesis, buffer-gas cooling is used to study the cooling of the conformational state of large organic molecules. We directly observe conformational relaxation of gas-phase 1,2-propanediol due to cold collisions with helium gas. Lastly, I present preliminary results on a variety of novel applications of buffer-gas cooling, such as mixture analysis, separation of chiral mixtures, the measurement of parity-violation in chiral molecules, and the cooling and spectroscopy of highly unstable reaction intermediates.
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.