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arXiv:1003.0948 · Physical Review Letters

Formation of van der Waals Molecules in Buffer-Gas-Cooled Magnetic Traps

Nathan Brahms, Timur V. Tscherbul, Peng Zhang, Jacek A. Kłos, H. R. Sadeghpour, A. Dalgarno, John M. Doyle, Thad G Walker

Published Jul 16, 2010• 33 citations
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Abstract

We predict that a large class of helium-containing cold polar molecules form readily in a cryogenic buffer gas, achieving densities as high as 10(12) cm(-3). We explore the spin relaxation of these molecules in buffer-gas-loaded magnetic traps and identify a loss mechanism based on Landau-Zener transitions arising from the anisotropic hyperfine interaction. Our results show that the recently observed strong T(-6) thermal dependence of the spin-change rate of silver (Ag) trapped in dense (3)He is accounted for by the formation and spin change of Ag(3)He van der Waals molecules, thus providing indirect evidence for molecular formation in a buffer-gas trap.

DOI
10.1103/physrevlett.105.033001

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Formation and dynamics of van der Waals molecules in buffer-gas traps

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We show that weakly bound He-containing van der Waals molecules can be produced and magnetically trapped in buffer-gas cooling experiments, and provide a general model for the formation and dynamics of these molecules. Our analysis shows that, at typical experimental parameters, thermodynamics favors the formation of van der Waals complexes composed of a helium atom bound to most open-shell atoms and molecules, and that complex formation occurs quickly enough to ensure chemical equilibrium. For molecular pairs composed of a He atom and an S-state atom, the molecular spin is stable during formation, dissociation, and collisions, and thus these molecules can be magnetically trapped. Collisional spin relaxation is too slow to affect trap lifetimes. However, (3)He-containing complexes can change spin due to adiabatic crossings between trapped and untrapped Zeeman states, mediated by the anisotropic hyperfine interaction, causing trap loss. We provide a detailed model for Ag(3)He molecules, using ab initio calculation of Ag-He interaction potentials and spin interactions, quantum scattering theory, and direct Monte Carlo simulations to describe formation and spin relaxation in this system. The calculated rate of spin-change agrees quantitatively with experimental observations, providing indirect evidence for molecular formation in buffer-gas-cooled magnetic traps. Finally, we discuss the possibilities for spectroscopic detection of these complexes, including a calculation of expected spectra for Ag(3)He, and report on our spectroscopic search for Ag(3)He, which produced a null result.

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Anomalous modes of electronic-spin relaxation of optically pumped Rb atoms observed in He and Ne buffer gases may result from the modification of the Rb hyperfine interaction, induced by the formation of Rb-He and Rb-Ne Van der Waals molecules. Isolation of anomalous effects also resolves disagreements in cross sections previously reported for the spin relaxation of Cs in Ne.

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Publisher’s Note: Formation of van der Waals Molecules in Buffer-Gas-Cooled Magnetic Traps [Phys. Rev. Lett.105, 033001 (2010)]

Nathan Brahms, Timur V. Tscherbul, P. Zhang +5 more

Received 19 July 2010DOI:https://doi.org/10.1103/PhysRevLett.105.059901©2010 American Physical Society

Physical Review Letters•Jul 26, 2010•1 citation

Cold Anisotropically Interacting van der Waals Molecule: TiHe

N. Quirós, Naima Tariq, Timur V. Tscherbul +2 more

We have used laser ablation and helium buffer-gas cooling to produce titanium-helium van der Waals molecules at cryogenic temperatures. The molecules were detected through laser-induced fluorescence spectroscopy. Ground-state Ti(a^{3}F_{2})-He binding energies were determined for the ground and first rotationally excited states from studying equilibrium thermodynamic properties, and found to agree well with theoretical calculations based on newly calculated ab initio Ti-He interaction potentials, opening up novel possibilities for studying the formation, dynamics, and nonuniversal chemistry of van der Waals clusters at low temperatures.

arXiv:1702.04116•Physical Review Letters•May 26, 2017•19 citations
We predict that a large class of helium-containing cold polar molecules form readily in a cryogenic buffer gas, achieving densities as high as 10(12) cm(-3). We explore the spin relaxation of these molecules in buffer-gas-loaded magnetic traps and identify a loss mechanism based on Landau-Zener transitions arising from the anisotropic hyperfine interaction. Our results show that the recently observed strong T(-6) thermal dependence of the spin-change rate of silver (Ag) trapped in dense (3)He is accounted for by the formation and spin change of Ag(3)He van der Waals molecules, thus providing indirect evidence for molecular formation in a buffer-gas trap.
We show that weakly bound He-containing van der Waals molecules can be produced and magnetically trapped in buffer-gas cooling experiments, and provide a general model for the formation and dynamics of these molecules. Our analysis shows that, at typical experimental parameters, thermodynamics favors the formation of van der Waals complexes composed of a helium atom bound to most open-shell atoms and molecules, and that complex formation occurs quickly enough to ensure chemical equilibrium. For molecular pairs composed of a He atom and an S-state atom, the molecular spin is stable during formation, dissociation, and collisions, and thus these molecules can be magnetically trapped. Collisional spin relaxation is too slow to affect trap lifetimes. However, (3)He-containing complexes can change spin due to adiabatic crossings between trapped and untrapped Zeeman states, mediated by the anisotropic hyperfine interaction, causing trap loss. We provide a detailed model for Ag(3)He molecules, using ab initio calculation of Ag-He interaction potentials and spin interactions, quantum scattering theory, and direct Monte Carlo simulations to describe formation and spin relaxation in this system. The calculated rate of spin-change agrees quantitatively with experimental observations, providing indirect evidence for molecular formation in buffer-gas-cooled magnetic traps. Finally, we discuss the possibilities for spectroscopic detection of these complexes, including a calculation of expected spectra for Ag(3)He, and report on our spectroscopic search for Ag(3)He, which produced a null result.
We have trapped large numbers of copper (Cu) and silver (Ag) atoms using buffer-gas cooling. Up to $3\ifmmode\times\else\texttimes\fi{}{10}^{12}\text{ }\text{ }\mathrm{Cu}$ atoms and $4\ifmmode\times\else\texttimes\fi{}{10}^{13}\text{ }\text{ }\mathrm{Ag}$ atoms are trapped. Lifetimes are as long as 5 s, limited by collisions with the buffer gas. Ratios of elastic to inelastic collision rates with He are $\ensuremath{\gtrsim}{10}^{6}$, suggesting Cu and Ag are favorable for use in ultracold applications. The temperature dependence of the $\mathrm{Ag}\mathrm{\text{\ensuremath{-}}}^{3}\mathrm{He}$ collision rate varies as ${T}^{5.8\ifmmode\pm\else\textpm\fi{}0.4}$. We find that this temperature dependence is inconsistent with the behavior predicted for relaxation arising from the spin-rotation interaction, and conclude that the $\mathrm{Ag}\mathrm{\text{\ensuremath{-}}}^{3}\mathrm{He}$ system displays anomalous collisional behavior in the multiple-partial wave regime. Gold (Au) was ablated into 3He^{3}\mathrm{He}3He buffer gas, however, atomic Au lifetimes were observed to be too short to permit trapping.
We have used laser ablation and helium buffer-gas cooling to produce titanium-helium van der Waals molecules at cryogenic temperatures. The molecules were detected through laser-induced fluorescence spectroscopy. Ground-state Ti(a^{3}F_{2})-He binding energies were determined for the ground and first rotationally excited states from studying equilibrium thermodynamic properties, and found to agree well with theoretical calculations based on newly calculated ab initio Ti-He interaction potentials, opening up novel possibilities for studying the formation, dynamics, and nonuniversal chemistry of van der Waals clusters at low temperatures.