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arXiv:0901.3129 · Physical Review A

Feshbach resonances in ultracold atom-molecule collisions

Andrea Simoni, Jean–Michel Launay, Pavel Soldán

Published Mar 3, 2009• 37 citations
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Abstract

We investigate the presence of Feshbach resonances in ultracold alkali-dialkali reactive collisions. A potential-energy surface of ${\text{Na}}_{3}$ in the lowest quartet state is constructed and used in quantum scattering calculations. An analysis of scattering features is performed through a systematic variation in the nonadditive three-body interaction potential. Our results should provide useful information for interpreting future atom-molecule collision experiments.

DOI
10.1103/physreva.79.032701

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We investigate the presence of Feshbach resonances in ultracold alkali-dialkali reactive collisions. A potential-energy surface of Na3{\text{Na}}_{3}Na3​ in the lowest quartet state is constructed and used in quantum scattering calculations. An analysis of scattering features is performed through a systematic variation in the nonadditive three-body interaction potential. Our results should provide useful information for interpreting future atom-molecule collision experiments.
Compared to purely atomic collisions, ultracold molecular collisions potentially support a much larger number of Fano-Feshbach resonances due to the enormous number of rovibrational states available. In fact, for alkali-metal dimers we find that the resulting density of resonances cannot be resolved at all, even on the sub-$\ensuremath{\mu}$K temperature scale of ultracold experiments. As a result, all observables become averaged over many resonances and can effectively be described by simpler, nonresonant scattering calculations. Two particular examples are discussed: nonchemically reactive RbCs and chemically reactive KRb. In the former case, the formation of a long-lived collision complex may lead to the ejection of molecules from a trap. In the latter case, chemical reactions broaden the resonances so much that they become unobservable.
Collisional resonances of molecules can offer a deeper understanding of interaction potentials and collision complexes, and allow control of chemical reactions.Here, we experimentally map out the spectrum of Feshbach resonances in collisions between ultracold triplet rovibrational ground-state NaLi molecules and Na atoms over a range of 1400 G. Preparation of the spin-stretched state puts the system initially into the nonreactive quartet potential.A total of 25 resonances are observed, in qualitative agreement with quantum-chemistry calculations using a coupled-channels approach.Although the theory cannot predict the positions of resonances, it can account for several experimental findings and provide unprecedented insight into the nature and couplings of ultracold, strongly interacting complexes.Previous work has addressed only weakly bound complexes.We show that the main coupling mechanism results from spin-rotation and spin-spin couplings in combination with the anisotropic atom-molecule interaction, and that the collisional complexes which support the resonances have a size of 30a 0 -40a 0 .This study illustrates the potential of a combined experimental and theoretical approach.
Compared to purely atomic collisions, ultracold molecular collisions potentially support a much larger number of Fano-Feshbach resonances due to the enormous number of ro-vibrational states available. In fact, for alkali-metal dimers we find that the resulting density of resonances cannot be resolved at all, even on the sub-μμμK temperature scale of ultracold experiments. As a result, all observables become averaged over many resonances and can effectively be described by simpler, non-resonant scattering calculations. Two particular examples are discussed: non-chemically reactive RbCs and chemically reactive KRb. In the former case, the formation of a long-lived collision complex may lead to the ejection of molecules from a trap. In the latter case, chemical reactions broaden the resonances so much that they become unobservable.
Ultracold collisions between spin-polarized Na atoms and vibrationally excited Na2{\mathrm{N}\mathrm{a}}_{2}Na2​ molecules are investigated theoretically, using a reactive scattering formalism (including atom exchange). Calculations are carried out on both pairwise additive and nonadditive potential energy surfaces for the quartet electronic state. The Wigner threshold laws are followed for energies below ${10}^{\ensuremath{-}5}\text{ }\mathrm{K}$. Vibrational relaxation processes dominate elastic processes for temperatures below ${10}^{\ensuremath{-}3}--{10}^{\ensuremath{-}4}\text{ }\mathrm{K}$. For temperatures below ${10}^{\ensuremath{-}5}\text{ }\mathrm{K}$, the rate coefficients for vibrational relaxation ($v=1\ensuremath{\rightarrow}0$) are $4.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ and $5.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\text{ }{\mathrm{c}\mathrm{m}}^{3}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$ for the additive and nonadditive potentials, respectively. The large difference emphasizes the importance of using accurate potential energy surfaces for such calculations.
Abstract Compared to purely atomic collisions, ultracold molecular collisions potentially support a muchlarger number of Fano-Feshbach resonances due to the enormous number of ro-vibrational statesavailable. In fact, for alkali-metal dimers we nd that the resulting density of resonances cannotbe resolved at all, even on the sub-K temperature scale of ultracold experiments. As a result, allobservables become averaged over many resonances and can e ectively be described by simpler, non-resonant scattering calculations. Two particular examples are discussed: non-chemically reactiveRbCs and chemically reactive KRb. In the former case, the formation of a long-lived collisioncomplex may lead to the ejection of molecules from a trap. In the latter case, chemical reactionsbroaden the resonances so much that they become unobservable. PACS numbers: 34.50.-s, 34.50.Cx 1 Present address: Laboratoire Aime Cotton, Universite Paris-Sud, CNRS, B^at. 505, 91405, Orsay, France 1 arXiv:1210.4955v1 [physics.atom-ph] 17 Oct 2012
Magnetic Feshbach resonances play a central role in experimental research on atomic gases at ultracold temperatures, as they allow one to control the microscopic interactions between ultracold atoms by tuning an applied magnetic field. These resonances arise due to strong hyperfine interactions between the unpaired electron and the nuclear magnetic moment of the alkali metal atoms. A major thrust of current research is to create an ultracold gas of diatomic alkali metal molecules in the ground rovibrational state of the ground electronic ${}^{1}\ensuremath{\Sigma}$ state. Unlike alkali metal atoms, ${}^{1}\ensuremath{\Sigma}$ diatomic molecules have no unpaired electrons. However, the hyperfine interactions of molecules may give rise to magnetic Feshbach resonances. We use quantum scattering calculations to study the possible width of these resonances. Our results show that the widths of magnetic Feshbach resonances in ultracold molecule-molecule collisions for ${}^{1}\ensuremath{\Sigma}$ molecules may exceed 1 mG, rendering such resonances experimentally detectable. We hope that this work will stimulate the experimental search of these resonances.