Bulletin of the American Physical Society
2005 36th Meeting of the Division of Atomic, Molecular and Optical Physics
Tuesday–Saturday, May 17–21, 2005; Lincoln, Nebraska
Session L5: Ultracold Collisions and Photoassociation Processes |
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Chair: Brett DePaola, Kansas State University Room: Burnham Yates Conference Center Arbor |
Friday, May 20, 2005 1:30PM - 1:42PM |
L5.00001: On the Nature of Feshbach Resonances Paul Julienne Magnetically tunable Feshbach resonances that have been used in various experiments with cold bosonic and fermionic quantum gases differ widely in their 2-body molecular physics properties. Some resonances tend to be “open channel dominated” for near-resonance magnetic field tuning spanning a large fraction of the resonance width. Such resonances are well- characterized by a single scattering length for a field- dependent effective potential. Others are “closed channel dominated” for tuning spanning most of their width, and the scattering length alone provides an inadequate parameterization of near-threshold scattering and bound state properties. Coupled channels calculations of near-threshold bound and scattering properties using a quantitative model [1] of (F=1/2, M=1/2) + (F=1/2, M=-1/2)6Li atomic collisions illustrate these differences well. The results are consistent with the picture in [2]. The 834 G s-wave resonance is strongly “open channel dominated”, whereas the 543 G s-wave resonance is strongly “closed channel dominated”. Using formulas in [3], a general criterion for the near-resonance magnetic detuning range of open channel dominance can be stated in terms of the resonance width, the background scattering length, the difference in magnetic moment between open and closed channels, and the reduced mass of the atom pair. [1] M. Bartenstein, et. al., cond-mat/0408673; [2] S. Simonucci, et al., cond-mat/0407600; [3] Goral et al., J. Phys. B 37, 3457 (2004) [Preview Abstract] |
Friday, May 20, 2005 1:42PM - 1:54PM |
L5.00002: Observation of Feshbach resonances in an ultracold gas of chromium atoms Juergen Stuhler, Joerg Werner, Axel Griesmaier, Sven Hensler, Tilman Pfau, Andrea Simoni, Eite Tiesinga We have observed Feshbach resonances (FRs) in collisions between ultracold $^{52}$Cr atoms by monitoring atom loss from a crossed optical dipole trap as a function of applied magnetic offset fields (0-60 mT). This is the first observation of FRs in an atomic species with more than one valence electron. Chromium has the electronic configuration [Ar]3d$^{5}$4s$^{1}$, which gives rise to its large magnetic moment of 6 Bohr magnetons. The zero nuclear spin of $^{52}$Cr leads to regularly-spaced resonance sequences. Comparing the experimental FR positions with multi-channel scattering calculations of A. Simoni and E. Tiesinga (NIST Gaithersburg, USA), we are able to assign all eleven FR that are expected up to second order in the dipole-dipole coupling for the S=6, MS=-6 s-wave entrance channel and to determine the s-wave scattering lengths of the lowest $^{2S+1}\Sigma ^{+}_{g}$ potentials to be 112(14) a0, 58(6) a0 and -7(20) a0 (a0 = 0.0529 nm) for S = 6, 4, and 2, respectively. Exploiting one of the FRs to tune the isotropic contact interaction in our recently realized Bose-Einstein condensate (BEC) of $^{52}$Cr should allow us to create a dipolar BEC in which the anisotropic (magnetic) dipole-dipole interaction is dominant. [Preview Abstract] |
Friday, May 20, 2005 1:54PM - 2:06PM |
L5.00003: Optical Feshbach resonances in a Rubidium BEC Matthias Theis, Gregor Thalhammer, Klaus Winkler, Rudolf Grimm, Johannes Hecker Denschlag We demonstrate optical tuning of the scattering length in a Bose-Einstein condensate with a so-called optical Feshbach resonance as predicted by Fedichev et al. [1]. This resonance is based on optically coupling pairs of atoms to a bound molecular state. We show that this scattering resonance can be induced via one-photon transitions to an excited molecular state [2] or via two-photon transitions to a molecular ground state [3]. By controlling the laser power and the detuning from the optical resonance we are able to change the atomic scattering length over a wide range and we can describe the results with the theoretical model by Bohn and Julienne [4]. [1] P. Fedichev et al., Phys. Rev. Lett. 77, 2913 (1996) [2] M. Theis et al., Phys. Rev. Lett. 93, 123001 (2004) [3] G. Thalhammer et al., Phys. Rev. A in press, cond-mat 0409552 (2004) [4] J. L. Bohn et al., Phys. Rev. A 60, 414 (1999) [Preview Abstract] |
Friday, May 20, 2005 2:06PM - 2:18PM |
L5.00004: Optically tunable Feshbach resonances for control of ultracold alkaline earth atomic gases Roman Ciurylo, Eite Tiesinga, Paul Julienne Optical Feshbach resonances due to photoassociation of two colliding ground state atoms using laser frequencies to the red of the atomic intercombination line may provide a way to control the scattering length of ultracold alkaline earth atomic collisions, such as Ca or Sr, and the related species Yb. Since the line width of the intercombination line is very small due to the long lifetime of the excited state, it is possible to use large detuning from the photoassociation resonance to control loss processes due to excited state decay. The small linewidth also allows the detuning from atomic resonance to be small enough while molecular Franck-Condon factors are large enough to allow significant changes in scattering length. Our model calculations for cold Ca collisions use standard expressions from photoassociation theory to calculate the real and imaginary parts of the scattering length and develop criteria for changing the scattering length without excessive losses. Unlike alkali atom systems where rapid loss processes accompany optical Feshbach control of the scattering length, losses can remain acceptably small for the Group II atoms, while retaining the ability to change the magnitude and sign of the scattering length. [Preview Abstract] |
Friday, May 20, 2005 2:18PM - 2:30PM |
L5.00005: Photoassociative spectroscopy at long range in ultracold strontium P.G. Mickelson, S.B. Nagel, A.D. Saenz, Y.N. Martinez, Y.C. Chen, T.C. Killian, P. Pellegrini, R. C\^{o}t\'{e} We discuss photoassociative spectroscopy of Strontium in a magneto-optical trap operating on the $^{1}$S$_{0}$ --$>$ $^{3}$P$_{1}$ intercombination line at 689 nm. A laser detuned from the 461 nm atomic resonance by hundreds of MHz to a few GHz drives the photoassociative transitions. The photoassociation occurs at long range, and relativistic retardation strongly influences the photoassociative spectrum. The results resolve differences between experiment and recent theoretical calculations for the $^{1}$P$_{1}$ atomic lifetime. [Preview Abstract] |
Friday, May 20, 2005 2:30PM - 2:42PM |
L5.00006: Zeeman relaxation and magnetic Feshbach resonances in bosonic $^{52}$Cr gas. Z. Pavlovi\'c$^{1,2}$, R. Krems$^{2,3}$, R. C\^ot\'e$^1$, H. R. Sadeghpour$^2$ We present spin-change collision cross sections and rate constants in cold and ultracold chromium gas in the presence of a magnetic field. By performing coupled-channels calculations, we obtain positions and widths of magnetic Feshbach resonances in the ultracold collisions of Cr atoms. We specifically address scattering of cold chromium in relation with two separate experiments$^{1, 2}$\footnotetext[1]{J. D. Weinstein {\it et al.}, Phys.Rev.A {\bf 65}, 021604(2002).}\footnotetext[2]{J. Werner {\it et al.}, arXiv: cond-mat/0412049.} and present detailed results for resonances in higher partial waves as well as partial Zeeman relaxation cross sections and rate coefficients. [Preview Abstract] |
Friday, May 20, 2005 2:42PM - 2:54PM |
L5.00007: Theoretical analysis of the photoassociative spectroscopy of ultracold strontium atoms Philippe Pellegrini, Robin C\^ot\'e, Thomas Killian Photoassociation (PA) of two colliding ultracold atoms in their ground state occurs when a laser field excites resonantly a ro-vibrational bound molecular level of an excited electronic state. By driving the frequency of the PA laser, high-precision spectroscopy of excited molecular states at large internuclear distances can be performed. The accuracy of the PA spectra allows a precise determination of important parameters such as the atomic radiative life time or the $s$-wave scattering length. Here, we present results on PA spectroscopy of the excited $^1\Sigma_u^+(^1S_0+^1P_1)$ potential of the Sr$_2$ molecule. A full quantum theoretical analysis of the measured PA spectra will be presented. Accurate determination of scattering lengths and atomic radiative lifetime will be shown and the role of relativistic retardation effects at very large internuclear distances will be discussed. [Preview Abstract] |
Friday, May 20, 2005 2:54PM - 3:06PM |
L5.00008: State-selective detection of ultracold KRb molecules in near-dissociation vibrational levels D. Wang, E.E. Eyler, P.L. Gould, W.C. Stwalley Using resonance-enhanced one-color two-photon ionization, we have detected ultracold polar KRb molecules with vibrational selectivity. The KRb molecules are produced by photoassociation (PA) followed by radiative decay to either $X\,\,^1\Sigma^+$ ground state or the metastable triplet $a\,\,^3\Sigma^+$ state[1]. The distribution of final states can be modified by selecting different PA resonances, which radiate to various near-dissociation vibrational levels in the $X$ and $a$ states, following the Franck-Condon principle. We have obtained long range bound-bound excitation spectra of these molecules, which also provide the first spectroscopic data on the $(4)\,\,^1\Sigma^+$ shelf state, the $(4)\,\,^3\Sigma^+$ state and the $(3)\,\,^3\Pi$ state. Analysis of our spectra shows vibrational spacings in these excited states in good agreement with those calculated from $ab$ $initio$ potential curves. We acknowledge support from NSF.\newline [1] D. Wang et al., Phys. Rev. Lett., 93, 243005(2004). [Preview Abstract] |
Friday, May 20, 2005 3:06PM - 3:18PM |
L5.00009: Trapping and interactions of an ultracold gas of Cs$_{2}$ molecules T. Kraemer, M. Mark, P. Waldburger, J. Herbig, C. Chin, H.C. Naegerl, R. Grimm We investigate dynamics and interactions of Cs$_{2}$ dimers in a CO$_{2}$-laser dipole trap. Starting with a Bose-Einstein condensate (BEC) of 2.2*10$^{5}$ Cs atoms, we create ultracold molecules in a single, weakly bound quantum state by sweeping the magnetic field across a narrow Feshbach resonance. When the molecules are created in free space, the conversion efficiency exceeds 30{\%}, yielding up to 40000 molecules. In our trapping experiments, about 6000 ultracold Cs$_{2}$ dimers are prepared in the optical trap at a temperature of 200 nK. We transfer the trapped molecules from the initial molecular state to other molecular states by following avoided crossings. We find two magnetically tunable resonances in collisions between the molecules for one of the molecular states. We interpret these Feshbach-like resonances as being induced by Cs$_{4}$ bound states near the molecular scattering continuum. Further, we have discovered a new molecular state with very large orbital angular momentum of $l$=8. This state is very weakly coupled to one of the initial molecular states. We use the corresponding avoided crossing as a beam splitter to realize a molecular interferometer. We acknowledge support by the Austrian Science Fund (FWF) within SFB 15 and by the European Union in the frame of the Cold Molecules TMR network under Contract No. HPRN-CT-2002-00290. [Preview Abstract] |
Friday, May 20, 2005 3:18PM - 3:30PM |
L5.00010: Dynamics of association in ultra-cold Fermi gases in spherically symmetric harmonic traps Takahiko Miyakawa, Pierre Meystre We consider the photoassociation of a low-density gas of quantum-degenerate trapped fermionic atoms into bosonic molecules in a spherically symmetric harmonic potential. For a dilute system and the photoassociation coupling energy small compared to the level separation of the trap, only those fermions in the single shell with Fermi energy are coupled to the bosonic molecular field. Introducing a collective pseudo-spin operator formalism we show that this system can then be mapped onto the Tavis-Cummings Hamiltonian of quantum optics, with an additional pairing interaction. We first show that the filling factor - fraction of total number of pairs to the degeneracy in the Fermi level - has strong impact on coherent dynamics of the Bose-Fermi system. In a semiclassical description of the system, the pairing interaction between fermions is shown to result in a self-trapping transition in the photoassociation, with a sudden suppression of the coherent oscillations between atoms and molecules. We also show that the full quantum dynamics of the system is dominated by quantum fluctuations in the vicinity of the self-trapping solution. [Preview Abstract] |
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L5.00011: One-dimensional three-body problem with Feshbach resonance interactions Vladimir Yurovsky A problem concerning collisions of cold atoms under tight cylindrical confinement is considered. A Feshbach resonance in two-body collisions is described by a two-channel zero-range interaction, one of them being a closed (molecular) channel. The interaction parameters are related to three-dimensional scattering parameters [1]. A neglect of collisions of the closed channel molecule with the third atom allows the elimination of the closed channel in the three-body problem, reducing the interaction to a one-channel zero-range one with an energy dependent strength. Unlike the problem with energy independent strength (the exactly soluble Lieb-Liniger-McGuire problem [2,3]) the resonant problem does not allow an exact Bethe-ansatz solution. This problem is analyzed by a numerical solution of the Faddeev-Lovelace equations. The results demonstrate that the internal symmetry of the Lieb-Liniger-McGuire problem is broken, and the resonant scattering allows for reflection in elastic atom-molecule collisions, dissociation in inelastic collisions, and three-body association. 1. V. A. Yurovsky, Phys. Rev. A {\bf 71}, 012709 (2005). 2. E. H. Lieb and W. Liniger, Phys. Rev. {\bf 130}, 1605 (1963). 3. J. B. McGuire, J. Math. Phys. {\bf 5}, 622 (1964). [Preview Abstract] |
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