Bulletin of the American Physical Society
39th Annual Meeting of the APS Division of Atomic, Molecular, and Optical Physics
Volume 53, Number 7
Tuesday–Saturday, May 27–31, 2008; State College, Pennsylvania
Session C4: Ultracold Molecules |
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Chair: Phil Gould, University of Connecticut Room: Nittany Lion Inn Ballroom AB |
Wednesday, May 28, 2008 2:00PM - 2:12PM |
C4.00001: A quantum defect theory of near-threshold molecular Feshbach resonance states Paul Julienne, Cheng Chin, Eite Tiesinga The framework provided by multichannel quantum defect theory (MQDT) [1,2] provides an excellent way to classify the near-threshold molecular vibrational states associated with magnetically tunable Feshbach resonances of two cold atoms. The separation of energy and length scales between long- and short-range interactions permit the definition of a dimensionless resonance strength parameter that determines, along with the analytic properties of the long-range potential, near threshold scattering and bound state properties. The MQDT bound state equations based on the long-range potential [2] yield a simple theory for near-threshold bound states. While resonance strengths span order of magnitude in practice, near-threshold molecular bound states fall into two broad classes, those where the bound state has primarily entrance channel character or those where it has primarily closed channel character as magnetic field is tuned over the width of the resonance. Most resonances tend to be closed channel dominated, including the very broad $^{7}$Li resonance near 720 G, whereas the $^{85}$Rb, $^{6}$Li, and $^{40}$K resonances successfully used in many quantum degenerate gas experiments are open channel dominated. See also [3]. 1. F. H. Mies, J. Chem. Phys. 80, 2514(1984) 2. F. H. Mies and M. Raoult, Phys. Rev. A 62, 012708(2000) 3. T. K\"{o}hler, K. G\'{o}ral, and P. S. Julienne, Rev. Mod. Phys.78, 1311 (2006) [Preview Abstract] |
Wednesday, May 28, 2008 2:12PM - 2:24PM |
C4.00002: Energy structure of weakly bound molecules near Feshbach resonances Cheng Chin, Paul Julienne, Eite Tiesinga Recent experiments on ultracold molecules created near a Feshbach resonance have allowed high precision measurements on molecular binding energy near the continuum.~ To date, measurements on molecules such as~ 6Li$_2$, 40K$_2$, 87Rb$_2$, 133Cs$_2$ and 40K-87Rb have been reported and provided the critical information to pin down the long-range interaction parameter of atoms and to test atomic scattering theories. We show that binding energies of molecules near the continuum can be well-approximated based on simplified interaction models, such as the van der Waals potential or a square-well potential. These simplified models allow for self-consistent, analytic expressions for molecular binding energies and scattering lengths near Feshbach resonances. We report excellent agreement between our results and the full multi-channel calculation within and beyond the quantum threshold regime.~ Examples include 6Li$_2$, 40K$_2$, 133Cs$_2$ and 40K-87Rb. In the latter two cases, we will present the comparison in the regime where several resonances overlap. [Preview Abstract] |
Wednesday, May 28, 2008 2:24PM - 2:36PM |
C4.00003: Quantum dynamics of F + HCl and F + DCl reactions at ultralow energies Balakrishnan Naduvalath, Goulven Qu{\'e}m{\'e}ner Quantum scattering calculations of F + HCl $\to$ HF + Cl and F + DCl $\to$ DF + Cl reactions have been carried out at cold and ultracold temperatures. The effect of rotational and vibrational excitations of the HCl molecule on the reactivity is investigated. The reactivity of the F + DCl system is found to be significantly lower than that of the F + HCl reaction indicating the importance of quantum tunneling at low energies. For both reactions, Feshbach resonances occur at low energies due to decay of quasibound states of the FHCl and FDCl complexes supported by the van der Waals interaction potential. We also explored the validity of the coupled-states approximation for cold collisions and we found that it is generally valid for the background scattering but it fails to reproduce the resonances or to correctly describe the Wigner threshold behavior of cross sections when the diatom is initially in an excited rotational level. [Preview Abstract] |
Wednesday, May 28, 2008 2:36PM - 2:48PM |
C4.00004: Weakly bound molecules. Analysis by the Lu-Fano method coupled to the LeRoy-Bernstein model. Laurence Pruvost, Haikel Jelassi, Bruno Viaris de Lesegno We have realized the photo-associative spectroscopy of cold 87Rb atoms, below the (5s1/2+5p1/2) dissociation limit. Recorded spectra exhibit 3 molecular vibrational series : 0g-, 0u+ and 1g. Such weakly bound molecules (WBM) are described by the dipole-dipole interaction (1/R3 or as 1/R6). WBM energies are described by the Le Roy-Bernstein (LRB) model. The discrepancies to LRB law are due to the short distance behaviour of molecular potentials or to couplings between molecular series due to interactions such as spin-orbit or spin-spin interactions. To analyse the data, we have adapted the Lu-Fano (LF) method - well-known for Rydberg atoms - to WBM. Using the LRB law, a molecular quantum defect is defined and plotted versus the energy. The obtained LF graph allows us to characterise the molecular potential and the interactions. The 0g- LF graph is a linear, signature of the short range behaviour of the molecular potential. A model connects the slope to short range behaviour [1]. The 0u+ LF graph exhibits sharp variations, signatures of a coupling with a neighbouring series. A two series model allows us to evaluate the coupling, identify two perturbing levels of the (5s1/2+5p3/2) 0u+ series and do predictions about its first pre-dissociated level [2]. [1] H. Jelassi, B.Viaris De Lesegno, L. Pruvost, Phys. Rev. A. 73, 32501 (2006) [2] H. Jelassi, B.Viaris De Lesegno, L. Pruvost, Phys. Rev. A. 74, 12510 (2006) [Preview Abstract] |
Wednesday, May 28, 2008 2:48PM - 3:00PM |
C4.00005: Vibrational energy transfer in ultracold molecule-molecule collisions Goulven Qu{\'e}m{\'e}ner, Balakrishnan Naduvalath, Roman Krems The success in creating Bose-Einstein condensates of molecules has spurred much interest in atom-molecule and molecule-molecule collisions at cold and ultracold temperatures. We present here a rigorous study of vibrational relaxation in H$_2$ + H$_2$ collisions at cold and ultracold temperatures and identify an efficient mechanism of ro-vibrational energy transfer [1]. If the colliding molecules are in different rotational and vibrational levels, the internal energy may be transferred between the molecules through an extremely state-selective process involving simultaneous conservation of internal energy and total rotational angular momentum. The same transition in collisions of distinguishable molecules corresponds to the rotational energy transfer from one vibrational state of the colliding molecules to another. [Preview Abstract] |
Wednesday, May 28, 2008 3:00PM - 3:12PM |
C4.00006: Vibrationally dependent interactions of cold molecules Svetlana Kotochigova Controllable dipole-dipole interaction between polar molecules in an optical trap lies at the heart of many proposals to exploit entanglement as an essential resource for strongly correlated many-body states and quantum information processing. Even though polar molecules have a permanent electronic dipole moment and interact at large internuclear separations $R$ via dipole-dipole interactions, at shorter distances Van der Waals forces prevail. Here we study the vibrationally dependent short-range interactions between cold molecules and cold molecules with their constituent atoms. The interaction coefficients are obtained by integrating the product of the dynamic polarizabilities $\alpha(i\omega)$ over imaginary frequencies. Both the parallel and perpendicular components of the dynamic polarizability are calculated. The calculation is done for the polar RbCs and KRb molecules and its constituent atoms. This data can be used to estimate limits on the collisional lifetime of molecules in optical traps and find microscopic mechanisms by which the losses occur. The ultimate goal is to find optimal experimental conditions to diminish these many-body losses. The author acknowledges support of this work by ARO. [Preview Abstract] |
Wednesday, May 28, 2008 3:12PM - 3:24PM |
C4.00007: Electric field control of chemical reactions Timur Tscherbul, Roman Krems We develop the formalism for rigorous quantum scattering calculations of probabilities for chemical reactions in the presence of an external electric field. The approach is based on the Fock-Delves hyperspherical coordinates and adiabatic partitioning of the total Hamiltonian. The adiabatic channel wave functions are expanded in basis sets of hyperangular functions corresponding to different reaction arrangements and the effects of external fields are included in each chemical arrangement separately. We show that the chemical reaction of vibrationally excited LiF molecules with H atoms at temperatures below 1 K can be significantly modified by external electric fields. In particular, we demonstrate that (i) electric fields may enhance the $s$-wave reaction cross section by several orders of magnitude, (ii) reactive scattering resonances at low collision energies may be shifted and suppressed by electric fields of $\sim100$ kV/cm; (iii) the chemical reaction becomes more probable than inelastic energy transfer at high electric fields. [Preview Abstract] |
Wednesday, May 28, 2008 3:24PM - 3:36PM |
C4.00008: Interactions between Dipolar Molecules and Rubidium L Paul Parazzoli, Noah Fitch, Daniel Lobser, Heather Lewandowski The development of techniques to produce cold molecules has opened up the possibility to study molecular collisions in a new regime. The ability to control molecules externally allows researchers to investigate specific interactions between species by selecting particular states or orientations. In addition, by tuning the relative velocities, the collision energy can be controlled precisely. We are investigating the interactions of cold dipolar molecules with magnetically trapped rubidium atoms. Using Stark deceleration, we can very precisely control the velocity of the molecules, as well as select a particular internal state of the molecule for the interaction. [Preview Abstract] |
Wednesday, May 28, 2008 3:36PM - 3:48PM |
C4.00009: Trapping of Stark-decelerated neutral molecules Steven Hoekstra, J.J. Gilijamse, S.Y.T. van de Meerakker, G. Meijer Stark-decelerated and trapped molecules can be used for cold collision studies, the measurement of metastable lifetimes and high resolution spectroscopy. We have trapped OH, OD, NH and CO molecules an electrostatic trap, at typical temperatures of $\sim 50$ mK and densities of $\sim 10^7$ cm$^{-3}$. The trapping time is typically a few seconds, limited by excitation due to room-temperature blackbody radiation. The deceleration and trapping of NH molecules will be discussed in detail. NH molecules in the long-lived metastable $a^{1}\Delta (v=0, J=2)$ state are ideally suited for Stark deceleration experiments because of their relatively large Stark shift and low mass. Following the deceleration and trapping, the metastable NH molecules are detected by the excitation of a spin-forbidden transition, resulting in spontaneous decay to the electronic ground state ($X^{3}\Sigma^{-}$). The electronic ground state has a negligible Stark shift, but can be trapped magnetically. First experiments towards the accumulation of ground state NH molecules in a magnetic trap will be presented.\newline \it{S.Hoekstra et al, Phys. Rev. Lett. 98 133001 (2007)}\newline \it{S.Hoekstra et al, Phys. Rev. A 76 063408 (2007)} [Preview Abstract] |
Wednesday, May 28, 2008 3:48PM - 4:00PM |
C4.00010: Collision experiments utilizing trapped neutral OH molecules Brian Sawyer, Benjamin Stuhl, Mark Yeo, Dajun Wang, Benjamin Lev, Jun Ye Advances in cold molecule production promise to profoundly impact research on precision measurement, quantum information, and controlled chemistry. To this end, we employ a Stark decelerator to remove 99.5{\%} of the center-of-mass kinetic energy of a supersonic beam of ground-state OH molecules. We subsequently trap a 70 mK sample of the decelerated molecules at a density of 10$^{6}$ cm$^{-3}$ within a magnetic quadrupole whose center lies 1cm from the decelerator exit. Our magnetoelectrostatic trap (MET) design allows for the addition of an electric field of variable magnitude to the trapped sample to facilitate polar-molecule collision studies. We report progress toward observation of collisions between trapped OH and different atomic and molecular beams. [Preview Abstract] |
Wednesday, May 28, 2008 4:00PM - 4:12PM |
C4.00011: Toward trapping cold molecules produced via ``kinematic'' cooling Jeffrey J. Kay, Sebastiaan Y. T. van de Meerakker, Kevin E. Strecker, David W. Chandler ``Kinematic'' cooling is a relatively simple technique by which a vast array of molecules can be translationally cooled using crossed atomic and molecular beams. The success of the technique relies primarily on the existence of an approximate mass degeneracy between the molecule to be cooled and its atomic (or molecular) collision partner. Here, we discuss progress toward electrostatic trapping of cold molecules produced using this technique. Schemes that allow optimization of cold molecule production by tuning the velocity of the molecular beam will also be discussed. [Preview Abstract] |
Wednesday, May 28, 2008 4:12PM - 4:24PM |
C4.00012: Kinematic cooling of molecules in a magneto-optical trap Ken Takase, David W. Chandler, Kevin E. Strecker We will present our current progress on a new experimental technique aimed at slowing and cooling hot molecules using a single collision with magneto-optically trapped atoms. Kinematic cooling, unlike buffer gas and sympathetic cooling, relies only on a single collision between the molecule and atom to stop the molecule in the laboratory frame. This technique has recently been demonstrated in a crossed atomic and molecular beam machine to produce 35mK samples of nitric oxide via a single collision with argon [1]. In this technique we replace the atomic beam with a sample magneto-optically trapped atoms. We are currently designing and building a new apparatus to attempt these experiments. [1] Kevin E. Strecker and David W. Chandler (to be published) [Preview Abstract] |
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