Bulletin of the American Physical Society
41st Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 55, Number 5
Tuesday–Saturday, May 25–29, 2010; Houston, Texas
Session J3: Ultracold Chemistry |
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Chair: Marjatta Lyyra, Temple University Room: Imperial West |
Thursday, May 27, 2010 8:00AM - 8:12AM |
J3.00001: Ultracold chemistry with ground-state KRb molecules B. Neyenhuis, S. Ospelkaus, K.-K. Ni, D. Wang, M.H.G. de Miranda, G. Quemener, J.L. Bohn, D.S. Jin, J. Ye We prepare a near-quantum-degenerate gas of fermionic KRb molecules, with all the molecules in the absolute lowest energy state. We observe atom-exchange chemical reactions in a regime where the reaction rates are determined by the quantum statistics of the molecules, single partial wave scattering, and quantum threshold laws [1].\\[4pt] [1] S. Ospelkaus, K.-K. Ni, D. Wang, M. H. G. de Miranda, B. Neyenhuis, G. Qu\'{e}m\'{e}ner, P. S. Julienne, J. L. Bohn, D. S. Jin, J. Ye, Quantum-State Controlled Chemical Reactions of Ultracold KRb Molecules, Science (in press). [Preview Abstract] |
Thursday, May 27, 2010 8:12AM - 8:24AM |
J3.00002: Bi-alkali chemical pathways at ultracold temperatures Edmund Meyer, John Bohn The production of ultracold polar molecules such as KRb \footnotemark[2] has reinvigorated the need for an understanding of the chemical pathways available to the collision of 2 KRb molecules. We present a systematic study of the the alkali-metal family of dimers (Li-Cs) and their respective trimer states. We take as an example the K$_2$Rb molecule and construct surfaces for the two lowest doublet states in C$_{\rm 2v}$ geometries. We find that the only possible pathway for chemical reactions is through the bond switching mechanism, i.e. the two heteronuclear molecules collide and produce two homonuclear molecules. Even this pathway is closed for some species of bi-alkali-metal dimers. \footnotetext[2]{K.-K. Ni, S. Ospelkaus, M. H. G. de Miranda, A. Pe'er, B. Neyenhuis, J. J. Zirbel, S. Kotochigova, P. S. Julienne, D. S. Jin, and J. Ye, Science 322: 231-235, (2008)} [Preview Abstract] |
Thursday, May 27, 2010 8:24AM - 8:36AM |
J3.00003: Universal reaction rates for ultracold molecular collisions Paul Julienne, Zbigniew Idziaszek We offer a simple yet general model of reactive collisions using a quantum defect framework based on the separation of the collision dynamics into long-range and a short-range parts [1]. Two dimensionless quantum defect parameters $s$ and $y$ are used to characterize the S-matrix for a given entrance channel; $s$ represents a phase parameter and $y$ the probability of short-range reaction. The simple analytic expressions we obtain give universal values for $s$-wave and $p$-wave collision rates for a van der Waals potential when $y$ approaches unity. In this limit, reaction rates are governed entirely by the threshold laws governing the quantum transmission of the long range potential and depend only on the van der Waals coefficient. The universal rate constants explain the magnitude of the observed rate constants for reactive collisions of fermionic KRb + KRb or K + KRb [2]. In contrast, reaction rates will be non-universal and depend strongly on the phase parameter $s$ if the short range reaction probability is low, $y <<$ 1. [1] Z. Idziaszek and P. S. Julienne, arXiv:0912.0370. [2] S. Ospelkaus, K.-K. Ni, D. Wang, M. H. G. de Miranda, B. Neyenhuis, G. Qu\'{e}m\'{e}ner, P. S. Julienne, J. L. Bohn, D. S. Jin, and J. Ye, arXiv:0912.3854. [Preview Abstract] |
Thursday, May 27, 2010 8:36AM - 8:48AM |
J3.00004: Cold N - NH Collisions in a Magnetic Trap Matthew Hummon, Timur Tscherbul, Jacek Klos, Hsin-I Lu, Edem Tsikata, Wesley Campbell, Alexander Dalgarno, John Doyle Direct cooling and deceleration techniques typically produce molecular samples with temperatures in the range of 10 to 500~mK. Sympathetic cooling of these molecular samples via collisions with a cotrapped atomic species may be a route to attaining temperatures below 1 mK. We present a combined experimental and theoretical study of cold collisions between magnetically trapped atomic nitrogen and NH at temperatures of $\sim 500$~mK and discuss its implications for sympathetic cooling of molecules to ultracold temperatures. [Preview Abstract] |
Thursday, May 27, 2010 8:48AM - 9:00AM |
J3.00005: Cold collision of co-trapped ND$_{3}$ molecules and Rb atoms Noah Fitch, Paul Parazzoli, Heather Lewandowski Interest in cold molecular collisions has increased in recent years due to the development of new techniques that allow for a high level of control over both the internal and external degrees of freedom of molecular beams. Our chosen technique for creating and controlling cold molecules is Stark deceleration. This technique utilizes time-varying inhomogenous electric fields to slow a beam of polar moleucles for subsequent trapping. After deceleration, our ND$_{3}$ molecules are loaded into a trap consisting of a static quadrupole electric field. A magnetic trap of Rb atoms is then physically superimposed with the molecular trap where collisions between the two species can be studied. We report on studies using both $^{14}$ND$_{3}$ and $^{15}$ND$_{3}$ with collision partners consisting of $^{87}$Rb$_{F=1}$, $^{87}$Rb$_{F=2}$, $^{85}$Rb$_{F=2}$, and $^{85}$Rb$_{F=3}$. [Preview Abstract] |
Thursday, May 27, 2010 9:00AM - 9:12AM |
J3.00006: Inelastic collisions of CaH molecules at cryogenic temperatures Vijay Singh, Mei-Ju Lu, Kyle Hardman, Muir Morrision, Jonathan Weinstein We are interested in using doublet-sigma molecules to investigate the role of electon spin in chemical reactions [1]. As a first step towards this goal, we are creating polarized samples of CaH molecules and studying their collisional properties. We create CaH molecules by laser ablation of a solid CaH$_2$ target and cool them by helium buffer gas cooling. To study the polarization changing collisions of CaH molecules with He atoms, we spin polarize CaH by optical pumping. We measure the spin depolarization rate by fitting the return to equilibrium. At 4 Kelvin, our measurement shows a depolarization rate coefficient of $10^{-11}$~cm$^3$~s$^{-1}$, which is very large compared to the measured spin depolarization rate at 0.4 Kelvin[2]. This large depolarization rate is likely due to excited rotational states [3]. Progress towards measuring this rate coefficient as a function of temperature will be discussed. \\[4pt] [1] R. V. Krems, Physical Chemistry Chemical Physics 10, 4079 (2008)\\[0pt] [2] Weinstein et.al., Nature 395, 148 (1998).\\[0pt] [3] Maussang et.al., Phys.Rev.Lett. 94, 123002 (2004) [Preview Abstract] |
Thursday, May 27, 2010 9:12AM - 9:24AM |
J3.00007: Quantum calculations of H2-H2 collisions in the cold and ultracols regimes Samantha Fonseca dos Santos, Stephen Leep, Balakrishnan Naduvalath, Goulven Quemener, Robert Forrey, Phillip Stancil Quantum scattering calculations of H$_2$ + H$_2$, both ortho-para and ortho-ortho cases, and HD + H$_2$ have been carried out at cold and ultracold temperatures using ab inito potential energy surfaces for the H$_4$ system developed by Hinde and Boothroyd and coworkers. The calculations were performed in the rigid rotor approximation using the MOLSCAT code as well as the vibrating rotor scheme implemented in the TwoBC code [1]. Elastic, inelastic and state-to-state cross sections obtained from our calculations are compared with available theoretical and experimental results. We also report preliminary results of differential cross sections for para-H$_2$ + para-H$_2$ collisions. \\[4pt] [1] R. V. Krems, TwoBC-quantum scattering program, University of Britsh Columbia, Vancouver, Canada (2006). [Preview Abstract] |
Thursday, May 27, 2010 9:24AM - 9:36AM |
J3.00008: Interference of Feshbach and zero-energy resonances in ultracold chemical reactions Subhas Ghosal, Ionel Simbotin, Robin C\^ot\'e We present results of quantum mechanical scattering calculations to determine the behavior of reactive scattering at ultracold temperatures. The presence of a virtual state or a bound state associated with the van der Waals well in the entrance channel can enhance the zero temperature rate coefficient for reactive systems with a barrier. Particularly, if the virtual or bound state is very close to the threshold and form zero kinetic energy resonance, the reaction rate can be enhanced by many orders of magnitude. Feshbach resonances associated with the bound or quasi-bound states of the van der Waals well in the product arrangement can also increase or decrease the final outcome of the result. We have explored this by examining two chemical reactions in the ultracold temperature limit; F + H$_2$ $\rightarrow$ FH + H and D + H$_2$ $\rightarrow$ DH + H. The position of the virtual or bound states is shifted by changing the reduced mass of the system. This can also be achieved with an external electric field. [Preview Abstract] |
Thursday, May 27, 2010 9:36AM - 9:48AM |
J3.00009: Trap losses in slow atom-molecule (He-OH) collisions T.V. Tscherbul, Z. Pavlovic, H.R. Sadeghpour, R. Cote, A. Dalgarno We present a theoretical analysis of molecular trap loss induced by collisions with slow atomic beams in external fields. The theory explicitly accounts for the effects of non-uniform trapping fields on collision dynamics by analyzing the kinetic energy of collision products in the lab frame. The cross sections for trap loss in collisions of OH($J=3/2,M=3/2,f$) molecules with $^4$He atoms in a beam are sensitive to the lab-frame beam orientation. We find that when the beam kinetic energy is comparable to the gas thermal energy, the OH trap loss is mainly dominated by inelastic collisions. In this regime, increasing the trap depth suppresses both elastic and inelastic collisions to a comparable extent. In contrast, when the beam energy is large, the total cross section for trap loss is dominated by elastic scattering. The elastic contribution decreases with increasing trap depth due to small-angle quantum diffractive collisions, whereas the inelastic contribution shows no significant variations. Our results indicate that varying the trap depth in cold beam-gas collision experiments might be used to manipulate inelastic collisions and probe the angular dependence of atom-molecule scattering. [Preview Abstract] |
Thursday, May 27, 2010 9:48AM - 10:00AM |
J3.00010: Quantum defect model of reactive collisions in quasi-1D and quasi-2D traps Zbigniew Idziaszek, Paul S. Julienne, Andrea Micheli, Guido Pupillo, Mikhail Baranov, Peter Zoller We study reactive collisions of ultracold molecules in quasi-one-dimensional and quasi-two-dimensional traps. Based on the quantum defect model developed for reactive collisions in three dimensions [1], we derive analytic results for elastic and loss rates induced by some short-range reaction process. Our model is characterized by two dimensionless quantum defect parameters: $y$ and $s$. The former describes probability of reaction, and the latter gives the phase of the wave function at short range. For $y$ close to unity we obtain universal collision rates determined only by the quantum reflection process from the long-range potential, and dependent only on the van der Waals coefficient and the trap frequency. At small reaction probabilities the collision rates are not universal and exhibit resonances induced by the confining potential. We discuss generalization of our model to include dipole-dipole interactions for collisions of polar molecules and present results for reactive dipolar collisions in quasi-two-dimensional systems. [1] Z. Idziaszek and P. S. Julienne, arXiv:0912.0370. [Preview Abstract] |
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