Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 65, Number 4
Monday–Friday, June 1–5, 2020; Portland, Oregon
Session S03: Cold Collisions of Atoms, Molecules, and IonsLive
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Sponsoring Units: GFB Chair: Eric Hudson, UCLA Room: D135-136 |
Friday, June 5, 2020 8:00AM - 8:12AM Live |
S03.00001: Quantum Diffractive Collision Universality James Booth, Pinrui Shen, Roman Krems, Kirk Madison We have demonstrated that quantum diffractive collisions are governed by a universal scaling law characterized by a single, \textit{experimentally determined} parameter [1]. We report the quantitative form of the universal, cumulative energy distribution transferred to stationary sensor particles via quantum diffractive collisions. The distribution’s characteristic energy scale is the localization length associated with the collision-induced quantum measurement and its characteristic shape is determined solely by the form of the long-range interaction potential between the collision partners. We have observed the universal function specific to van der Waals collisions by using cold $^{87}$Rb confined in a shallow magnetic trap as an energy/momentum spectrometer for quantum diffractive collisions. This universal function realizes a \textit{self-defining} particle flux/pressure sensor for any ambient gas species. This work represents the first primary and quantum definition of the Pascal, a fundamental advance for vacuum and pressure metrology. This new standard was compared to an existing orifice flow standard by calibrating an ionization gauge for N$_2$ gas against each standard. The two values agree at the 0.5\% level. [1]Booth \textit{et al}, New J. Phys. \textbf{21} (2019). [Preview Abstract] |
Friday, June 5, 2020 8:12AM - 8:24AM Live |
S03.00002: High-resolution Energy Control of Ultracold Atom-ion Collisions Meirav Pinkas, Ruti Ben-Shlomi, Tomas Sikorsky, Ziv Meir, Nitzan Akerman, Roee Ozeri In the low collision energy regime, the quantum nature of the collision can modify the classical cross-section energy dependence, for example as a result of shape resonances. We report on a method for controlling cold atom-ion collision energy in the single collision regime. This is done by sweeping a one-dimensional optical lattice with cold Rb atoms, on a Sr$^{+}$ ion in a Paul trap. When the ion is cooled down to its ground state, this technique permits a delicate control over the collision energy, with a resolution of $\sim$100$\mu$K, limited only by the excess-micromotion energy of the ion. This resolution is one order-of-magnitude better than in previous experiments in cold atom-ion collisions. We demonstrate this method by measuring the energy dependence of the cross-section of the Electronic-Excitation-Exchange (EEE) process, when the ion is prepared in the metastable electronic excited state decay into the electronic ground state after a collision. We found that for collision energies ranging between 0.2-12 mK, the EEE cross-section obeys the classical Langevin cross-section. This method can be extended for measuring various inelastic processes cross-sections and its high energy resolution enables detection of subtle changes in the classical cross-section. [Preview Abstract] |
Friday, June 5, 2020 8:24AM - 8:36AM Live |
S03.00003: Light assisted charge exchange interaction between potassium (K) atoms and calcium ions (Ca$^{\mathrm{+}})$ ions in an ion-atom hybrid trap Jyothi Saraladevi, Eric Pretzsch, Kenneth Brown In the past decade, hybrid ion-atom traps enabled us to study rich chemical interactions between laser cooled and trapped ions and atoms. Controlling these chemical interactions by manipulating the internal states of ions and atoms is an exciting new research direction in this field. In this talk, we present our investigation of charge exchange interaction between laser cooled potassium (K) atoms in a magneto optical trap (MOT) and calcium (Ca$^{\mathrm{+}})$ ions in a linear Paul trap. The charge exchange interaction is observed to be photon mediated, and the interaction rate coefficient can be controlled by manipulating the electronic state population of Ca$^{\mathrm{+}}$ ions. Our experimental observations are in good agreement with theoretical calculations. [Preview Abstract] |
Friday, June 5, 2020 8:36AM - 8:48AM Live |
S03.00004: Resonant dipolar collisions of microwave-dressed ultracold molecules Zoe Yan, Jee Woo Park, Yiqi Ni, Huanqian Loh, Sebastian Will, Tijs Karman, Martin Zwierlein We apply microwave dressing to ultracold, fermionic ${}^{23}$Na${}^{40}$K ground-state molecules and observe resonant dipolar collisions with cross sections exceeding three times the $s$-wave unitarity limit. The origin of these collisions is the resonant alignment of the approaching molecules' dipoles along the intermolecular axis, leading to strong attraction. We perform coupled-channels calculations that agree well with the experimentally observed collision rates. While collisions are here observed as laser-induced loss, microwave dressing on chemically stable molecules trapped in box potentials may enable the creation of strongly interacting dipolar gases of molecules. For molecules trapped in optical lattices, the strong induced interactions provide a crucial tool for applications in quantum computing and quantum simulation. [Preview Abstract] |
Friday, June 5, 2020 8:48AM - 9:00AM Live |
S03.00005: Quantum statistical properties in the three-body recombination rate of ultracold bosonic and fermionic atoms Hui Li, Eite Tiesinga, Svetlana Kotochigova We theoretically investigate collisions among ultracold bosonic and fermionic atoms and molecules in an external magnetic field. We study three-body recombination processes near magnetic Feshbach resonances, where the rate coefficients are resonantly enhanced. Our simulations show that this enhancement is controlled by quantum statistics leading to line shapes with a unique partial wave and temperature behavior. In particular, we obtained a striking difference in the temperature-dependence of the three-body recombination rate of $s$- and $d$- wave entrance-channel Feshbach resonances for colliding bosonic Er atoms [1], whereas in $p$-wave fermionic mixtures of Li+Li+Yb, the rate has a maximum value that is independent of temperature [2]. [1] T. Maier, H. Kadau, M. Schmitt, M. Wenzel, I. Ferrier-Barbut, T. Pfau, A. Frisch, S. Baier, K. Aikawa, L. Chomaz, M. J. Mark, F. Ferlaino, C. Makrides, E. Tiesinga, A. Petrov, and S. Kotochigova, Phys. Rev. X {\bf 5}, 041029 (2015). [2] A. Green, H. Li, J.H.S. Toh, X.X. Tang, K. McCormick, M. Li, E. Tiesinga, S. Kotochigova, and S. Gupta, submitted to PRX (2019), arXiv:1912.04874v. [Preview Abstract] |
Friday, June 5, 2020 9:00AM - 9:12AM Live |
S03.00006: Controlling the stereodynamics of cold molecular collisions Masato Morita, Qian Yao, Changjian Xie, Hua Guo, N. Balakrishnan We report explicit quantum calculations of stereodynamic control of rotational transitions in cold molecular collisions by taking H$_2$+HCl as an illustrative example. It is found that low-energy resonances in the rotational quenching cross sections of HCl due to collisions with para-H$_2$ can be significantly enhanced or suppressed by controlling the orientation of the HCl molecule against the initial relative velocity vector between the collision partners. Remarkably, the stereodynamic control is possible even when multiple (overlapping) shape-resonances exist within a relatively narrow energy range, indicating that such control is not limited to an isolated shape resonance. We demonstrate a striking case where two overlapping resonance can be switched off simultaneously by tuning stereodynamics. Such promising controllability of resonances originates from the significant initial helicity dependence of the cross section as well as of the strong angular distribution of the scattered HCl molecule. [Preview Abstract] |
Friday, June 5, 2020 9:12AM - 9:24AM On Demand |
S03.00007: Trapped ion-molecule reactions of sympathetically cooled C$_{2}$H$_{2}^{+}$ and CCl$^{+}$ with nitriles Olivia Krohn, Katherine Catani, James Greenberg, Heather Lewandowski Cold ion-molecule reactions in the laboratory allow for detailed investigations of collision dynamics and reactions relevant to the chemical evolution of the Interstellar Medium (ISM). Our experimental system uses trapped laser-cooled atomic ions to sympathetically cool molecular ions important in ISM chemistry. Product ions from reactions with neutral organic species are monitored as a function of reaction time using a time-of-flight mass spectrometer. Two systems (C$_{2}$H$_{2}^{+}$ and CCl$^{+}$ with CH$_{3}$CN) were studied for their potential contributions to our understanding of chemistry in the ISM and planetary atmospheres. Isotopologue substitutions and quantum chemical calculations aid in interpretation of experimental data. The branching ratios and rate constants of these systems provide insight into the mechanisms of bonding and behavior of the functional groups included in each reaction --- particularly the activity of the CN bond. [Preview Abstract] |
Friday, June 5, 2020 9:24AM - 9:36AM On Demand |
S03.00008: Ultracold Reaction Rates of Triplet NaLi Molecules with Na Atoms in a Magnetic Field Rebekah Hermsmeier, Timur Tscherbul Nearly all of the open-shell molecular radicals trapped in recent experiments (such as SrF, CaF, and NaLi) are chemically reactive, motivating the study of spin-dependent chemical reaction rates in the presence of external magnetic fields. We calculated the rates of the ultracold chemical reaction of triplet-state NaLi($^3\Sigma$) molecules with Na atoms, including the fine and hyperfine structure of the reactants. To this end, we first determine the energy level structure of NaLi as a function of magnetic field. We then use coupled-channel statistical theory to calculate the reaction rates for the different hyperfine states of NaLi and Na. We examine the regimes where the chemical reactions can be most efficiently controlled by switching the hyperfine states and applying an external magnetic field. [Preview Abstract] |
Friday, June 5, 2020 9:36AM - 9:48AM On Demand |
S03.00009: Experimental control of Quantum Chemistry with Interfering Pathways for Rb$_{\mathrm{2}}$ Molecular Formation in a Bose-Einstein Condensate Hasan Esat Kondakci, Chuan-Hsun Li, Yong P. Chen Ultracold atoms are amongst the best test beds to study coherent quantum chemistry due to the excellent ability to control the quantum states of the atoms. Here, we show coherent control based on quantum interference between reaction pathways in the ultracold molecule formation induced by a laser light, a process known as photoassociation (PA). We prepare optically trapped Rb-87 Bose-Einstein condensates (BECs) where the atoms are prepared in superpositions between different m$_{\mathrm{F}}$ spin states in the F$=$1 hyperfine state. By exploiting the quadratic Zeeman shift at low magnetic bias fields and a free evolution following a $\pi $/2 RF pulse inducing a spin population transfer, the evolution time controls the cumulative relative phase between the two reaction pathways (one for (m$_{\mathrm{F}}=$0, m$_{\mathrm{F}}=$0) pairs and the other for ($+$1,-1) pairs), resulting in an interferometric control of the normalized PA rate with near perfect visibility. Our method also provides a robust measurement technique to determine the quadratic Zeeman shift taking advantage of the cancellation effect due to the fast-oscillating phases in the scattering channel involving spin ($+$1,-1) pairs. [Preview Abstract] |
Friday, June 5, 2020 9:48AM - 10:00AM |
S03.00010: Modelling loss processes in ultracold molecular collisions James Croft, John Bohn, Goulven Quéméner Experiments on non-reactive ultracold molecules, appear to have observed two-body collisional losses, even when the molecules are in their absolute ground state. It has been proposed that these losses are due to the formation of long-lived collision complexes. However interpreting the experimental results is challenging—the usual time-independent scattering methods do not treat the formation of long-lived complexes as a loss process and yield a unitary S-matrix. Using ideas taken from nuclear physics I will discuss how direct information about the complex itself can be extracted from the experimental results using an approach based on appropriately averaged cross-sections. [Preview Abstract] |
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