Bulletin of the American Physical Society
48th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 62, Number 8
Monday–Friday, June 5–9, 2017; Sacramento, California
Session C4: Atom-Atom and Atom-Molecule Collisions |
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Chair: Daniel Fisher, Missouri University of Science and Technology Room: 309 |
Tuesday, June 6, 2017 2:00PM - 2:12PM |
C4.00001: Wavepacket dynamics in the scattering of hydrogen anions off vicinally nano-stepped metal surfaces John Shaw, Himadri Chakraborty, David Monismith We study the electron dynamics in monocrystalline Cu, Au and Pd surfaces [1] with stepped vicinal structures modeled in a Kronig-Penney scheme [2]. The unoccupied bands of the surface is resonantly excited \textit{via} the charge transfer interaction of the surface with a moving hydrogen anion. The interaction dynamics are simulated in a quantum mechanical wavepacket propagation approach [3] that used parallel computations [4]. The survival probability of the interacting ion is calculated as well as the electron probability density at all times during the interaction. Animated videos are produced of the electron probability density which show that, when the electron is transferred to the metal, the first two image states are the most likely locations of the electron as it evolves through the superlattice. The survival probability shows peaks at those energies that produce standing waves between the steps on the surface when the electron is in the image state subbands. [1] Chulkov et al, \textit{Surf. Sc.} 437, 330 (1999); [2] Mugarza and Ortega, \textit{J. Phys. Cond. Matt.} 15, S3281 (2003); [3] Schmitz et al, \textit{Phys. Rev. A} 81, 042901 (2010); [4] Monismith et al, submitted, \textit{High Performance Distributed Computing (HPDC2017)}. [Preview Abstract] |
Tuesday, June 6, 2017 2:12PM - 2:24PM |
C4.00002: Bayesian optimization for constructing potential energy surfaces of polyatomic molecules with the smallest number of ab initio calculations. Rodrigo A Vargas-Hernandez, Roman V Krems We examine the application of kernel methods of machine learning for constructing potential energy surfaces (PES) of polyatomic molecules. In particular, we illustrate the application of Bayesian optimization with Gaussian processes as an efficient method for sampling the configuration space of polyatomic molecules. Bayesian optimization relies on two key components: a prior over an objective function and a mechanism for sampling the configuration space. We use Gaussian processes to model the objective function and various acquisition functions commonly used in computer science to quantify the accuracy of sampling. The PES is obtained through an iterative process of adding ab initio points at the locations maximizing the acquisition function and re-trainig the Gaussian process with new points added. We sample different PESs with one or many acquisition functions and show how the acquisition functions affect the construction of the PESs. [Preview Abstract] |
Tuesday, June 6, 2017 2:24PM - 2:36PM |
C4.00003: Rate Constants for Fine-Structure Excitations in O - H Collisions with Error Bars Obtained by Machine Learning Daniel Vieira, Roman Krems Fine-structure transitions in collisions of O($^3P_j$) with atomic hydrogen are an important cooling mechanism in the interstellar medium; knowledge of the rate coefficients for these transitions has a wide range of astrophysical applications. The accuracy of the theoretical calculation is limited by inaccuracy in the ab initio interaction potentials used in the coupled-channel quantum scattering calculations from which the rate coefficients can be obtained. In this work we use the latest ab initio results for the O($^3P_j$) + H interaction potentials to improve on previous calculations of the rate coefficients. We further present a machine-learning technique based on Gaussian Process regression to determine the sensitivity of the rate coefficients to variations of the underlying adiabatic interaction potentials. To account for the inaccuracy inherent in the ab initio calculations we compute error bars for the rate coefficients corresponding to 20 \% variation in each of the interaction potentials. We obtain these error bars by fitting a Gaussian Process model to a data set of potential curves and rate constants. We use the fitted model to do sensitivity analysis, determining the relative importance of individual adiabatic potential curves to a given fine-structure transition. [Preview Abstract] |
Tuesday, June 6, 2017 2:36PM - 2:48PM |
C4.00004: Multi-band scattering of two atoms in a one-dimensional lattice with on-site interactions Seth Rittenhouse, Panagiotis Giannakeas, Nirav Mehta We examine a system of two-particles confined to a one-dimensional lattice described by a multi-band Hubbard model with on-site interactions. Asymptotically the two particles are relegated to remain in bands that are energetically accessible. However, when the particles occupy the same state, they can virtually scatter into bands that are energetically closed. We incorporate this virtual scattering by solving the Lippmann-Schwinger equation for the reactance matrix (K-matrix) using a lattice Green's operator. The resulting formula for the K-matrix for open band scattering bears a striking similarity to that which arrises from channel closing formulas in standard multi-channel scattering theory. We then apply this formula for two-body scattering in the lowest and first excited bands within a two band approximation. Within this approximation, virtual scattering into closed bands can create scattering resonances in the presence of bound states attached to closed bands in analogy to Feshbach or confinement induced resonances. [Preview Abstract] |
Tuesday, June 6, 2017 2:48PM - 3:00PM |
C4.00005: Theoretical Investigations in Support of a Cold-Atom Based UHV Pressure Sensor Constantinos Makrides, Eite Tiesinga Recent efforts are underway in the development of a prototype cold-atom vacuum sensor, which make use of cold atoms to measure pressures in the Ultra-High Vacuum (UHV) and Extreme-High Vacuum (XHV) regimes where modern sensors cannot function or produce accurate measurements. These cold-atom devices would correlate loss of atoms in a cold atomic gas to the pressure of the room-temperature environment in which it is placed. Essential in making this connection is having the best understanding of the various collision processes that can lead to losses from both the background gasses, typically the H$_2$ molecule, as well as the collisions among the cold atoms. Here, we present our investigations into these collisional processes using Lithium atoms as the cold constituent. For a complete description, we determine collisional phase shifts and elastic cross sections for collision energies that spans several times room temperature. We compare various semiclassical approximations with full quantum simulations where possible. [Preview Abstract] |
Tuesday, June 6, 2017 3:00PM - 3:12PM |
C4.00006: Excited State Atom-Ion Charge-Exchange Ming Li, Constantinos Makrides, Alexander Petrov, Svetlana Kotochigova We theoretically investigate the exothermic charge-exchange reaction between an excited atom and a ground-state positive ion. In particular, we focus on MOT-excited Ca*(4s4p $^1$P) atoms colliding with ground-state Yb$^+$ ions, which are under active study by the experimental group of E. Hudson at UCLA [1]. Collisions between an excited atom and an ion are guided by two major contributions to the long-range interaction potentials, the induction $C_4/R^4$ and charge-quadrupole $C_3/R^3$ potentials, and their coupling by the electron-exchange interaction. Our model of these forces leads to close-coupling equations for multiple reaction channels. We find several avoided crossings between the potentials that couple to the nearby asymptotic limits of Yb*+Ca$^+$, some of which can possibly provide large charge exchange rate coefficients above 10$^{-10}$ cm$^3/$s. [1] W. G. Rellergert, S. T. Sullivan, S. Kotochigova, A. Petrov, K. Chen, S. J. Schowalter, and E. R. Hudson, Phys. Rev. Lett. 107, 243201 (Dec 2011). [Preview Abstract] |
Tuesday, June 6, 2017 3:12PM - 3:24PM |
C4.00007: Radiative association of a carbon atom and a proton via triplet molecular states J. F. Babb, B. M. McLaughlin Collisions between a ground state C atom and H${}^+$ along triplet molecular states with photon emission leading to a bound CH+ molecular ion (radiative association process) are studied theoretically. Cross sections and rate coefficients are calculated. The present results are compared to those for collisions of C and H${}^+$ leading to C${}^+$ and H with photon emission (radiative charge transfer process) along triplet molecular states and to those for radiative association of a carbon ion and a hydrogen atom along the singlet molecular state. We investigate the photon emission spectra and discuss possible astrophysical applications [Preview Abstract] |
Tuesday, June 6, 2017 3:24PM - 3:36PM |
C4.00008: Spin and electronic-excitation exchange in ultracold ion-atom collisions Ruti Ben shlomi, Tomas Sikorsky, Ziv Meir, Nitzan Akerman, Yehonatan Dallal, Meirav Pinkas, Roee Ozeri We experimentally study the dynamics of single and many inelastic collisions between ultracold ${ }^{87}Rb$ atoms and a single ${ }^{88}Sr{ }^{+}$ ion. A single ion is trapped in a linear Paul trap, laser cooled to 1 mK, and initially optically pumped to the higher excited metastable D-state. Then the ion is immersed in an ultracold ${ }^{87}Rb$ cloud. We investigated relaxation rates of the ion from the D-state due to collisions with atoms. We measured relaxation to the S-state after two Langevin collisions on average, followed by an energy release of 1500 K. This can be explained by a non-adiabatic excitation-exchange process: $Sr^{+}(D)+Rb(S)\to Sr^{+}(S)+Rb(P)$ . We further studied the dependence of this process on the mutual spin orientation of the ion and atoms and on initializing the ion in the different spin-orbit split $D_{5/2} $ and $D_{3/2} $ levels. We also initially spin polarized the ion and atoms in their electronic ground state and investigated the spin dynamics of the ion after one to several collisions. We observed that after 7 Langevin collisions on average, the spin of the ion aligned with the spin direction of the cloud, indicating that the dominant interaction between the ion and atoms spins during a collision is that of spin-exchange. Since the steady-state spin population of the ion reached only 90{\%}, we conclude that a spin-relaxation mechanism is involved as well. [Preview Abstract] |
Tuesday, June 6, 2017 3:36PM - 3:48PM |
C4.00009: Quantum Chemistry and Non-equilibrium Thermodynamics in an Atom-Ion Hybrid Trap Prateek Puri, Michael Mills, Steven Schowalter, Alex Dunning, Christian Schnieder, Kuang Chen, Eric Hudson Hybrid atom-ion traps allow for the precise control and investigation of atom-ion collisions in the ultracold regime. Recently our group has utilized these platforms for the study of quantum chemistry and non-equilibrium thermodynamics. With the long interrogation times associated with the ion trap environment and precisely tunable entrance channels of both the atom and ion via laser excitation, LQT-MOT hybrid traps are a convenient platform for the study of quantum state resolvable cold chemistry. We describe a recent study of excited state chemistry between cold Ca atoms and the BaOCH3+ molecular ion, which has resulted in the product BaOCa+, the first observed mixed hypermetallic alkaline earth oxide molecule. Further, due to the complexity of ion-ion heating within an LQT and micromotion interruption collisions, there remain many open questions about the thermodynamics of ions in a hybrid trap environment. We describe an analytical model that explains the thermodynamics of these systems as well an experimental effort confirming one of the more interesting hallmarks of this model, the bifurcation in steady state energy of ions immersed in an ultracold gas, as parameterized by total ion number. [Preview Abstract] |
Tuesday, June 6, 2017 3:48PM - 4:00PM |
C4.00010: Quantum-State-Resolved Ion-Molecule Chemistry Gary Chen, Tiangang Yang, Wesley Campbell, Eric Hudson We are working towards a new platform for quantum-state-resolved ion-molecule chemistry by utilizing a combination of cryogenic buffer gas cooling, laser-cooled ion sympathetic cooling, and integrated mass spectrometry in an RF Paul trap. Cold molecular species produced in a cryogenic buffer gas beam collide with target atomic carbon ions in an linear quadrupole trap. Ion imaging and time of flight mass spectrometry are then used to observe the resulting reaction rates and products. We can utilize the precision control over quantum states allowed by this neutral-plus-ion chemistry environment (N+ICE) to resolve state-resolved quantum chemical reactions without high-density molecular sample production; proposed extensions suggest true state-to-state chemistry is possible in this system. We report progress towards cold carbon and water chemistry, including co-trapping and sympathetic cooling of carbon ions with laser-cooled beryllium ions. [Preview Abstract] |
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