Bulletin of the American Physical Society
2006 37th Meeting of the Division of Atomic, Molecular and Optical Physics
Tuesday–Saturday, May 16–20, 2006; Knoxville, TN
Session E5: Ultracold Plasmas and Molecules |
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Chair: Nicholas Bigelow, University of Rochester Room: Knoxville Convention Center 301AB |
Wednesday, May 17, 2006 1:30PM - 1:42PM |
E5.00001: Confinement of strongly magnetized ultracold plasmas J.-H. Choi, X. Zhang, A. P. Povilus, G. Raithel We report on the trapping and evolution dynamics of strongly magnetized quasi-neutral ultracold plasmas. By applying a quadrupole electric field in addition to a Penning trap field~(B=2.9T), both ion and electron components of the plasmas have been confined over several milliseconds in a nested trapping configuration. An ultracold plasma is first created by photoionizing a cloud of laser-cooled $^{85}$Rb atoms. After a variable time delay, an electric-field ramp is applied to extract electrons from the trap. The measurements allow us to determine the depth of the potential well in which the electron component of the plasma is trapped. We observe a periodic modulation of the electron trap depth, caused by a breathing-mode oscillation of the ionic component in the trap. In addition, evaporative cooling leads to a decrease of the electron gas temperature over several milliseconds. The long-term loss of particles from the trap is mostly due to a slow ${\bf E}\times{\bf B}$ drift motion. In the future, simultaneous confinement of ion and electron components might be important in studying strongly coupled neutral plasmas. [Preview Abstract] |
Wednesday, May 17, 2006 1:42PM - 1:54PM |
E5.00002: Microwave Ionization of Rydberg Atoms in an Ultracold Plasma Robert Fletcher, Xianli Zhang, Steven Rolston Expanding ultracold neutral plasmas are dynamic systems, driven by electron pressure that is proportional to the electron temperature. This leads to adiabatic cooling, which is counteracted by heat produced by three-body recombination into Rydberg atoms. To date, the study of Rydberg production in ultracold plasmas has relied on field ionization techniques, which destroy the plasma. We are investigating the use of microwave ionization of Rydberg atoms in the plasma. The plasma does not respond at the microwave frequencies ($\sim$ 2 GHz), so Rydberg populations can be probed repeatedly during the plasma evolution. We apply multiple pulsed microwave fields at varying times to an expanding neutral ultracold Xenon plasma, followed by a final field ionization ramp. This technique provides a good tool for the investigation of the time- dependent populations of Rydberg atoms in the plasma, allowing for a better understanding of collisional processes in expanding ultracold plasmas and the role of Rydbergs in the equilibration of the plasma electron temperature. We also investigate the application of a continuously applied microwave field on the evolution of the plasma. [Preview Abstract] |
Wednesday, May 17, 2006 1:54PM - 2:06PM |
E5.00003: Observation of Collective Modes of Ultracold Plasmas Xianli Zhang, Robert Fletcher, Steven Rolston Applying a radio-frequency electric field to an expanding ultracold neutral plasma leads to the observation of as many as six peaks in the emission of electrons from the plasma. These are identified as collective modes of the plasma and are in qualitative agreement with a model of Tonks-Dattner resonances, electron sound waves propagating in a finite-sized, inhomogeneous plasma. The existing theories that describe Tonks-Dattner resonances assume a fixed outer boundary condition, which is absent from our plasmas that expand freely into vacuum. In calculating the mode frequencies, we assume an outer boundary related to the size of the plasma. These modes are not predicted within cold plasma theory and require in inclusion of the electron pressure term, which is proportional to the electron temperature , in the fluid equations. Such modes may provide an accurate method to determine the time- dependent electron temperature, applicable over a large range in time and density of the expanding plasmas. [Preview Abstract] |
Wednesday, May 17, 2006 2:06PM - 2:18PM |
E5.00004: Gaussian expansion and electron temperature determination in ultracold Sr plasma Sampad Laha, Priya Gupta, Clayton Simien, Sarah Nagel, Natali Martinez, Pascal Mickelson, Thomas Killian In an ultracold neutral plasma, absorption images show the details of the self-similar expansion of a Gaussian density distribution. As the plasma evolves, the initial electron temperature (T$_{e})$ plays a critical role and calculating the electron temperature is necessary to predict which processes will be dominant in the evolution. Here, we present a study to calculate the electron temperature of ultracold Sr plasma when the ion cloud undergoes a Gaussian expansion. Calculating T$_{e}$ will tell us how many ions are we losing due to three body recombination (TBR). [Preview Abstract] |
Wednesday, May 17, 2006 2:18PM - 2:30PM |
E5.00005: Rydberg Atom - Rydberg Atom Dipole-Dipole Potentials Arne Schwettmann, Jeff Crawford, K. Richard Overstreet, James P. Shaffer We present numerical methods to calculate dipole-dipole interaction potential curves for Rydberg atom pairs that include a background electric field, orientational effects, and do not make use of the two-state approximation. As an example, we present dipole-dipole potentials of pairs of Cs Rydberg atoms in the 89D and adjacent electronic states for internuclear separations in the $\mu$m range, for a background electric field of $\sim 28\;$mV/cm as found in a typical cold Rydberg gas created in a magneto-optical trap. It is shown that an accurate calculation of dipole-dipole potentials for Rydberg atoms requires a large basis set of atom pair states. [Preview Abstract] |
Wednesday, May 17, 2006 2:30PM - 2:42PM |
E5.00006: Stark Slowing of Asymmetric Rotors Arne Schwettmann, Jack Franklin, K. Richard Overstreet, Jonathan Tallant, James P. Shaffer Stark deceleration is one of the few methods that can be used to slow polyatomic molecules. We present calculations of Stark shift energies, a quantitative analysis of nonadiabatic transition probabilities, and orientational distribution functions applicable to typical Stark slowing conditions for the two small asymmetric rotors nitromethane and acetaldehyde. We show that asymmetric polyatomic molecules are good candidates for Stark slowing. [Preview Abstract] |
Wednesday, May 17, 2006 2:42PM - 2:54PM |
E5.00007: Collision cooling of molecules Kevin E. Strecker, Jamie Ramirez, Dave W. Chandler We report a new technique using single collisions to cool molecules from supersonic speeds down into the milliKelvin regime. Geometrically orientated collision between partners with equal or near equal masses results in a small fraction (of atoms or molecules) coming to rest in the laboratory frame. When one collision partner is a molecule, excess collision energy can be funneled into the rotational modes of that molecule, forming cold ground vibrational but rotationally excited molecules. Using this technique, we have cooled nitric oxide (NO J= 7.5) to below 200 mK, the current resolution of our system [1]. This technique has recently been expanded to cool ammonia (NH$_{3}$ J = 2), via collisions with neon and hydrogen bromide (HBr J = 1) via collisions with Kryton. The cooling limit of this technique is only limited by the mass defect between the molecule and colliding atom. We are currently attempting to improve or ability to measure the final temperature of the molecules, as our measured final velocities are a factor of four greater then theoretically predicted. [1] D.W. Chandler, J. Valentini, M. Ellioff, Science v.302, 1940 (2003) [Preview Abstract] |
Wednesday, May 17, 2006 2:54PM - 3:06PM |
E5.00008: Cold Molecule Experiments and Production Eric R. Hudson, Brian C. Sawyer, Benjamin L. Lev, Jun Ye Cold molecules promise to revolutionarily impact atomic physics with studies of cold molecular collisions and quantum chemistry, implementation of quantum information processing, and possibilities for ferro-electric phase transitions. Our research efforts have focused on providing cold, trapped molecules through Stark deceleration of supersonic beams of hydroxyl radicals (OH) and, recently, formaldehyde molecules (H$_{2}$CO). Specifically, our work has been used to uncover the dynamics governing the evolution of the molecules within the decelerator, as well as to efficiently produce molecules for subsequent study. In our current experiments, we accelerate/decelerate a supersonic beam of molecules to a mean speed adjustable between 500 m/s to rest, with a translational temperature tunable from 1mK to 1K. Recently, we performed high resolution microwave spectroscopy of the lowest $\lambda $-doublet lines of OH, which along with astronomical observation of OH mega-masers can be used to constrain the time variation of the fine structure constant. We will present our latest results on deceleration and trapping, high resolution microwave spectroscopy of OH, and discuss our work on the study of cold molecular collisions/reactions as well as techniques that should allow extension to the ultra-cold regime. [Preview Abstract] |
Wednesday, May 17, 2006 3:06PM - 3:18PM |
E5.00009: One-photon Assisted Formation of Ultracold LiH and NaH Elizabeth Juarros, Kate Kirby, Robin C\^{o}t\'{e} Alkali hydride molecules have large dipole moments in their ground electronic states. We explore the possibility of forming such molecules from a mixture of the ultracold atomic gases, employing a one-photon stimulated radiative association process. Using accurate molecular potential energy curves and dipole moments, we have calculated the rate coefficients for populating each of the vibrational levels of the X$^1\Sigma^+$ state of LiH and NaH. We have found that significant molecule formation rates into the upper vibrational levels can be realized with laser intensities and MOT densities that are easily attainable experimentally. We examine the spontaneous emission cascade which takes place from these upper vibrational levels on a timescale of milliseconds, and calculate the resulting rotational populations in $v=0$. We show that photon emission in the cascade process does not contribute to trap loss. [Preview Abstract] |
Wednesday, May 17, 2006 3:18PM - 3:30PM |
E5.00010: ``Molecule chips'' Michaela Tscherneck, Michael Holmes, Amy Wakim, Nicholas Bigelow Cooling and trapping of atoms close to a reflective surface -- the so-called atom chip -- and creating, manipulating, and detecting molecules are two exciting and promising fields. The tightness of atom chip traps and the ability to include optical elements on the chip surface allow for precise positioning and manipulation of the trapped atoms. So far, molecules have not been studied in a chip environment. In this talk, we will present our first results of such a ``molecule chip''. [Preview Abstract] |
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