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 S2: Rydberg Physics and Ultracold Plasmas |
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Chair: Georg Raithel, University of Michigan Room: Knoxville Convention Center Ballroom EFG |
Friday, May 19, 2006 8:00AM - 8:36AM |
S2.00001: Expansion Dynamics of Ultracold Neutral Plasmas Invited Speaker: Ultracold neutral plasmas [1], formed by photoionizing laser-cooled atoms near the ionization threshold, stretch the boundaries of traditional neutral plasma physics. The electron temperature in these plasmas is from 1-1000K and the ion temperature is around 1 K. The density can be as high as 10$^{10}$ cm$^{-3}$. Fundamental interest stems from the possibility of creating strongly-coupled plasmas, but recent experimental and theoretical work has focused on the equilibration and expansion dynamics. Using optical absorption imaging [2], we study expansion dynamics during the first 30 microseconds after photoionization. Images record the spatial extent of the plasma, while the Doppler broadened absorption spectrum measures the ion velocity spectrally. The expansion is driven by the pressure of the electron gas, so the ion acceleration depends on the electron temperature. Evidence for terminal ion velocity supports predictions of adiabatic cooling of electrons during expansion [3]. Images confirm the self-similar nature of a Gaussian density distribution. Understanding expansion dynamics is important for plans to laser cool and trap the plasma. This work is supported by the National Science Foundation and David and Lucille Packard Foundation. \newline \newline [1] T. C. Killian, S. Kulin, S. D. Bergeson, L. A. Orozco, C. Orzel, and S. L. Rolston, Phys. Rev. Lett. \textbf{83}, 4776 (1999). \newline [2] C. E. Simien, Y.C. Chen, P. Gupta, S. Laha, Y. N. Martinez, P. G. Mickelson, S. B. Nagel, T. C. Killian, Phys. Rev. Lett. \textbf{92}, 143001 (2004). \newline [3] F. Robicheaux and J. D. Hanson, Phys. Plasmas \textbf{10}, 2217 (2003), T. Pohl, T. Pattard, and J. M. Rost, Phys. Rev. A \textbf{70}, 033416 (2004). [Preview Abstract] |
Friday, May 19, 2006 8:36AM - 9:12AM |
S2.00002: Rydberg gases as thermostats for ultracold neutral plasmas Invited Speaker: Thomas Pattard Ultracold neutral plasmas produced by photoionization of laser-cooled neutral atoms are very appealing for a number of different reasons. One of them is the prospect of creating, in a table-top experiment, a strongly coupled two-component plasma where the electrostatic potential energy greatly exceeds the thermal kinetic energy of the plasma particles. In this talk, I will discuss two examples of the potential of Rydberg gases for manipulating the electronic {\em and} ionic temperature of ultracold plasmas. In the first part, I will show that the addition of Rydberg atoms to a plasma permits controlling the electronic temperature to a significant extent. Depending on the level of excitation of the atoms and the timing of their creation, both cooling and heating of the plasma electrons can be achieved. In the second part, I will discuss the effect of a ``dipole blockade'', i.e.the interaction-induced suppression of Rydberg excitations in a gas, as a means of creating spatial correlations in the initial state (i.e.\ before the plasma is created by photoionization) in order to suppress the disorder-induced heating of the ions. This blockade effect, which has recently been demonstrated in a number of experiments, is of significant current interest beyond its application for the creation of strongly coupled plasmas, e.g.\ in certain proposed schemes for quantum information processing. In the talk, I will introduce a microscopic theoretical approach for the simulation of the Rydberg excitation dynamics in a dense gas of interacting atoms. It allows for a detailed investigation of the excitation process, including its statistical properties and spatial correlation properties. [Preview Abstract] |
Friday, May 19, 2006 9:12AM - 9:48AM |
S2.00003: Enhanced Recombination Rate in Ion Storage Rings: Formation of Rydberg Atoms during the Beam Merging Invited Speaker: Electron-ion recombination plays an important role in many areas of physics, such as astrophysics, fusion plasma, and accelerator physics. With the aid of electron coolers in ion storage rings it has become possible to measure the recombination rate at very low energies (typically $<$ 1 meV). For the experiments involving bare ions, the dominant recombination mechanism was attributed to radiative recombination. The measured rates\footnote{G. Gwinner {\it et al.}, Phys. Rev. Lett. {\bf 84}, 4822 (2000).} showed, however, a significant enhancement beyond the theoretical prediction\footnote{H. A. Kramers, Philos. Mag. {\bf 46}, 836 (1923); H. Bethe and E. Salpeter, {\it Quantum Mechanics of One- and Two-Electron Atoms}, Springer, Berlin (1957).}. We will discuss additional pathways for recombination due to the presence of guiding magnetic fields in the electron cooler. In the merging section a toroidal magnetic field guiding the electron beam crosses the ion beam with a finite angle and a transient motional electric field is induced in the rest frame of ion. This electric field forms Rydberg atoms and their radiative stabilization significantly contributes to the measured rate. The scaling of the rate with the ion charge and the magnetic guiding field is discussed. Understanding the origin of the observed enhancement is of importance for the analog process of antihydrogen formation presently studied in several experiments\footnote{C. Wesdorp, F. Robicheaux and L. D. Noordam, Phys. Rev. Lett {\bf 84}, 3799 (2000).}. [Preview Abstract] |
Friday, May 19, 2006 9:48AM - 10:24AM |
S2.00004: Microwave manipulation of an atomic electron. Invited Speaker: Applying a microwave field to an atom at its Kepler orbital frequency, the frequency of the $\Delta $ n = 1 transition, phase locks the motion of the electron, producing a non dispersing wave packet. Such wavepackets are robust and do not disperse for thousands of orbits. The phase locked electron's motion can be sped up or slowed down by chirping the frequency of the microwave field, and changes in n of 10 have been observed. In quantum mechanical terms such a process is adiabatic passage through a series of overlapping single photon resonances. It is also possible to produce large changes in n by chirping the frequency backwards, in which case the population transfer occurs by a single multiphoton adiabatic passage, and only small frequency chirps are required. These process are all readily understood in terms of the evolution of the Floquet, or dressed energy levels during the microwave pulse. It is a pleasure to acknowledge the invaluable contributions of H. Maeda, D. V. L. Norum, and J. H. Gurian to this work. This work has been supported by the National Science Foundation. [Preview Abstract] |
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