Bulletin of the American Physical Society
40th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 54, Number 7
Tuesday–Saturday, May 19–23, 2009; Charlottesville, Virginia
Session J6: Focus Session: Exotic States in Rydberg Atoms |
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Chair: Tom Killian, Rice University Room: Minor Hall 125 |
Thursday, May 21, 2009 8:00AM - 8:30AM |
J6.00001: Observation of ultralong range Rydberg molecules Invited Speaker: In 1934, Enrico Fermi described the scattering of a low energy electron from a neutral atom by using the ideas of scattering length and pseudopotential. Although the long range potential for an electron-atom interaction is always attractive, Fermi realized that the s-wave scattering length that characterizes the low energy collision can be either positive or negative. For a positive scattering length, the wavefunction of the electron is shifted away from the atom, the electron is repelled; whereas for a negative scattering length, the wavefunction of the electron is shifted to the atom, the electron is attracted. Based on Fermi's approach, Greene and co-workers predicted a novel molecular binding mechanism where a low energy Rydberg electron is scattered from a ground state atom in the case of negative scattering length. In this situation, the interaction between the electron and ground state atom is attractive and results in the formation of bound states of the ground state atom and the Rydberg atom. Molecules bound by electron scattering can have an internuclear separation of several thousand Bohr radii and are very different from molecules formed by 2 Rydberg atoms where the binding is the result of multipolar forces between the atoms alone. In this talk, we present experimental data on the observation of these exotic molecular states for Rb Rydberg atoms in S states for principal quantum numbers n between 34 and 40. The spectroscopic results for the vibrational ground and first excited state of the dimer Rb(5S)-Rb(nS) are presented and the s-wave scattering length for electron-Rb(5S) scattering in the low energy regime where the kinetic energy is less than 100 meV. Finally, we discuss and present data on the lifetimes and decay mechanisms of these molecules in a magnetic trap. [Preview Abstract] |
Thursday, May 21, 2009 8:30AM - 8:42AM |
J6.00002: Observation of Cs Rydberg atom macrodimers Arne Schwettmann, K. Richard Overstreet, Jonathan Tallant, Donald W. Booth, James P. Shaffer We report the observation of cold Cs Rydberg atom molecules bound at internuclear separations of R$\sim $3-9 $\mu $m. The bound states result from avoided crossings between Rydberg atom pair interaction potentials in an applied electric field. The molecular states can be modified by changing the applied electric field. The molecules are observed by mapping the radial separation of the two Rydberg atoms as a function of time delay between excitation and detection using the Coulomb repulsion of the ions after pulsed field ionization. Measurements were performed for 63D+65D, 64D+66D, 65D+67D, and 66D+68D pairs. The experiment is in good agreement with calculations of the pair interactions for these states. \newline [Preview Abstract] |
Thursday, May 21, 2009 8:42AM - 8:54AM |
J6.00003: Four-Wave Mixing and Coherent Processes in Ultracold Atoms Using Intermediate Rydberg States. E. Brekke, J.O. Day, L. Hardy, T.G. Walker Continuous 5S-5P-$n$D two-photon excitation to the Rydberg state was combined with an $n$D-6P tuned laser to explore coherent processes using intermediate Rydberg states. In a phase-matched geometry, four-wave mixing was demonstrated in good agreement with theory. The directional emission was optimized to 50{\%} for off-resonant Rydberg excitation. Further coherent schemes have been explored using small excitation volumes, showing promise for studying quantum effects in blockaded atom clouds. [Preview Abstract] |
Thursday, May 21, 2009 8:54AM - 9:06AM |
J6.00004: Properties of the ultracold molecular plasma formed in a seeded supersonic beam of NO Edward Grant, Jonathan Morrison, Christopher Rennick We have prepared an ultracold plasma of NO$^{+}$ molecular cations and electrons, entrained in a seeded supersonic molecular beam as a 1 mm$^{3}$ volume element with a charge density exceeding 10$^{12}$~cm$^{-3}$. Crossed laser beams defining this volume element produce a dense gas of NO molecules excited to a single rovibrationally selected $nf$ Rydberg state. Rydberg-Rydberg Penning interactions initiate the evolution to a plasma on a 100 ns timescale. The first of these electrons escape, after which Rydberg molecules and electrons \--- now trapped in the potential well formed by the macroscopic space charge \--- undergo an avalanche of ionizing collisions. The reservoir of Rydberg binding energy appears to moderate free electron temperature, and the high charge density acts to suppress exothermic three-body recombination. We have measured a rate of plasma expansion over 30 $\mu$s that accords with the Vlasov equations for a quasi-neutral plasma with an electron temperature that falls from an initial 8 K to 1 K, corresponding to an electron correlation, $\Gamma_{e}$, as high as 10. [Preview Abstract] |
Thursday, May 21, 2009 9:06AM - 9:18AM |
J6.00005: Using Population Echoes to Explore Coherent Interactions in a Nearly Frozen Rydberg Gas M.R. Kutteruf, R.R. Jones Coherent interactions between atoms in a nearly frozen Rydberg gas have been investigated using an echo technique. Distinct Rydberg atom populations, $\vert $25s$_{1/2}>$ and $\vert $33s$_{1/2}>$, are laser-excited in a Rb MOT. A dipole-dipole coupling between pair states, $\vert $25s$_{1/2}>\vert $33s$_{1/2}>$ and $\vert $24p$_{1/2}>\vert $34p$_{3/2}>$, is then applied for a time,$\tau $, using an electric field pulse which Stark shifts the pairs into resonance. After a delay, T, a second, identical field pulse is applied and the total population in the $\vert $24p$_{1/2}>\vert $34p$_{3/2}>$ state is measured. Interference fringes are observed as a function T, with a frequency determined by the zero-field energy separation, E, between the two pairs of states. As shown by Anderson et al [Phys. Rev. A 65, 063404], the interference fringes decay in a time T$_{c} \quad \sim $ 100 ns, which decreases for increasing Rydberg atom density. We show that a fast electric field step applied midway between the two interaction periods diabatically transports the pair state across the resonance, reversing the sign of E. In the absence of atom motion or decoherence, our signal should be identical to that for T=0, independent of static variations in E in the MOT. The coherence time can be extracted from measurements of this echo signal vs. T. This work has been supported by the NSF and the AFOSR. [Preview Abstract] |
Thursday, May 21, 2009 9:18AM - 9:30AM |
J6.00006: Rotary echo tests of coherence in Rydberg-atom excitation Kelly Cooper Younge, Georg Raithel Rotary echoes are employed to study excitation dynamics in many-body Rydberg systems. In this method, a phase reversal of a narrow-band excitation field, applied at a variable time during the excitation pulse, results in echo signals the visibility of which reveal the degree of coherence of the excitation process. Rotary echoes are measured for several $n$D$_{5/2}$ Rydberg levels of rubidium with principal quantum numbers near $n=43$, where the strength of electrostatic Rydberg-atom interactions is modulated by a F\"orster resonance. The Rydberg-atom interactions are shown to diminish the echo visibility, in agreement with recent theoretical work. The equivalence of echo signals with spectroscopic data is demonstrated. [Preview Abstract] |
Thursday, May 21, 2009 9:30AM - 10:00AM |
J6.00007: Rydberg atoms in ultracold plasmas Invited Speaker: Ultracold plasmas are formed through the photoionization of laser-cooled atoms, or spontaneous ionization of a dense cloud of Rydberg atoms or now molecules[1]. Ultracold plasmas are inherently metastable, as the ions and electrons would be in a lower energy state bound together as atoms. The dominant process of atom formation in these plasmas is three-body recombination, a collision between two electrons and an ion that leads to the formation of a Rydberg atom. This collisional process is not only important in determining the lifetime and density of the plasma, but is also critical in determining the time evolution of the temperature. The formation of the Rydberg atoms is accompanied by an increase in electron energy for the extra electron in the collision, and is a source of heating in these plasmas. Classical three-body recombination theory scales as T$^{-9/2}$, and thus as a plasma cools due to a process such as adiabatic expansion, recombination-induced heating turns on, limiting the temperature [2]. The Rydberg atoms formed live in the plasma and contribute to the temperature dynamics, as collisions with plasma electrons can change the principal quantum number of the Rydberg atom, driving it to more tightly bound states (a source of plasma heating) or to higher states (a source of plasma cooling). If the plasma is cold and dense enough to be strongly coupled, classical three-body recombination theory breaks down. Recent theoretical work [3] suggests that the rate limits as the plasma gets strongly coupled. I will review the role of Rydberg atoms in ultracold plasmas and prospects for probing Rydberg collisions in the strongly coupled environment. \\[4pt] [1] J. P. Morrison, \textit{et al.}, Phys. Rev. Lett. \textbf{101}, 205005 (2008 \\[0pt] [2] R. S. Fletcher, X. Zhang, and S. L. Rolston, Phys. Rev. Lett. \textbf{99}, 145001 (2007 \\[0pt] [3] T. Pohl, private communication. [Preview Abstract] |
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