Bulletin of the American Physical Society
39th Annual Meeting of the APS Division of Atomic, Molecular, and Optical Physics
Volume 53, Number 7
Tuesday–Saturday, May 27–31, 2008; State College, Pennsylvania
Session P6: Ultracold Rydberg Atoms and Plasmas |
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Chair: Peter Schmelcher, University of Heidelberg Room: Nittany Lion Inn Boardroom II |
Friday, May 30, 2008 11:00AM - 11:12AM |
P6.00001: Van der Waals interaction in the cold Rydberg gas Jianing Han, Thomas Gallagher The two photon microwave transitions between Rb $ns$ and $(n+1)s$ states exhibit density dependent frequency shifts and broadenings far in excess of what would be expected from the small difference in their van der Waals coefficients. We attribute the large shifts to the fact that what we observe is transitions not in isolated atoms, but predominantly in pairs of atoms, specifically, the $nsns$ to $ns(n+1)s$ transitions. The latter states have a strong dipole dipole coupling the nearly degenerate $np_{3/2}np_{3/2}$ state. Over the range of n we have studied the $ns(n+1)s$ state passes through the resonance with the $np_{3/2}np_{3/2}$ state, reversing the sign of the asymmetry. The resonance occurs at n=38, where the spectrum is very different, and we attribute at least part of the difference to the fact that in this case many body effects must be taken into account. Similar observations have been made for the $ns$ to $(n+2)s$ transitions. The observed spectra can in almost all cases be fit to a lineshape model based on the density distribution in the trap and the calculated van der Waals coefficients. [Preview Abstract] |
Friday, May 30, 2008 11:12AM - 11:24AM |
P6.00002: Transform-limited Rydberg-Rydberg Collisions in a Thermal Atomic Beam M.R. Kutteruf, R.R. Jones The expansion of a localized volume of Rydberg atoms in a thermal atomic beam allows us to reduce the relative velocities of atoms undergoing resonant dipole-dipole collisions. Three collinear 5 ns dye laser pulses are tightly focused at right angles to a thermal potassium beam, exciting atoms to the 29s$_{1/2}$ and 27d$_{3/2}$ Rydberg states. Following the excitation, the excited volume expands and the relative velocities between nearby atoms decrease to a small fraction of the average velocity of the beam. A fast rising electric field pulse then Stark tunes the excited atoms into collisional resonance, 29s+27d =$>$ 29p+28p. The collision probability is determined by measuring the 29p yield using state-selective field ionization. The energy width of the collisional resonance is found to be independent of the field-pulse duration provided this duration is much longer than the time required to complete a typical collision. By reducing the field-pulse duration to less than this collision time, we specify the precise times for starting and stopping the collisions, and observe a broadening of the coherent collisional resonance as previously seen [Thomson et al., PRL 65, 3273 (1990)] using a considerably more complicated velocity selection approach. This work has been supported by NSF. [Preview Abstract] |
Friday, May 30, 2008 11:24AM - 11:36AM |
P6.00003: Electric field effects on cold Rydberg atom pair excitation Arne Schwettmann, James P. Shaffer, Valter A. Nascimento, Lucas L. Caliri, Luis G. Marcassa We present experimental results that show a significant yield of nP atoms after excitation of nS Rb Rydberg atoms from a MOT using a pulsed dye laser, where $27\leq n\leq39$. Such results are naturally attributed to binary collisions. This cannot be the case here, because the interaction between Rb nS atoms is repulsive. In this experiment, the AC-Stark effect, dipole- dipole interactions, and DC Stark effect work together to create a non-vanishing final population of nP(n-1)P pairs. The background electric field and multipole interactions cause an admixture of ns-ns character into the nP(n-1)P pairs. The AC Stark shift from the laser pulse shifts the intermediate state into resonance with the nP(n-1)P final pair. We compare our results to calculations done by numerically solving the density matrix equations for a two-photon excitation of the nP(n-1)P pair state at $0.55\leq R\leq1.8$ microns. [Preview Abstract] |
Friday, May 30, 2008 11:36AM - 11:48AM |
P6.00004: Long-Range Wells in Rydberg-Rydberg Potential Curves Nolan Samboy, Jovica Stanojevic, Robin C\^ot\'e We explore the interaction of Rydberg atom pairs including dipole, quadrupole, and spin-orbit couplings. We find that some of the resulting potential curves exhibit wells deep enough to support many bound states. We investigate the properties of these wells as well as the influence that small electric fields may have on the potential curves. [Preview Abstract] |
Friday, May 30, 2008 11:48AM - 12:00PM |
P6.00005: Long Range, Cold Cs Rydberg Atom-Rydberg Atom Molecules K. Richard Overstreet, Arne Schwettmann, Jonathan Tallant, James P. Shaffer The interaction between atoms in a cold Rydberg gas depends strongly on electric field and principle quantum number. When the density of states becomes large enough at large n, avoided crossings between different molecular states occur frequently. The character of these crossings can be controlled with a background electric field. At some background electric fields, bound molecular Rydberg states appear. We present experimental work on detecting these bound Rydberg molecules in a cold Rydberg gas using the Coulomb repulsion of the Ryberg atoms after pulsed field ionization. [Preview Abstract] |
Friday, May 30, 2008 12:00PM - 12:12PM |
P6.00006: Dipole-Dipole Interactions in a Cold Cs Rydberg Gas Jonathan Tallant, K. Richard Overstreet, Arne Schwettmann, James P. Shaffer We present experimental measurements of absorption line shifts in a cold Cs Rydberg gas. The spectral line shape of Cs at n=66, 89, and 120 (p and d states) is measured as a function of Rydberg atom density. The experimental results are compared to Monte Carlo simulations. The Monte Carlo simulations use Rydberg atom interaction potentials calculated in a background electric field. The magnitude of the line shift and its dependence on Rydberg atom density indicate the onset of many-body dipole-dipole interactions. [Preview Abstract] |
Friday, May 30, 2008 12:12PM - 12:24PM |
P6.00007: Rydberg Atom Population Distributions in Ultracold Plasmas Robert Fletcher, Xianli Zhang, Steven Rolston Despite being an important collisional process in cold plasmas, three-body recombination (3BR) is described by a rate expression that has yet to be strongly confirmed for very low electron temperatures. While the low temperature 3BR rates have recently been measured in an ultracold plasma, the strong temperature dependence (T$^{-9/2})$ of the 3BR rate expression, combined with the difficulty of accurately measuring electron temperatures of ultracold plasmas, makes it difficult to verify the 3BR rate expressions at low temperatures. In this work, we use microwave ionization to measure the Rydberg population distributions in an ultracold plasma in an attempt to test the predictions of 3BR rate theory. This method of testing 3BR rate equations has a clear advantage over measuring the overall 3BR rate, as it is far less dependent on the particular electron temperature of the plasma. [Preview Abstract] |
Friday, May 30, 2008 12:24PM - 12:36PM |
P6.00008: Using projection imaging to study ultracold plasmas Xianli Zhang, Robert Fletcher, Steven Rolston We have developed a time-of-flight projection imaging technique to study ultracold plasma dynamics, such as plasma expansion and instabilities. We image the charged particle (electrons or ions) distribution by extracting them with a high-voltage pulse and accelerating them onto a position-sensitive detector. Measuring the 2-D Gaussian width of the ion images, we can extract the final asymptotic expansion velocity of the ultracold plasma. The plasma expansion velocities at different initial electron temperatures match earlier results obtained by measuring the plasma oscillation frequency, providing further support for this method as a means to extract ultracold plasma densities. We observe that the transverse expansion velocity in a uniform longitudinal magnetic field scales as B$^{-1/2}$, explained by a nonlinear ambipolar diffusion model that involves anisotropic diffusion in two directions. We also observe that the electron projection images split into two or three lobes with ExB field configuration, coincident with the observation of large oscillations in the electron emission signals. We identify the electron images as a signature of plasma instabilities due to electrons drifting relative to ions across the magnetic field. This work is supported by the NSF. [Preview Abstract] |
Friday, May 30, 2008 12:36PM - 12:48PM |
P6.00009: Highly excited state Rydberg recombination and consequences for ultracold neutral plasmas Hossein Sadeghpour, Thomas Pohl, Daniel Vrinceanu Three-body recombination is a fundamental collision process for laboratory and astrophysical plasmas and in ultracold plasmas, it becomes the dominant formation mechanism, because of the inverse 9/2 dependence with temperature. In calculations and modeling of recombination and electron-impact excitation/de-excitation of Rydberg atoms, it is generally agreed that the level population of a Rydberg atom comes into thermodynamic equilibrium with the plasma electrons at kT. This is not strictly valid and will be discussed in the context of ionic microfield population of highest Rydberg levels. Plasma dynamics simulations of three-body recombination will be discussed which demonstrate that the recently measured rate of recombination in an ultracold plasma can be quantitatively described, only when the rates for electron-Rydberg atom scattering, as provided in literature, are revised. [Preview Abstract] |
Friday, May 30, 2008 12:48PM - 1:00PM |
P6.00010: Adiabatic Cooling and Electron-Ion Collisions during Self-Similar Expansion in Ultracold Neutral Plasmas Jose Castro, Hong Gao, Thomas Killian Plasma expansion dynamics are discussed for Ultracold Neutral Plasmas (UNP). Previous measurements of the ion kinetic energy and plasma size of UNP's have shown that the expansion is self-similar. This self-similar expansion follows an analytic solution of the Vlasov equations, the central equations in the kinetic theory of plasmas. As the plasma expands, both ion and electron species undergo adiabatic cooling. Spatially resolved fluorescence spectroscopy of the UNP shows that at early times the plasma undergoes electron-ion collisions resulting in initial heating of the ions. This effect is combined with adiabatic cooling which dominates at later times in the expansion. [Preview Abstract] |
Friday, May 30, 2008 1:00PM - 1:12PM |
P6.00011: Rydberg atom interactions in high density samples Kelly Younge, Aaron Reinhard, Thomas Pohl, Paul Berman, Georg Raithel We present a study of state mixing and counting statistics of $^{85}$Rb Rydberg atoms in small, high density samples. The atoms are collected in an optical dipole trap, excited into Rydberg states with 100ns laser pulses, and then immediately ionized with an electric-field ramp. At sufficiently high density, the electrostatic interaction between the atoms allows population of atomic states that would not be populated by direct photo-excitation alone. The percentage of atoms quasi-instantaneously excited into these product states is measured for sample densities up to approximately $3 \times 10^{11}$cm$^{-3}$. The experiments are performed for target states with principal quantum numbers near n=43, where a Foerster resonance enhances the state-mixing ratios. The large amount of instantaneous mixing can be explained only within the context of a many-body theory. We have also studied statistical distributions of Rydberg atoms in small ensembles, where an electric field is used to tune the collisional interaction into resonance. Probability distributions for the number of Rydberg excitations are clearly sub-Poissonian, indicating a blockade of Rydberg atom excitation. [Preview Abstract] |
Friday, May 30, 2008 1:12PM - 1:24PM |
P6.00012: Many-Body Ionization in a Frozen Rydberg Gas Edward Shuman, Paul Tanner, Jianing Han, Tom Gallagher Here we present measurements of spontaneous ionization in a dense gas of 300 $\mu $K $^{85}$Rb atoms of \textit{n$\sim $} 50. At densities of $\sim $10$^{10}$ cm$^{-3}$ ionization occurs on a 100 ns time scale, far too fast to be explained by the motion of the atoms or photoionization by 300 $\mu $K blackbody radiation. Rapid ionization is accompanied by spectral broadening and by the emergence of new features, which we attribute to multiple atom absorptions. We attribute the rapid ionization to a sequence of near resonant dipole-dipole transitions through virtual states in this intrinsically many-body system, culminating in the ionization of some of the atoms. [Preview Abstract] |
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