### Session C5: Rydberg Atoms and Cold Plasmas

Chair: Thomas Gallagher, University of Virginia
Room: Arboretum I-III

 Wednesday, May 26, 2010 2:00PM - 2:12PM C5.00001: Electric field effects on decay of Rb Rydberg atom pairs Donald Booth , Jonathan Tallant , Arne Schwettmann , James Shaffer , Jader Cabral , Jorge Kondo , Luis Gon\c{c}alves , Luis Marcassa We discuss the effects of the dc Stark effect on the decay of pairs of Rb Rydberg atoms. The decay of nD+nD Rydberg pair states, where 29 $\le$ n $\le$ 41, is observed by measuring the (n+2)P atomic products after a delay of 100 ns following their excitation in a Rb MOT. The background dc electric field has a significant effect on the decay of these pairs. The experimental results are compared to theoretical calculations which use the Landau-Zener approximation to calculate the probability of a transfer of population between the two states, taking into account dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interactions as well as the dc Stark effect. The experimental results rule out the possibility of free atom collisions being responsible for the transfer of population. The results show agreement with a model of non-adiabatic decay from the nD+nD state, and suggests that, rather than being the results of free-atom collisions, the pairs are excited directly. Wednesday, May 26, 2010 2:12PM - 2:24PM C5.00002: Near Threshold Resonant Processes in Plasmas Francis Robicheaux , Stuart Loch , Connor Ballance , Mitch Pindzola We present a theoretical reformulation of near threshold processes in plasmas that will, in many circumstances, greatly decrease the uncertainty of the calculation. An example process is dielectronic recombination where an electron collides with an ion so that it is captured into a resonant state and then is subsequently stabilized by the emission of a photon. In many situations, a low energy resonance can cause anomalously large possible errors in the calculation because the uncertainty in the resonance energy could cause a resonance to be classified as a bound state (or a bound state to be classified as a resonance). We will present a formulation of near threshold processes that could remove much of this computational uncertainty. Wednesday, May 26, 2010 2:24PM - 2:36PM C5.00003: Dielectronic recombination and autoionization yields in weak static electric fields Jirakan Nunkaew , Tom Gallagher It should be possible to measure the contribution to dielectronic recombination (DR) of energetically unresolved high $\ell$ states by measuring the recombination rate as a function of electric field. As the field is raised lower $\ell$ states are converted to Stark states. Autoionization rates increase as $\ell$ is decreased, and when the field is raised to the point that an $\ell$ state with an autoionization rate in excess of the radiative decay rate is added to the manifold of Stark states the DR rate will exhibit an observable increase. We measure the autoionization yields of the autoionizing Ba $6p_{j}nk$, $j=1/2$, $3/2$ states. The autoionization yield is complementary to DR, and the measurements indicate that the proposed approach should work well. While it is not surprising that this approach works for excited ion states which are isotropic, such as the Ba$^+$ $6p_{1/2}$, it is less obvious that it should work for an anisotropic ion, Ba$^+$ $6p_{3/2}$ where there are four quantum defects for each $\ell$, and the Stark effect of the Ba $6p_{3/2}nk$ is more complex than that of the Ba $6p_{1/2}nk$. Calculations of the Stark effect reveal that, while there are four times as many $6p_{3/2}nk$ levels as $6p_{1/2}nk$ levels, many of the interactions of the $6p_{3/2}nk$ levels are negligible, and the problem is no more complicated than the $6p_{1/2}nk$ problem. Wednesday, May 26, 2010 2:36PM - 2:48PM C5.00004: Ion Acoustic Waves in Ultracold Neutral Plasmas Jose Castro , Patrick McQuillen , Thomas Killian Ion acoustic waves were observed in Strontium Ultracold Neutral Plasmas (UNP) through fluorescence imaging of the Sr ions. Ion acoustic waves were excited on the plasma by imprinting modulations on the density distribution with a periodic transmission grating placed along the path of the ionization beam. Fluorescence images show the presence of density waves on the plasma and spatially-resolved fluorescence spectra show oscillations in the velocity distribution of the ions. The dispersion relation and effect of strong coupling will be discussed. Wednesday, May 26, 2010 2:48PM - 3:00PM C5.00005: Observation of the electric field of a cold neutral plasma Hyunwook Park , Raheel Ali , Thomas Gallagher The electric field of a cold nearly neutral plasma in a Rb magneto-optical trap is investigated by observing the Stark shift of the 42S-42P microwave transition. The plasma field consists of micro- and macro-fields which originate from the nearest ion and the macroscopic charge imbalance, respectively. Although the system is nearly electrically neutral, the microfield is always present. The macrofield arises from the charge imbalance created by the fact that some electrons leave the plasma and the ones which remain have to be on the outside of the plasma. We suggest initial electron distributions at different initial temperatures of the electron from which the charge imbalance, or spatial distribution of the excess ions, can be estimated. The combination of the computed micro- and macro-fields agrees well with the observed data. Wednesday, May 26, 2010 3:00PM - 3:12PM C5.00006: Electron Distribution of an Ultracold Neutral Plasma Kevin Twedt , Steven Rolston Electrons in an expanding ultracold plasma are expected to be in quasi-equilibrium, since the collision times are short compared to the plasma lifetime, yet we observe electrons evaporating out as the ion density decreases during expansion.~ We observe that a small electric field that shifts the electron cloud with respect to the ions increases the evaporation rate.~ We have calculated the spatial distribution of a zero-temperature electron cloud as a function of applied field and ion density, which is assumed to be Gaussian at all times.~ This calculation allows us to predict the flux of cold electrons from the plasma at all times, and is in good agreement with our observed electron signal.~ In addition, short electric field pulses can dump a fraction of plasma electrons without affecting the ion expansion.~ Evaporation ceases for several microseconds before quickly refilling to match the shape of the unperturbed signal.~ The dynamics of this process should yield insight into the actual electron distribution, especially for high-energy electrons, where deviations from a Boltzmann distribution should be most pronounced. This work is partially supported by the NSF. Wednesday, May 26, 2010 3:12PM - 3:24PM C5.00007: Spectroscopy of high-L Rydberg levels of Fr-like U and Th Ions Mark E. Hanni , Stephen R. Lundeen , Charles W. Fehrenbach High-L Rydberg levels of Fr-like U$^{5+}$ and Th$^{3+}$ have been studied using the Resonant Excitation Stark Ionization Spectroscopy (RESIS) technique. Beams of Rn-like U$^{6+}$ and Th$^{4+}$, obtained from a 14 GHz permanent magnet ECR source at Kansas State University, capture a single electron from a dense Rb Rydberg target, becoming Fr-like ions in very highly excited states, n $\sim$ 50. These Rydberg states are selectively excited to much higher levels, using a Doppler-tuned CO$_{2}$ laser, partially resolving the fine structure of the lower Rydberg level. Analysis of the spectrum with the long-range polarization model can be used to determine the polarizability of the Rn-like ion cores. The technique is analogous to previous studies of Kr$^{5+}$ Rydberg levels [1], but is more challenging because of the presence of background due to auto-ionizing Rydberg levels bound to metastable Rn-like ions.\\[4pt] [1] S.R. Lundeen and C.W. Fehrenbach, Phys. Rev. A 75, 032523(2007) Wednesday, May 26, 2010 3:24PM - 3:36PM C5.00008: Measurement of the Polarizability of Ba$^{2+}$ Erica L. Snow , Shannon L. Woods , Mark E. Hanni , Stephen R. Lundeen , Charles W. Fehrenbach The dipole polarizability of Ba$^{2+}$ was determined by spectroscopy of high-L Rydberg levels of Ba$^{+}$, using the Resonant Excitation Stark Ionization Spectroscopy (RESIS) method. Beams of Ba$^{2+}$, obtained by sputtering solid Ba inside a 14 GHz permanent magnet ECR source at Kansas State University, captured a single electron from a dense Rb 12F Rydberg target, forming highly excited Rydberg levels of Ba$^{+}$. Rydberg levels of Ba$^{+}$ with n=19 or 20 and L=5,6,7,8, and 9 were excited to a much higher level using a Doppler-tuned CO$_{2}$ laser and then detected by Stark ionization. The resolved fine structure of these levels, analyzed with the long-range polarization model, determined the polarizability of the ground state of Ba$^{2+}$. Wednesday, May 26, 2010 3:36PM - 3:48PM C5.00009: Ionization rates computed from classical periodic orbit theory Kevin Mitchell We consider the ionization dynamics of a Rydberg atom in applied electric and magnetic fields, an inherently chaotic process which has served as an important laboratory model for both classical and quantum chaos. Such classically chaotic systems often decay exponentially over intermediate time scales, before turning over into an algebraic tail at long times. (The algebraic tail is due to residual stickiness'' of the regular, i.e. nonchaotic, region of the classical phase space.) We demonstrate how the intermediate exponential decay rate can be directly computed from the classical periodic orbits of the electron. This computation requires a detailed knowledge of the chaotic electron dynamics, made possible by recently developed theoretical tools. Wednesday, May 26, 2010 3:48PM - 4:00PM C5.00010: State-dependent Energy Shift of Rydberg Atoms in a Ponderomotive Optical Lattice S.E. Anderson , K.C. Younge , B. Knuffman , G. Raithel We investigate, experimentally and in calculations, the state-dependence of the ponderomotive energy shift of Rydberg atoms in an optical lattice. In the utilized lattice, the size of the Rydberg wavefunctions approaches the lattice period. As a result, Rydberg atoms in the lattice experience energy shifts that are state-dependent and deviate from the free-electron ponderomotive shift. We report measurements of these energy shifts obtained via microwave spectroscopy of nS$\rightarrow$(n+1)S transitions of cold $^{85}$Rb Rydberg atoms. The lattice is formed using a one Watt, retro-reflected 1064~nm laser beam, focused into a spot of 13~$\mu$m diameter, resulting in ponderomotive lattices with depths up to 50~MHz. The observed microwave spectra of Rydberg atoms in the lattice exhibit side-bands that are shifted by several hundred kHz from the lattice-free transition. The spectra depend strongly on both the Rydberg state and the lattice depth. The experimental results are supported by a semi-classical simulation that reproduces all features of the spectra.