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arXiv:1211.3450 · Physical Review A

Imaging the evolution of an ultracold strontium Rydberg gas

Patrick McQuillen, X. Zhang, Trevor Strickler, F. B. Dunning, T. C. Killian

Published Jan 7, 2013• 81 citations
arXivHTMLPDF

Abstract

Clouds of ultracold strontium $5s48s\phantom{\rule{4pt}{0ex}}{}^{1}{S}_{0}$ or $5s47d\phantom{\rule{4pt}{0ex}}{}^{1}{D}_{2}$ Rydberg atoms are created by two-photon excitation of laser-cooled $5{s}^{2}\phantom{\rule{4pt}{0ex}}{}^{1}{S}_{0}$ atoms. The spontaneous evolution of the cloud of low orbital angular momentum (low-$\ensuremath{\ell}$) Rydberg states towards an ultracold neutral plasma is observed by imaging resonant light scattered from core ions, a technique that provides both spatial and temporal resolution. Evolution is observed to be faster for the $S$ states, which display isotropic attractive interactions, than for the $D$ states, which exhibit anisotropic, principally repulsive interactions. Immersion of the atoms in a dilute ultracold neutral plasma speeds up the evolution and allows the number of Rydberg atoms initially created to be determined.

DOI
10.1103/physreva.87.013407

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Clouds of ultracold strontium 5s48s1S05s48s\phantom{\rule{4pt}{0ex}}{}^{1}{S}_{0}5s48s1S0​ or 5s47d1D25s47d\phantom{\rule{4pt}{0ex}}{}^{1}{D}_{2}5s47d1D2​ Rydberg atoms are created by two-photon excitation of laser-cooled 5s21S05{s}^{2}\phantom{\rule{4pt}{0ex}}{}^{1}{S}_{0}5s21S0​ atoms. The spontaneous evolution of the cloud of low orbital angular momentum (low-\ensuremathℓ\ensuremath{\ell}\ensuremathℓ) Rydberg states towards an ultracold neutral plasma is observed by imaging resonant light scattered from core ions, a technique that provides both spatial and temporal resolution. Evolution is observed to be faster for the SSS states, which display isotropic attractive interactions, than for the DDD states, which exhibit anisotropic, principally repulsive interactions. Immersion of the atoms in a dilute ultracold neutral plasma speeds up the evolution and allows the number of Rydberg atoms initially created to be determined.
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We report a systematic experimental and numerical study of the expansion of ultracold Rydberg plasmas. Specifically, we have measured the asymptotic expansion velocities, v0{v}_{0}v0​, of ultracold neutral plasmas (UNPs) which evolve from cold, dense samples of Rydberg rubidium atoms using ion time-of-flight spectroscopy. From this, we have obtained values for the effective initial plasma electron temperature, Te,0=mionv02/kB{T}_{e,0}={m}_{\mathrm{ion}}{v}_{0}^{2}/{k}_{B}Te,0​=mion​v02​/kB​ (where mion{m}_{\mathrm{ion}}mion​ is the Rb+{\mathrm{Rb}}^{+}Rb+ ion mass), as a function of the original Rydberg atom density and binding energy, Eb,i{E}_{b,i}Eb,i​. We have also simulated numerically the interaction of UNPs with a large reservoir of Rydberg atoms to obtain data to compare with our experimental results. We find that for Rydberg atom densities in the range ${10}^{7}--{10}^{9} {\mathrm{cm}}^{\ensuremath{-}3}$, for states with principal quantum number n>40,Te,0n>40, {T}_{e,0}n>40,Te,0​ is insensitive to the initial ionization mechanism which seeds the plasma. In addition, the quantity kBTe,0{k}_{B}\phantom{\rule{0.16em}{0ex}}{T}_{e,0}kB​Te,0​ is strongly correlated with the fraction of atoms which ionize, and is in the range $0.6\ifmmode\times\else\texttimes\fi{}|{E}_{b,i}|\ensuremath{\lesssim}{k}_{B}{T}_{e,0}\ensuremath{\lesssim}2.5\ifmmode\times\else\texttimes\fi{}|{E}_{b,i}|$. On the other hand, plasmas from Rydberg samples with $n\ensuremath{\lesssim}40$ evolve with no significant additional ionization of the remaining atoms once a threshold number of ions has been established. The dominant interaction between the plasma electrons and the Rydberg atoms is one in which the atoms are deexcited, a heating process for electrons that competes with adiabatic cooling to establish an equilibrium where Te,0{T}_{e,0}Te,0​ is determined by their Coulomb coupling parameter, ${\mathrm{\ensuremath{\Gamma}}}_{e}\ensuremath{\sim}0.01$.
We study a supersonic beam of cold, dense, xenon Rydberg atoms as it evolves to an ultracold plasma. At early times, while the free electron density is low, d-series Rydbergs atoms undergo long-range ℓ-mixing collisions producing states of high orbital angular momentum. These high-ℓ states drive dipole-dipole interactions where Penning ionization provides a seed of electrons in a cloud of Rydberg atoms excited into the 51d state. The electron density increases and reaches the threshold for avalanche into plasma at 25 μs. After 90 μs the plasma becomes fully formed developing rigidity to a 432 V/cm ionizing field as well as sensitivity to a weak 500 mV/cm field. A shell model was developed to understand the dynamics behind this process. In addition, in collaboration with the Weidemüller group, a model was developed using Penning ionization to seed the spontaneous avalanche of a cloud of strongly blockaded Rydberg atoms in a MOT.
We present the first large-scale simulations of an ultracold neutral plasma, produced by photoionization of laser-cooled xenon atoms, from creation to initial expansion, using classical molecular-dynamics methods with open boundary conditions. We reproduce many of the experimental findings such as the trapping efficiency of electrons with increased ion number, a minimum electron temperature achieved on approach to the photoionization threshold, and recombination into Rydberg states of an anomalously low principal quantum number. In addition, many of these effects establish themselves very early in the plasma evolution ( similar ns) before the present experimental observations begin.
We present the first experiment to observe an ultracold neutral plasma. The plasma, which was created by photoionization of laser cooled atoms, has charge densities as high as 2×109 cm−3, and the temperatures of electrons and ions are as low as 100 mK and 10 μK, respectively. The plasma has a lifetime of about 100 μs, much longer than predicted by recombination rates. When the laser that excites the atoms is tuned below the ionization limit we create a sample of very highly excited cold Rydberg atoms. At our highest densities and during a time of a few microseconds, in which the Rydberg atoms are essentially stationary, the ensemble evolves towards an unbound plasma-like state.
There is considerable interest in the dynamics of ultracold plasmas and ultracold Rydberg gases. Ultracold plasmas are typically formed by photo-excitation of ultracold atoms to an energy region near (both above and below) an ionization threshold. Excitation to bound, highly-excited Rydberg states can lead to formation of a plasma via several processes, including collisions between Rydberg atoms. Three-body recombination in an ultracold plasma can also result in the production of ultracold Rydberg atoms. Understanding the dynamics of ultracold Rydberg gases is therefore important for understanding the dynamics of ultracold plasmas. In this dissertation, we have investigated the formation and survival of a particular class of Rydberg atoms. These atoms are known as ZEKE state Rydberg atoms, where the term ZEKE is derived from "Zero Kinetic Energy." ZEKE Rydberg states are high angular momentum and high angular momentum projection excited states which can be formed by laser excitation in the presence of electric fields. Without the electric fields, these states would be optically dark. We have investigated ZEKE Rydberg states in ultracold argon in an energy region just below the second ionization threshold of the atoms in our magneto-optical trap. Here, low angular momentum states decay very quickly (< 1 ns) by auto-ionization through a core spin flip due to the Rydberg electron-core interaction. This interaction has been significantly reduced due to the dilution of low l and m states as a result of l and m mixing during excitation. Hence, ZEKE Rydberg states live orders of magnitude longer than low angular momentum states. We are reporting on our experimental investigation of these states in the ultracold regime and prospects for future studies in which external control of these states may be used to control ultracold plasma dynamics.