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Journal of the Optical Society of America B

Back and forth between Rydberg atoms and ultracold plasmas

Thomas F. Gallagher, P. Pillet, Michael Robinson, B. Laburthe-Tolra, Michael W. Noel

Published May 1, 2003• 51 citations
Publisher

Abstract

By photoionizing cold, trapped atoms it is possible to produce ultracold plasmas with temperatures in the vicinity of 1 K, roughly 4 orders of magnitude colder than conventional cold plasmas. After the first photoelectrons leave, the resulting positive charge traps the remaining electrons in the plasma. Monitoring the dynamics of the expansion of these plasmas shows explicitly the flow of energy from electrons to the ionic motion, which is manifested as the expansion of the plasma. The electron energy can either be their initial energy from photoionization or can come from the energy redistribution inherent in recombination and superelastic scattering from recombined Rydberg atoms. If the cold atoms are excited to Rydberg states instead of being photoionized, the resulting cold Rydberg gas quickly evolves into an ultracold plasma. After a few percent of the atoms are ionized by collisions or blackbody radiation, electrons are trapped by the resulting positive charge, and they quickly lead to ionization of the Rydberg atoms, forming a plasma. While the source of this energy is not clear, a likely candidate is superelastic scattering, also thought to be important for the expansion of deliberately made plasmas.

DOI
10.1364/josab.20.001091

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•Jan 1, 2004•1 citation
By photoionizing cold, trapped atoms it is possible to produce ultracold plasmas with temperatures in the vicinity of 1 K, roughly 4 orders of magnitude colder than conventional cold plasmas. After the first photoelectrons leave, the resulting positive charge traps the remaining electrons in the plasma. Monitoring the dynamics of the expansion of these plasmas shows explicitly the flow of energy from electrons to the ionic motion, which is manifested as the expansion of the plasma. The electron energy can either be their initial energy from photoionization or can come from the energy redistribution inherent in recombination and superelastic scattering from recombined Rydberg atoms. If the cold atoms are excited to Rydberg states instead of being photoionized, the resulting cold Rydberg gas quickly evolves into an ultracold plasma. After a few percent of the atoms are ionized by collisions or blackbody radiation, electrons are trapped by the resulting positive charge, and they quickly lead to ionization of the Rydberg atoms, forming a plasma. While the source of this energy is not clear, a likely candidate is superelastic scattering, also thought to be important for the expansion of deliberately made plasmas.
We have observed the spontaneous evolution of a dense sample of Rydberg atoms into an ultracold plasma, in spite of the fact that each of the atoms may initially be bound by up to 100 cm(-1). When the atoms are initially bound by 70 cm(-1), this evolution occurs when most of the atoms are translationally cold, <1 mK, but a small fraction, approximately 1%, is at room temperature. Ionizing collisions between hot and cold Rydberg atoms and blackbody photoionization produce an essentially stationary cloud of cold ions, which traps electrons produced later. The trapped electrons rapidly collisionally ionize the remaining cold Rydberg atoms to form a cold plasma.
Dense samples of cold Rydberg atoms have previously been observed to spontaneously evolve to a plasma, despite the fact that each atom may be bound by as much as $100\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. Initially, ionization is caused by blackbody photoionization and Rydberg-Rydberg collisions. After the first electrons leave the interaction region, the net positive charge traps subsequent electrons. As a result, rapid ionization starts to occur after $1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$ caused by electron-Rydberg collisions. The resulting cold plasma expands slowly and persists for tens of microseconds. While the initial report on this process identified the key issues described above, it failed to resolve one key aspect of the evolution process. Specifically, redistribution of population to Rydberg states other than the one initially populated was not observed, a necessary mechanism to maintain the energy balance in the system. Here we report new and expanded observations showing such redistribution and confirming theoretical predictions concerning the evolution to a plasma. These measurements also indicate that, for high nnn states of purely cold Rydberg samples, the initial ionization process which leads to electron trapping is one involving the interactions between Rydberg atoms.
By photoionizing a sample of laser-cooled xenon atoms, we create ultracold neutral plasmas with initial temperatures of 1–1000 K and densities as high as 1010 cm−3. The plasma is formed by the trapping of electrons by the residual positive charge that is left after some electrons initially leave the sample. We excite plasma oscillations with applied radio frequency fields and use this to monitor the expansion of the unconfined plasma. We have observed significant recombination of the plasma into Rydberg atoms (up to 20%). At these low temperatures, the only traditional form of recombination that could be significant is three-body recombination (TBR), but we see a number of trends in direct contradiction to what one would expect from classical TBR theory.
By photoionizing a sample of laser‐cooled xenon atoms, we create ultracold neutral plasmas with initial temperatures of 1–1000 K and densities as high as 1010 cm−3. The plasma is formed by the trapping of electrons by the residual positive charge that is left after some electrons initially leave the sample. We excite plasma oscillations with applied radio frequency fields and use this to monitor the expansion of the unconfined plasma. We have observed significant recombination of the plasma into Rydberg atoms (up to 20 %). At these low temperatures, the only traditional form of recombination that could be significant is three‐body recombination (TBR). Recent theoretical studies of ultracold plasmas, and prospects for future experiments, are discussed.
We have observed the evolution of cold Rydberg gases and ultracold plasmas in an environment that allows for the control of interactions of the blackbody radiation temperature from 4K to 300K. The work in this thesis describes the influence of initial densities, populations, excitation state, evolution time and thermal background temperature on the evolution of cold Rydberg gases as well as directly excited ultracold plasmas. With some initial ionization, cold-Rydberg-atom gases easily develop a positive space charge that traps electrons for times on the order of tens of mus. This acts as a catalyst for electron-Rydberg atom collisions, increasing the collision probabilities by orders of magnitude and profoundly altering the final state of the Rydberg gas. Using state-selective field ionization, we have provided direct proof that these collisions cause changes in l-states, and we have seen the first experimental evidence that they also lead to inelastic n-changing collisions, as predicted by theory. We have also found evidence for Rydberg-Rydberg collisions, enabled by the attractive electric-dipole forces between atoms in high-l states and possibly by van-der-Waal's interactions at very high-n . Thermal interactions are seen to influence the formation of a plasma from a cold Rydberg gas. Plasmas form more easily in the presence of a 300K radiation field than at 4K, due to the contribution of thermal ionization to the plasma formation. The background temperature also appears to affect the electron temperature in both the directly excited plasma and the plasma spontaneously formed from a cold Rydberg gas. In Rydberg atom gases that have undergone partial ionization, electron temperatures were estimated using the dependence of n-changing collisions on the electron temperature.