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Journal of Physics B Atomic Molecular and Optical Physics

Evolution from Rydberg gas to ultracold plasma in a supersonic atomic beam of Xe

Jachin Hung, Hossein Sadeghi, Markus Schulz-Weiling, Edward R. Grant

Published Jul 28, 2014• 13 citations
Publisher

Abstract

A Rydberg gas of xenon, entrained in a supersonic atomic beam, evolves slowly to form an ultracold plasma. In the early stages of this evolution, when the free-electron density is low, Rydberg atoms undergo long-range -mixing collisions, yielding states of high orbital angular momentum. The development of high- states promotes dipole–dipole interactions that help to drive Penning ionization. The electron density increases until it reaches the threshold for avalanche. Ninety μs after the production of a Rydberg gas with the initial state, , a 432 V cm−1 electrostatic pulse fails to separate charge in the excited volume, an effect which is ascribed to screening by free electrons. Photoexcitation cross sections, observed rates of -mixing, and a coupled-rate-equation model simulating the onset of the electron-impact avalanche point consistently to an initial Rydberg gas density of .

DOI
10.1088/0953-4075/47/15/155301

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The evolution from Rydberg gas to plasma in an atomic beam of Xe : with comparative simulations to a strongly blockaded Rydberg gas of Rb

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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.

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Abstract Not long after metastable xenon was photoionized in a magneto-optical trap, groups in Europe and North America found that similar states of ionized gas evolved spontaneously from state-selected, high principal quantum number Rydberg gases. Studies of atomic xenon and molecular nitric oxide entrained in a supersonically cooled molecular beam subsequently showed much the same final state evolved from a sequence of prompt Penning ionization and electron-impact avalanche to plasma, well-described by coupled rate-equation simulations. But, measured over longer times, the molecular ultracold plasma was found to exhibit an anomalous combination of very long lifetime and very low apparent electron temperature. This review first summarizes early developments in the general study of ultracold plasmas formed by atomic and molecular Rydberg gases, and then turns to describe the particular properties of the nitric oxide molecular ultracold plasma that appear to call for an explanation beyond the realm of conventional plasma physics.

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Abstract Double-resonant transitions excite nitric oxide in a seeded supersonic molecular beam, yielding a state-selected Rydberg gas that evolves to form an ultracold plasma. This plasma propagates in z with the molecular beam over a variable distance as great as 600 mm to strike an imaging detector, which records the charge distribution in the dimensions, x and y . The laser-crossed molecular beam excitation geometry convolutes an axial Gaussian distribution of NO about z with the Gaussian intensity distribution of the laser beam about x to create an ellipsoidal volume of Rydberg gas. Plasma images provide evidence for the relaxation of this Rydberg gas volume in an electron impact avalanche that breaks the ellipsoidal symmetry in x to form repelling plasma volumes. We find that the energy deposited in the recoil velocity of mass transport, V x depends systematically on the initially selected Rydberg gas principal quantum number, n 0 , and the initial density of the Rydberg gas, ρ 0 . These quantities combine to determine ρ e , the initial density of electrons formed by the prompt Penning ionization of closely spaced pairs of Rydberg molecules. Above a threshold density of Penning electrons, we find that V x depends linearly on ρ e . We argue that this bifurcation occurs as a consequence of the initial geometry of the Rydberg gas. Ambipolar electron expansion accelerates initially formed core ions. Resonant charge transfer redistributes this ion energy to the column of Rydberg molecules on the long axis of the ellipsoid. The equalized velocities in each direction give rise to a ± x streaming motion that concentrates density in opposing plasma volumes, causing the symmetric gas volume to split like a rotating liquid drop. Significantly, these dynamics reduce electron temperature with little decrease in the ion density or increase in the ion temperature. This appears to facilitate the formation of a strongly coupled plasma.

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We report the sudden and spontaneous evolution of an initially correlated gas of repulsively interacting Rydberg atoms to an ultracold plasma. Under continuous laser coupling we create a Rydberg ensemble in the strong blockade regime, which at longer times undergoes an ionization avalanche. By combining optical imaging and ion detection, we access the full information on the dynamical evolution of the system, including the rapid increase in the number of ions and a sudden depletion of the Rydberg and ground state densities. Rydberg-Rydberg interactions are observed to strongly affect the dynamics of plasma formation. Using a coupled rate-equation model to describe our data, we extract the average energy of electrons trapped in the plasma, and an effective cross section for ionizing collisions between Rydberg atoms and atoms in low-lying states. Our results suggest that the initial correlations of the Rydberg ensemble should persist through the avalanche. This would provide the means to overcome disorder-induced heating, and offer a route to enter new strongly coupled regimes.

arXiv:1209.4728•Physical Review Letters•Jan 24, 2013•82 citations
A Rydberg gas of xenon, entrained in a supersonic atomic beam, evolves slowly to form an ultracold plasma. In the early stages of this evolution, when the free-electron density is low, Rydberg atoms undergo long-range -mixing collisions, yielding states of high orbital angular momentum. The development of high- states promotes dipole–dipole interactions that help to drive Penning ionization. The electron density increases until it reaches the threshold for avalanche. Ninety μs after the production of a Rydberg gas with the initial state, , a 432 V cm−1 electrostatic pulse fails to separate charge in the excited volume, an effect which is ascribed to screening by free electrons. Photoexcitation cross sections, observed rates of -mixing, and a coupled-rate-equation model simulating the onset of the electron-impact avalanche point consistently to an initial Rydberg gas density of .
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.
Abstract Not long after metastable xenon was photoionized in a magneto-optical trap, groups in Europe and North America found that similar states of ionized gas evolved spontaneously from state-selected, high principal quantum number Rydberg gases. Studies of atomic xenon and molecular nitric oxide entrained in a supersonically cooled molecular beam subsequently showed much the same final state evolved from a sequence of prompt Penning ionization and electron-impact avalanche to plasma, well-described by coupled rate-equation simulations. But, measured over longer times, the molecular ultracold plasma was found to exhibit an anomalous combination of very long lifetime and very low apparent electron temperature. This review first summarizes early developments in the general study of ultracold plasmas formed by atomic and molecular Rydberg gases, and then turns to describe the particular properties of the nitric oxide molecular ultracold plasma that appear to call for an explanation beyond the realm of conventional plasma physics.
Double-resonant photoexcitation of nitric oxide in a molecular beam creates a dense ensemble of 50f(2) Rydberg states, which evolves to form a plasma of free electrons trapped in the potential well of an NO + space charge. The plasma travels at the velocity of the molecular beam and, on passing through a grounded grid, yields an electron time-of-flight signal that gauges the plasma size and quantity of trapped electrons. This plasma expands at a rate that fits with an electron temperature as low as 5 K, colder than typically observed for atomic ultracold plasmas. The recombination of molecular NO + cations with electrons forms neutral molecules excited by more than twice the energy of the NO chemical bond, and the question arises whether neutral fragmentation plays a role in shaping the redistribution of energy and particle density that directs the short-time evolution from Rydberg gas to plasma. To explore this question, we adapt a coupled rate-equations model established for atomic ultracold plasmas to describe the energy-grained avalanche of electron–Rydberg and electron–ion collisions in our system. Adding channels of Rydberg predissociation and two-body, electron–cation dissociative recombination to the atomic formalism, we investigate the kinetics by which this relaxation distributes particle density and energy over Rydberg states, free electrons and neutral fragments. The results of this investigation point to conditions under which such processes can effect the steady-state temperature of plasma electrons.
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.
Abstract Double-resonant transitions excite nitric oxide in a seeded supersonic molecular beam, yielding a state-selected Rydberg gas that evolves to form an ultracold plasma. This plasma propagates in z with the molecular beam over a variable distance as great as 600 mm to strike an imaging detector, which records the charge distribution in the dimensions, x and y . The laser-crossed molecular beam excitation geometry convolutes an axial Gaussian distribution of NO about z with the Gaussian intensity distribution of the laser beam about x to create an ellipsoidal volume of Rydberg gas. Plasma images provide evidence for the relaxation of this Rydberg gas volume in an electron impact avalanche that breaks the ellipsoidal symmetry in x to form repelling plasma volumes. We find that the energy deposited in the recoil velocity of mass transport, V x depends systematically on the initially selected Rydberg gas principal quantum number, n 0 , and the initial density of the Rydberg gas, ρ 0 . These quantities combine to determine ρ e , the initial density of electrons formed by the prompt Penning ionization of closely spaced pairs of Rydberg molecules. Above a threshold density of Penning electrons, we find that V x depends linearly on ρ e . We argue that this bifurcation occurs as a consequence of the initial geometry of the Rydberg gas. Ambipolar electron expansion accelerates initially formed core ions. Resonant charge transfer redistributes this ion energy to the column of Rydberg molecules on the long axis of the ellipsoid. The equalized velocities in each direction give rise to a ± x streaming motion that concentrates density in opposing plasma volumes, causing the symmetric gas volume to split like a rotating liquid drop. Significantly, these dynamics reduce electron temperature with little decrease in the ion density or increase in the ion temperature. This appears to facilitate the formation of a strongly coupled plasma.
We report the sudden and spontaneous evolution of an initially correlated gas of repulsively interacting Rydberg atoms to an ultracold plasma. Under continuous laser coupling we create a Rydberg ensemble in the strong blockade regime, which at longer times undergoes an ionization avalanche. By combining optical imaging and ion detection, we access the full information on the dynamical evolution of the system, including the rapid increase in the number of ions and a sudden depletion of the Rydberg and ground state densities. Rydberg-Rydberg interactions are observed to strongly affect the dynamics of plasma formation. Using a coupled rate-equation model to describe our data, we extract the average energy of electrons trapped in the plasma, and an effective cross section for ionizing collisions between Rydberg atoms and atoms in low-lying states. Our results suggest that the initial correlations of the Rydberg ensemble should persist through the avalanche. This would provide the means to overcome disorder-induced heating, and offer a route to enter new strongly coupled regimes.