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arXiv:0806.3402

Ionization in a frozen Rydberg gas with attractive or repulsive potentials

Matthieu Viteau, Amodsen Chotia, D. Comparat, Duncan Tate, Thomas F. Gallagher, P. Pillet

Published Jun 20, 2008• 1 citation
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Abstract

We report clear evidence of the role of dipole-dipole interaction in Penning ionization of Rydberg atoms, leading to the formation of an ultracold plasma. Penning ionization of np Rydberg Cesium atoms is prevented for states with in n < 42, which correspond to a repulsive potential, but it does not occur for n larger than 42, corresponding to an attractive potential. Blackbody radiation is mostly responsible for the background and initial ionization, although ion-Rydberg collisions and population transfer due to limited superradiance may have to be considered.

DOI
10.48550/arxiv.0806.3402

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We report clear evidence of the role of dipole-dipole interaction in Penning ionization of Rydberg atoms, leading to the formation of an ultracold plasma. Penning ionization of np Rydberg Cesium atoms is prevented for states with in n < 42, which correspond to a repulsive potential, but it does not occur for n larger than 42, corresponding to an attractive potential. Blackbody radiation is mostly responsible for the background and initial ionization, although ion-Rydberg collisions and population transfer due to limited superradiance may have to be considered.
We report clear evidence of the role of the attractive dipole-dipole interaction in Penning ionization of Rydberg atoms, leading to the formation of an ultracold plasma. Penning ionization of npnpnp Rydberg cesium atoms does not occur for states with n<42n<42n<42, which have repulsive potentials, but it does occur for states of nnn larger than 42, which have attractive potentials. Blackbody radiation is mostly responsible for the background and initial ionization, although ion--Rydberg-atom collisions and population transfer due to limited superradiance may have to be considered.
We discuss the control of dipole-dipole interactions in a frozen assembly of Rydberg atoms. We report the evidence of dipole blockade of the Rydberg excitation for two configurations: dipole blockade induced by electric field and dipole blockade in Förster resonance. We demonstrate that two individual atoms separated by ∼ 4 μm can act as a collective dipole if their interaction is strong enough to be in the dipole blockade regime. This observation is crucial for the quantum entanglement of two or more atoms using dipole-dipole interaction. The dipole-dipole interactions between Rydberg atoms are also responsible for Penning ionization leading to the formation of an ultracold plasma. We have demonstrated that Penning ionization of np Rydberg cesium atoms can be prevented by considering repulsive dipole-dipole interactions.
We have observed the ionization of a cold gas of Rb Rydberg atoms which occurs when nsnsnsnsnsns van der Waals pairs of nsnsns atoms of $n\ensuremath{\approx}$ 40 on a weakly repulsive potential are transferred to an attractive dipole-dipole nsnpnsnpnsnp potential by a microwave transition. Comparing the measurements to a simple model shows that the initial 300-$\ensuremath{\mu}$K thermal velocity of the atoms plays an important role. Excitation to a repulsive dipole-dipole potential does not lead to more ionization on a 15-$\ensuremath{\mu}$s time scale than leaving the atoms in the weakly repulsive nsnsnsnsnsns state. This observation is slightly surprising since a radiative transition must occur to allow ionization in the latter case. Finally, by power broadening of the microwave transition, to allow transitions from the initial nsnsnsnsnsns state to the nsnpnsnpnsnp state over a broad range of internuclear spacings, it is possible to accelerate markedly the evolution to a plasma.
In cold dense Rydberg atom samples, the dipole-dipole interaction strength is effectively resonant at the typical interatomic spacing in the sample, and the interaction has a 1/R3 dependence on interatomic spacing R. The dipole-dipole attraction leads to ionizing collisions of initially stationary atoms, which produces hot atoms and ions and initiates the evolution of initially cold samples of neutral Rydberg atoms into plasmas. More generally, the strong dipole-dipole forces lead to motion, which must be considered in proposed applications.
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.
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.