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Physical Review A

Ionization of Rb Rydberg atoms in the attractive nsnp dipole-dipole potential

Hyunwook Park, E. S. Shuman, Thomas F. Gallagher

Published Nov 9, 2011• 30 citations
Publisher

Abstract

We have observed the ionization of a cold gas of Rb Rydberg atoms which occurs when $nsns$ van der Waals pairs of $ns$ atoms of $n\ensuremath{\approx}$ 40 on a weakly repulsive potential are transferred to an attractive dipole-dipole $nsnp$ 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 $nsns$ 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 $nsns$ state to the $nsnp$ state over a broad range of internuclear spacings, it is possible to accelerate markedly the evolution to a plasma.

DOI
10.1103/physreva.84.052708

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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≈n\ensuremath{\approx}n\ensuremath≈ 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-\ensuremathμ\ensuremath{\mu}\ensuremathμK 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-\ensuremathμ\ensuremath{\mu}\ensuremathμs 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.
We have observed the process $\mathrm{Rb}ns+\mathrm{Rb}\phantom{\rule{4pt}{0ex}}ns+2h\ensuremath{\nu}\ensuremath{\rightarrow}\mathrm{Rb}\phantom{\rule{4pt}{0ex}}n{p}_{1/2}+\mathrm{Rb}\phantom{\rule{4pt}{0ex}}n{p}_{3/2}$ from n=34n=34n=34 to n=40n=40n=40 in a frozen gas of Rb Rydberg atoms. It is resonant when the microwave frequency is halfway between the $ns\phantom{\rule{0.16em}{0ex}}\ensuremath{\rightarrow}\phantom{\rule{0.16em}{0ex}}n{p}_{1/2}$ and $ns\ensuremath{\rightarrow}n{p}_{3/2}$ frequencies, which range from 57 to 106 GHz. The process cannot occur in isolated atoms, nor can it occur if the magnetic quantum numbers are unchanged, an implicit assumption of one-dimensional models. A Floquet-Forster model shows that the coupling between the initial and final states involves the absorption of two microwave photons and the dipole-dipole interaction, which leads to a coupling proportional to the product of the density, the microwave field squared, and n∗14{n}^{*14}n∗14, where n∗{n}^{*}n∗ is the effective quantum number of the np3/2n{p}_{3/2}np3/2​ state. We have experimentally verified these dependences. The observed resonances are asymmetric, with a low-frequency tail, which we attribute to the van der Waals shift of the final np1/2np3/2n{p}_{1/2}n{p}_{3/2}np1/2​np3/2​ state due to its dipole-dipole interaction with the nearby ns(n+1)sns(n+1)sns(n+1)s state. While the van der Waals shift is negligible for most of the atoms in the Rydberg gas, it is not for the pairs of close atoms which undergo this transition.
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.
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.
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 investigate the initial ionization process of Rydberg cesium atoms prepared in repulsive van der Waals interaction potential. The time-of-flight spectrum is used to detect the free ions and ionized Rydberg atoms. Free ions are observed immediately after the production of Rydberg atoms and increase significantly in number on the blue detuned side of the resonance of 6P 3/2 →40D 5/2 with increasing delay time between the excitation laser pulse and the ramp ionization field. The initial ionization is explained as the excitation of pair states in attractive potential due to the ac Stark shifts induced by the excitation laser. The attractive dipole–dipole interaction results in fast collision process and penning ionization. We also measure free-ion fraction as a function of delay time. A rapid increase in the number of free ions is observed after a delay time of 1.8 µs. The experimental observations are in accordance with qualitative explanation of the initial ionization mechanism.