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Journal of Physics Conference Series

Features of triple recombination in ultracold plasma

V. S. Vorob’ev

Published Nov 1, 2017• 0 citations
Publisher

Abstract

We show that in the ultracold plasma, when the coupling parameter characterizes the interaction of electrons and ions exceeds unity, the collisional threebody (e – e – ion) recombination coefficient (TBR) becomes inversely proportional to the plasma frequency, whereas for the small values of this parameter it ∼ T −9/2 (T is the temperature). We compare the results obtained with different theoretical models and numerical simulation data of the recombination process.

DOI
10.1088/1742-6596/927/1/012074

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We show that in the ultracold plasma, when the coupling parameter characterizes the interaction of electrons and ions exceeds unity, the collisional threebody (e – e – ion) recombination coefficient (TBR) becomes inversely proportional to the plasma frequency, whereas for the small values of this parameter it ∼ T −9/2 (T is the temperature). We compare the results obtained with different theoretical models and numerical simulation data of the recombination process.
Ultracold neutral plasmas, created by photoionizing samples of laser-cooled atoms, have well-controlled initial density and temperature parameters. With initial particle peak densities of ~10^15 m^-3, initial ion temperatures in the tens of micro-Kelvin range, and initial electron temperatures with a controllable range of 1-1000 K, these systems provide a means to study otherwise laboratory-inaccessible parameter ranges for plasma research. Furthermore, these plasmas are inhomogeneous, unconfined, and freely expanding into a vacuum. Despite the extraordinarily low electron temperatures, the electron system remains weakly coupled, although the ion system exhibits strong coupling behavior. While the initial electron temperatures are very low in ultracold plasmas, the temperature evolution has only been measured indirectly, in the earliest ~5% of the plasma lifetime, and often with large uncertainties. We present a technique that, with further theoretical support, can provide straightforward temperature measurements throughout the first fifth of the plasma lifetime. By making use of collective modes of the plasma, we fit a model of Tonks-Dattner resonances (electron sound wave propagating in the plasma) to measurements of these resonances and obtain a time-dependent electron temperature measurement for the ultracold plasma. Three-body recombination, a plasma loss process that has a rate scaling with the -9/2 power of the electron temperature, is of obvious interest in these ultracold plasma systems. Several theoretical works have predicted that the three-body recombination rate expression would need to be modified at these low electron temperatures, although the validity of these changes often hinges on the electron system being strongly coupled. We have performed the lowest temperature measurements of three-body recombination rates in a plasma and show that these measurements potentially provide a low-uncertainty means to calculate electron temperatures.
In recent experiments ultracold plasmas were produced by photoionizing small clouds of laser cooled atoms. This paper presents the results of molecular dynamic simulations for the early time evolution of such plasmas. Contrary to earlier speculation, no evidence of strong electron–electron correlations is observed in the simulations even if the initial value of the coupling parameter (Γe=e2/akTe) is much larger than unity. As electron–electron correlations begin to develop, the correlation energy is released to heat the electrons, raising the electron temperature to the point where Γe∼1 and limiting further development of correlation. Further heating of the electrons occurs as a by-product of three-body recombination. When a model of laser cooling is added to the simulation, the formation of strong ion–ion correlation is observed. Contrary to earlier suggestion, the rate of three-body recombination is observed to be in reasonable agreement with the traditional formula, R=3.9×10−9 s−1[n(cm−3)]2[Te(K)]−9/2, but care must be taken to use the correct temporally evolving temperature, Te. The simulations are challenging because it is necessary to follow three-body recombination into weakly bound (high n quasiclassical) Rydberg states, and the time scale for such states is short compared to that for the plasma dynamics. This kind of problem was faced earlier in computational astrophysics when studying binary star formation in globular clusters and the simulation method used here is adapted from such studies.
We report the results of simulations that explain many properties of ultracold neutral plasmas. We find that three-body recombination is important at very low temperatures since it is a heating mechanism for the electron gas and it preferentially removes the slow ions from the plasma. We also find that collisions between cold electrons and Rydberg atoms are an important source of electron heating and deexcitation of atoms formed in the plasma. Simulations show that the Coulomb coupling constant does not become larger than similar1/5 for the reported experiments.
From the current ultracold plasma research, it is clear that the collective effects on this plasma have not been researched sufficiently. There is some recombination of electrons and positive ions into Rydberg states. In three body recombination, two electrons and a positive ion recombine to create a Rydberg atom and an electron, but this electron is a hotter electron. This electron causes the plasma to beat, and then the plasma coupling parameter would decrease away from one like in the conventional plasma. So then the question becomes whether or not we can use Rydberg atoms to coo] the plasmas. This would be done through using a second excitation at a different frequency.
Three-body recombination, an important collisional process in plasmas, increases dramatically at low electron temperatures, with an accepted scaling of Te(-9/2). We measure three-body recombination in an ultracold neutral xenon plasma by detecting recombination-created Rydberg atoms using a microwave-ionization technique. With the accepted theory (expected to be applicable for weakly coupled plasmas) and our measured rates, we extract the plasma temperatures, which are in reasonable agreement with previous measurements early in the plasma lifetime. The resulting electron temperatures indicate that the plasma continues to cool to temperatures below 1 K.
Signals of ultracold plasma are observed by two-photon ionization of laser-cooled caesium atoms in a magneto-optical trap. Recombination of ions and electrons into Rydberg atoms during the expansion of ultracold plasma is investigated by using state-selective field ionization spectroscopy. The dependences of recombination on initial electron temperature (1–70 K) and initial ion density (∼10 10 cm −3 ) are investigated. The measured dependence on initial ion density is N 1.547+0.004 at a delay time of 5 μs. The recombination rate rapidly declines as initial electron temperature increases when delay time is increased. The distributions of Rydberg atoms on different values of principal quantum number n, i.e. n = 30–60, at an initial electron temperature of 3.3 K are also investigated. The main experimental results are approximately explained by the three-body recombination theory.