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Journal of Physics Conference Series

Ion-collisional frequency measurements through optical pumping in Ultracold Neutral Plasmas

Jose Castro, T. C. Killian

Published Nov 1, 2009• 0 citations
Publisher

Abstract

Ion-collisional frequency measurements were performed in Strontium Ultracold Neutral Plasmas (UNP) through optical pumping between the two ground level spin states. Sr ground state ions can be optically pumped into a spin dark state through the use of appropriate circularly-polarized light. This optical pumping can be strongly affected by collisions between ions. Fluorescence measurements on UNPs show that velocity changing collisions between ions slow down the pumping rate into the dark ground level spin state.

DOI
10.1088/1742-6596/194/12/122002

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Ion-collisional frequency measurements were performed in Strontium Ultracold Neutral Plasmas (UNP) through optical pumping between the two ground level spin states. Sr ground state ions can be optically pumped into a spin dark state through the use of appropriate circularly-polarized light. This optical pumping can be strongly affected by collisions between ions. Fluorescence measurements on UNPs show that velocity changing collisions between ions slow down the pumping rate into the dark ground level spin state.
We present techniques to perturb, measure and model the ion velocity distribution in an ultracold neutral plasma produced by photoionization of strontium atoms. By optical pumping with circularly polarized light we promote ions with certain velocities to a different spin ground state, and probe the resulting perturbed velocity distribution through laser-induced fluorescence spectroscopy. We discuss various approaches to extract the velocity distribution from our measured spectra, and assess their quality through comparisons with molecular dynamic simulations.
The progress toward laser cooling an ultracold plasma is presented in this thesis. Ultracold neutral plasmas are created by photo-ionizing laser cooled Strontium atoms. Initially this system is very strongly coupled, however the ions rapidly heat up leaving the plasma on the borderline of the strongly coupled regime. To counteract the effects of this heating an attempt to laser cool the ions in the system is undertaken. However, from these experiments we discovered that velocity changing collisions occurring in the system prevented laser cooling. In order to determine the collisions rate, a collision model was developed to simulate the spectra of the plasma from optical pumping experiments. These experiments and simulations in fact demonstrate that collisions are occurring at a fast enough rate to prevent ion cooling on the time scale of our experiment.
Collisions between ultracold neutral atoms have for some time been the subject of investigation, initially with hydrogen and more recently with laser cooled alkali atoms. Advances in laser cooling and trapping of neutral atoms in a Magneto-Optic Trap (MOT) have made cold atoms available as the starting point for many laser cooled atomic physics investigations. The most spectacularly successful of these, the observation of Bose-Einstein Condensation (BEC) in a dilute ultra-cold spin polarized atomic vapor, has accelerated the study of cold collisions. Experimental and theoretical studies of BEC and the long range interaction between cold alkali atoms is at the boundary of atomic and low temperature physics. Such studies have been difficult and would not have been possible without the development and advancement of laser cooling and trapping of neutral atoms. By contrast, ion-atom interactions at low temperature, also very difficult to study prior to modern day laser cooling, have remained largely unexplored. But now, many laboratories worldwide have almost routine access to cold neutral atoms. The combined technologies of ion trapping, together with laser cooling of neutrals has made these studies experimentally feasible and several very important, novel applications might come out of such investigations . This paper is an investigation of ion-atom interactions in the cold and ultra-cold temperature regime. Some of the collisional ion-atom interactions present at room temperature are very much reduced in the low temperature regime. Reaction rates for charge transfer between unlike atoms, A + B(+) approaches A(+) + B, are expected to fall rapidly with temperature, approximately as T(sup 5/2). Thus, cold mixtures of atoms and ions are expected to coexist for very long times, unlike room temperature mixtures of the same ion-atom combination. Thus, it seems feasible to cool ions via collisions with laser cooled atoms. Many of the conventional collisional interactions, exploited as a useful tool at room temperature and higher, are greatly enhanced at low energy. For example, collisional spin transfer from one species of polarized atoms to another has long been a useful method for polarizing a sample of atoms where no other means was available. Because optical pumping cannot be used to polarize the nuclear spin of Xe-129 or He-3 (for use in nmr imaging of the lungs), the nuclear spins are polarized via collisions with an optically pumped Rb vapor in a cell containing both gases. In another case, a spin polarized thermal Cs beam was used to polarize the hyperfine states of trapped He(+)-3 ions in order to measure their hyperfine clock transition frequency. The absence of an x-ray light source to optically pump the ground state of the He(+)-3 ion necessitated this alternative state preparation. Similarly, Cd(+) and Sr(+) ions were spin-oriented via collisions in a cell with optically pumped Rb vapor. Resonant RF spin changing transitions in the ground state of the ions were detected by changes in the Rb resonance light absorption. Because cold collision spin exchange rates scale with temperature as T(sup -1/2) this technique is expected to be a far more powerful tool than the room temperature counterpart. This factor of 100 or more enhancement in spin exchange reaction rates at low temperatures is the basis for a novel trapped ion clock where laser cooled neutrals will cool, state select and monitor the ion clock transition. The advantage over conventional direct laser cooling of trapped ions is that the very expensive and cumbersome UV laser light sources, required to excite the ionic cooling transition, are effectively replaced by simple diode lasers.
Ultracold neutral plasmas are formed by photoionizing laser‐cooled atoms near the ionization threshold. Through the application of atomic physics techniques and diagnostics, these experiments stretch the boundaries of traditional neutral plasma physics. The electron temperature in these plasmas ranges from 1–1000 K and the ion temperature is around 1 K. The density can be as high as 1010 cm−3. Fundamental interest stems from the possibility of creating strongly‐coupled plasmas, but recombination, collective modes, and thermalization in these systems have also been studied. Optical absorption images of a strontium plasma, using the Sr+ 2S1/2 → 2P1/2 transition at 422 nm, depict the density profile of the plasma, and probe kinetics on a 50 ns time‐scale. The Doppler‐broadened ion absorption spectrum measures the ion velocity distribution, which gives an accurate measure of the ion dynamics in the first microsecond after photoionization.