Theses and Dissertations from UMD

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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM

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Author: search.filters.author.Fletcher, Robert SSubject: search.filters.subject.Physics, Atomic

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    Three-Body Recombination and Rydberg Atoms in Ultracold Plasmas
    (2008-04-18) Fletcher, Robert S; Rolston, Steven; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    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.