Bulletin of the American Physical Society
39th Annual Meeting of the APS Division of Atomic, Molecular, and Optical Physics
Volume 53, Number 7
Tuesday–Saturday, May 27–31, 2008; State College, Pennsylvania
Session B3: Current Topics in Gaseous Electronics (Co-Sponsored by GEC) |
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Chair: Rainer Johnson, University of Pittsburgh Room: Keller Building 104 |
Wednesday, May 28, 2008 11:00AM - 11:36AM |
B3.00001: Stark Spectroscopy: Measuring Electric Fields in Plasmas Invited Speaker: The electric field is one of the most important parameters in discharge plasmas. Many applications that use discharges depend on behavior at the boundary, and the electric field is closely connected with other discharge parameters, such as charge densities, fluxes of electrons and ions, and energy distribution functions. It is therefore necessary to understand the spatial distribution of electric field in the discharge, but this is difficult to measure. With this background, there has been considerable effort in recent years to develop a suite of spectroscopic methods capable of providing quantitative measurements of electric fields in glow discharges. The main techniques are laser-based active methods that seek to measure Stark effects in atoms or molecules in the discharge. Techniques have been developed for glow discharges based on the spectroscopy of atomic hydrogen and each of the noble gas atoms, using shifts and /or splitting of energy levels due to Stark effects. This paper will contain a brief overview of work in this area, and then explain in detail the development of a method based on spectroscopy of xenon atoms. The theory and experiments used to calibrate the method will be explained and the application of the technique to measure electric fields during plasma ignition will be presented. [Preview Abstract] |
Wednesday, May 28, 2008 11:36AM - 12:12PM |
B3.00002: Electron-Impact Excitation of Complex Atoms: Impact on Gaseous Electronics Applications Invited Speaker: In recent years there have been a number of experimental and theoretical investigations of electron-impact excitation (EIE) cross sections with improved measurement techniques and state-of-the-art calculations. These investigations not only reveal many interesting aspects of basic physics of electron collision with neutral atoms, but they are extremely important for many gaseous electronics applications such as lighting, plasma processing, and gas lasers. We will present the current status of many theoretical calculations of EIE cross sections using distorted-wave (DW) and close-coupling R-matrix and B-spline R-matrix methods for several noble gases such as Ar, Kr, and Xe in particular and compare them with recent improved measurements by the University of Wisconsin group using a Fourier-transform spectrometer. Although in some cases there is qualitative agreement between theoretical and experimental results, even with the advancements of both theoretical and experimental techniques and improved data, quantitative discrepancies still exist in most cases. We will present some of the issues regarding the disagreement and also discuss the significant effects of EIE data for various applications. In particular, results from this study have been used to model an electron beam pumped Ar-Xe laser at NRL. We will also comment on the complexity involved in calculating the structure and EIE cross sections of a complex open-shell element such as Mo and its implications for modeling a molybdenum discharge lamp. [Preview Abstract] |
Wednesday, May 28, 2008 12:12PM - 12:48PM |
B3.00003: A Puzzle in the Earth's Mesosphere: Runaway Electrons Invited Speaker: Gammay-ray flashes (GRF) of terrestrial origin with energy spectrum extending to 20 MeV and lasting in the order of a millisecond were detected in 2005 by the Reuven Ramaty High Energy Solar Spectroscopic Imager. These observations reveal the presence of relativistic electrons in the earth's mesosphere with 10$^{15}$ electrons/GRF and electron energies up to 40 MeV. Possible collisional avalanche mechanisms for generating such large pulses of relativistic electrons must overcome two dampening factors: 1) the peak ambient electric fields in the mesosphere, in the order of 5 V/cm during thunderstorms, are too low for electrons to overcome collisional losses and accelerate to these high energies, and 2) the secondary energy spectrum in ionizing collisions decreases with the square of the energy of the secondary so that collisional avalanches of high energy electrons tend to grow at lower energies. This talk examines the origin of these bursts in runaway electrons, electrons that on average are not in dynamical-equilibrium with the background gas and progressively move towards higher energies. [Preview Abstract] |
Wednesday, May 28, 2008 12:48PM - 1:24PM |
B3.00004: Bound States and Feshbach Resonances in Positron-Molecule Annihilation Invited Speaker: Considerable progress has been made recently in studying positron-molecule interactions using trap-based beams. Measurements of energy-resolved positron- on-molecule annihilation in alkane molecules have solved a four-decade mystery regarding the origin of very large positron-molecule annihilation rates [1-3]. Studies indicate that positrons can become temporarily bound to a variety of molecules via vibrational Feshbach resonances (VFR). These VFR manifest themselves as peaks of varying intensity in the annihilation spectra, with energies shifted below those of the vibrational modes by a molecule-dependant binding energy $\epsilon_b$. It is now possible to make quantitative predictions for the annihilation spectra in small, infrared-active molecules [2]. Furthermore, using this formalism, it is possible to discern broad empirical trends in the spectra of larger molecules. In particular, after normalizing out a weak dependence on $\epsilon_b$, resonant annihilation rates in nearly all hydrocarbons studied to date obey a `universal' scaling with the number of vibrational degrees of freedom [3]. The origin and significance of this scaling will be discussed. The only known outliers to this scaling, partially fluorinated alkanes, are shown to exhibit an additional inelastic channel, which diminishes the magnitudes of resonances above an energy threshold. Outstanding questions will be discussed, including understanding the chemical trends in positron-molecule binding energies and the role of intramolecular vibrational energy redistribution (IVR) in determining the magnitudes of the annihilation VFR peaks. \\ ~[1] L. D. Barnes, J. A. Young, and C. M. Surko, \textit{Phys. Rev. A} {\bf 74}, 012706 (2006). \\ ~[2] G. F. Gribakin and C. M. R. Lee, \textit{Phys. Rev. Lett.} {\bf 97}, 193201 (2006). \\ ~[3] J. A. Young and C. M. Surko, \textit{Phys. Rev. Lett.} {\bf 99}, 133201 (2007). [Preview Abstract] |
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