Bulletin of the American Physical Society
42nd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 56, Number 5
Monday–Friday, June 13–17, 2011; Atlanta, Georgia
Session H6: Advances in Gaseous Electronics |
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Chair: Michael Brunger, Flinders University Room: A706 |
Wednesday, June 15, 2011 8:00AM - 8:30AM |
H6.00001: Why isn't the atmosphere completely ionized? Invited Speaker: We have carried out experiments on electron attachment and positive ion / negative ion neutralization at thermal energies using the VENDAMS method (variable electron and neutral density attachment mass spectrometry, which is an extension of the flowing-afterglow Langmuir-probe technique). The VENDAMS method allows us to determine rate coefficients for ion-ion mutual neutralization along with neutral product branching fractions. The method is limited at present to noble gas positive ions. A major advantage of the method is that electron attachment to labile molecules such as POCl$_{2}$ or C$_{2}$F$_{5}$ may be studied, if these molecules are present in the apparatus as products of the primary neutralization reaction. Measurements have been made on electron attachment to SF$_{2}$, SF$_{3}$, SF$_{4}$, SF$_{5}$, and SF$_{6}$, for example, along with rate coefficients and neutral product distributions for SF$_{5}^{-}$ and SF$_{6}^{-}$ neutralization by Ar$^{+}$[1] and for SF$_{4}^ {-}$, SF$_{5}^{-}$, and SF$_{6}^{-}$ neutralization by Ar$^{+}$ and Kr$^{+}$ at 300 K. These and other electron attachment and ion-ion neutralization results, measured over a temperature range 300-600 K, will be presented. In addition, we have identified a new plasma process in which electrons act as a third body to remove energy from an orbiting ion-ion complex, which enhances the neutralization rate coefficient. Details of this process, electron-catalyzed mutual neutralization, have been recently published,[2] and new results will be presented for various species.\\[4pt] Collaborators in this work: Nicholas S. Shuman, Albert A. Viggiano, Jeffrey F. Friedman, Connor M. Caples, Raymond J. Bemish, and J\"{u}rgen Troe.\\[4pt] [1] N. S. Shuman, T. M. Miller, and A. A. Viggiano, J. Chem. Phys. \textbf{133}, 234304 (2010).\\[0pt] [2] N. S. Shuman, T. M. Miller, R. J. Bemish, and A. A. Viggiano, Phys. Rev. Lett. \textbf{106}, 018302 (2011). [Preview Abstract] |
Wednesday, June 15, 2011 8:30AM - 9:00AM |
H6.00002: Calculations of fast ion collisions with multi-center molecular targets Invited Speaker: The theoretical treatment of ion-molecule collisions is challenging for several reasons: the systems have many degrees of freedom, a rather complex geometry, and the electron dynamics might be nonperturbative and involve electron-electron interaction effects. However, the interest in accurate calculations has been growing recently. An important reason for this development is the relevance of ion-molecule collisions for a number of fields, such as atmospheric science, and the understanding of radiation damage of biological tissue. We have developed a new approach to meet these challenges [1]. It disregards rovibrational motion, but it does address the multi-center geometry of the system and the generally nonperturbative nature of the electron dynamics. The key ingredients are an expansion of the initially populated molecular orbitals in terms of a single-center basis and a spectral representation of the molecular Hamiltonian. This facilitates a separation of molecular geometry and collision dynamics and makes it possible to use well-established ion-atom methods with relatively minor modifications. We have extended our basis generator method to deal with the collision dynamics and report on results for ionization and fragmentation of water molecules by proton and He$^+$ ion impact over wide ranges of collision energies. For the case of He$^+$ impact this will include a discussion of effects due to the presence of the projectile electron. \\[4pt] [1] H. J. L\"udde {\it et al.}, Phys. Rev. A {\bf 80}, 060702(R) (2009) [Preview Abstract] |
Wednesday, June 15, 2011 9:00AM - 9:30AM |
H6.00003: Optical Diagnostics of Electron Energy Distributions in Low Temperature Plasmas Invited Speaker: Passive, non-invasive optical emission measurements provide a means of probing important plasma parameters without introducing contaminants into plasma systems. We investigate the electron energy distribution function (EEDF) in argon containing inductively-coupled plasmas due to dominant role in rates of gas-phase reactions for processing plasmas. EEDFs are determined using measurements of $3p^54p\rightarrow 3p^54s$ emissions in the 650-1150 nm wavelength range and measured metastable and resonant level concentrations, in conjunction with a radiation model that includes contributions from often neglected but critical processes such as radiation trapping and electron-impact excitation from metastable and resonant levels. Measurements over a wide range of operating conditions (pressure, RF power, Ar/Ne/N$_2$ gas mixtures) show a depletion of the EEDF relative to the Maxwell- Boltzmann form at higher electron energy, in good agreement with measurements made with Langmuir probes and predictions of a global discharge model. This result is consistent with predictions of electron kinetics and can be explained in terms of reduced life times for energetic electrons due to wall losses and inelastic collisions. This example highlights the potential utility of this method as a tool for probing kinetics of many types of low-temperature plasma systems, which are typically characterized by non-Maxwellian EEDFs. [Preview Abstract] |
Wednesday, June 15, 2011 9:30AM - 10:00AM |
H6.00004: Gaseous Positronics -- Positron interactions with atoms and molecules and their applications Invited Speaker: The advent of new technologies for accumulating, trapping and cooling positrons has led to a range of new experimental measurements of low energy positron interactions, and also prompted new, state-of-the-art theoretical advances in describing such interactions. This talk will present some of the recent experimental highlights of our program including the observation of threshold Wigner cusps, a search for quasi-bound positronic complexes or ``resonances,'' and measurements of positron interactions with biologically relevant molecules. The latter are an important precursor to the development of models of positron transport in soft matter and, ultimately, a positron dosimetry for techniques such as Positron Emission Tomography. [Preview Abstract] |
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