Bulletin of the American Physical Society
63rd Annual Gaseous Electronics Conference and 7th International Conference on Reactive Plasmas
Volume 55, Number 7
Monday–Friday, October 4–8, 2010; Paris, France
Session QR3: Collision Processes in Plasmas |
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Chair: Thomas Miller, Boston College Room: Petit Amphitheatre |
Thursday, October 7, 2010 4:00PM - 4:30PM |
QR3.00001: Collision induced water fragmentation: multiple ionisation and isotopic effects Invited Speaker: |
Thursday, October 7, 2010 4:30PM - 4:45PM |
QR3.00002: Quantum-mechanical capture and ionization calculations for ion-water-molecule collisions Tom Kirchner, Hans Juergen Luedde, Mitsuko Korobkin, Marko Horbatsch We elaborate on a nonperturbative quantum-mechanical approach to ion collisions from molecular targets introduced recently. Its 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. The approach amounts to a separation of molecular geometry and collision dynamics and offers the possibility 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 address ionization and fragmentation of water molecules by ion impact in the few-keV to few-MeV regime. We consider different geometries and study the variation of the results with respect to the orientation of the molecule. In order to compare the results with experimental data and other calculations we have performed a (partial) average of the orientation-dependent cross sections. The results for net electron transfer and ionization in proton-water-molecule collisions are in remarkably good agreement with measurements. Furthermore, fragmentation cross sections will be presented. The results indicate a stronger dependence on the molecular geometry than found in the case of the net ionization and transfer cross sections. [Preview Abstract] |
Thursday, October 7, 2010 4:45PM - 5:15PM |
QR3.00003: Teaching an old dog new tricks: Using the Flowing Afterglow Langmuir Probe apparatus to measure electron attachment to radicals and ion-ion neutralization Invited Speaker: Accurate kinetics of plasma processes are necessary for modeling of the chemistry occurring in the upper atmosphere, reentry, combustion, and discharges. While a great deal of data exists in the literature for many types of plasma processes, there remain gaps for reactions less amenable to traditional measurements due to the difficulty in preparing well-defined initial conditions. In particular ion-ion mutual neutralization reactions have received relatively little study, and essentially no detailed product branching fractions are known. Similarly, while hundreds of electron attachment rates to stable species have been reported, only one measurement of an electron attachment rate to an unstable radical species exists in the literature. We report several measurements of involving these classes of reactions using a novel flowing afterglow technique which we have called Variable Electron and Neutral Density Attachment Mass Spectrometry (VENDAMS). The technique takes advantage of these processes occurring as secondary and tertiary chemistry in high density plasmas, and uses excess electrons as chemical ionization agents to monitor neutral product concentrations. Systems starting with a variety of neutrals have been studied over a temperature range of 300 to 550 K, including SF$_{6}$, SF$_{5}$Cl, SF$_{5}$C$_{6}$H$_{5}$, SF$_{4}$, PSCl$_{3}$, and POCl$_{3}$. Electron attachment rate constants to the radical species SF$_{5}$, SF$_{3}$, SF$_{2}$, PSCl$_{2}$, and POCl$_{2}$, are reported; an unusual negative temperature dependence in the attachment rate constants for several of the species is seen. Product branching fractions in the mutual neutralization reactions of SF$_{6}^{-}$ and SF$_{5}^{-}$ are reported, showing little temperature dependence and a correlation between the fraction of dissociative product and the total energy available to the dissociation. Additionally, we present evidence of an electron catalyzed mutual neutralization process (Ar$^{+}$ + M$^{-}$~+ e$^{-}\diamondsuit $ neutrals + e$^{-})$ not previously reported in or speculated on in the literature. Typical rate constants for the process are on the order of 10$^{-18}$ cm$^{6}$ s$^{-1}$, meaning that the catalyzed process becomes competitive with two-body mutual neutralization at electron densities above 10$^{10}$~cm$^{-3}$, and may be the dominant mechanism in plasmas containing monatomic cations at higher electron densities. [Preview Abstract] |
Thursday, October 7, 2010 5:15PM - 5:30PM |
QR3.00004: Collisional dissociative recombination of H$_{3}^{+}$ ions with electrons Rainer Johnsen Plasma afterglow measurements have consistently yielded either much lower (by factors of 10 or more) or higher (by factors of 3 to 4) electron-H$_{3}^{+}$ recombination coefficients then those observed in ion storage rings, and calculated by \textit{ab-initio} theory. The origin of this long-standing discrepancy has not been clearly identified. I will show here that ``collisional dissociative recombination'' in conjunction with angular momentum $l$-mixing can account for the observed increase of recombination rates in plasma experiments at higher neutral densities. In this model, the enhancement of the recombination results from three-body electron capture into Rydberg states of high angular momentum $l$, followed by $l$-reducing collisions with neutral atoms that induce predissociation. Hence, while there is no true ``discrepancy'' between afterglow and storage ring H$_{3}^{+}$ recombination coefficients, recombination in a plasma is not a purely binary process. The same may be true for other ions that recombine by the ``indirect process.'' I also propose that the very low values obtained in some afterglows at low concentrations of neutral hydrogen are flawed by the presence of ion species other than H$_{3}^{+}$, rather than being due to different spin modifications, or vibrational excitation of H$_{3}^{+}$, as has been suggested. [Preview Abstract] |
Thursday, October 7, 2010 5:30PM - 5:45PM |
QR3.00005: Small Molecule Processes in ITER J. Brian A. Mitchell, Sophie Carles, Jean-Luc LeGarrec The International Thermonuclear Experimental Reactor (ITER), will present new problems related to energy management in plasma devices. In particular the management of the very high power loads encountered in the divertor and edge plasmas requires a thorough knowledge of atomic and molecular processes in these areas as these are critical to the necessary cooling of the plasma before it encounters solid surfaces that would otherwise be destroyed. These processes include excitation, ionization and recombination but also two and three body association reactions that can produce small molecular species from atomic constituents. Modern tokamaks have exposed surfaces that are commonly composed of carbon bricks and ITER will probably start off with this. Carbon, however, presents serious problems of dust production and tritium trapping so current thinking is to replace it with beryllium. The IAEA has initiated a Coordinated Research Project (CRP) specifically addressing A{\&}M processes for light elements including specifically hydrogen, helium, lithium and beryllium but also with an interest in carbon, nitrogen and oxygen. Reactions responsible for the formation and destruction of small molecular ions will be reviewed and needs for further work will be highlighted. [Preview Abstract] |
Thursday, October 7, 2010 5:45PM - 6:00PM |
QR3.00006: Low energy elastic electron scattering from furan Murtadha A. Khakoo, John Muse, Kevin Ralphs, Romarly F. da Costa, Marcio H.F. Bettega, Marco A.P. Lima We report normalized-experimental and theoretical differential cross-sections for elastic electron scattering by C$_{4}$H$_{4}$O (furan) molecules. The experimental data were taken at incident electron energies of 1eV, 1.5eV, 1.73eV, 2eV, 2.7eV, 3eV, 5eV, 7eV, 10eV, 20eV, 30eV and 50eV and covered the angular range between 10 to 130 degrees. The calculations employed the Schwinger multichannel method with pseudopotentials and were performed in the static-exchange and in the static-exchange plus polarization approximations. The calculated integral and momentum transfer cross sections clearly revealed the presence of two shape resonances ascribed to the B$_{1}$ and A$_{2}$ symmetries of the C$_{2v}$ point group, in very good agreement with the experimental findings. Overall agreement between theory and experiment is very good, especially for energies below 10eV. [Preview Abstract] |
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