Bulletin of the American Physical Society
61st Annual Gaseous Electronics Conference
Volume 53, Number 10
Monday–Friday, October 13–17, 2008; Dallas, Texas
Session WF2: Recombination and Attachment |
Hide Abstracts |
Chair: A. Orel, University of California, Davis Room: Salon A-D |
Friday, October 17, 2008 10:00AM - 10:30AM |
WF2.00001: Cold electron collisions with atomic and molecular ions in merged beams: high-resolution collision spectroscopy in storage rings Invited Speaker: Down to the lowest collision energies, free electrons efficiently react with atomic and molecular cations. Atomic ions can bind the colliding electrons by the emission of photons (radiative and dielectronic recombination), while molecular ions are efficiently broken up by slow free electrons without an energy barrier (dissociative recombination). For most atomic and molecular species, the cross sections for recombination and other inelastic cross sections show important resonances reflecting the energetic positions as well as the autoionization and pre-dissociation of quasibound intermediate states formed in the electron collision. High resolution experiments revealing such resonances as well as the underlying atomic and molecular properties and the rich dynamics are performed with merged beams of ions and electrons in ion storage rings, using event-by event counting and imaging methods. Recently, monochromatic electron impact energies down into the few-meV range have been realized by intense and cold merged electron beams from photocathode sources. Ion beam storage controls the internal vibrational and, to some extent, rotational state of the cation. Fast-beam multiparticle imaging is used to reconstruct the molecular fragmentation events and to monitor the initial ionic ro-vibrational state. Examples of recent measurements with multicharged atomic ions and with smaller molecules, from the hydrogen ions to di- and triatomic heavier species (such as CF$^+$ and CH$_2^+$) are presented. [Preview Abstract] |
Friday, October 17, 2008 10:30AM - 10:45AM |
WF2.00002: Dissociative recombination of molecular ions in the plasma environment Rainer Johnsen The two broad categories of experimental methods that are used to determine rates and products of dissociative recombination, single-collision techniques (e.g. merged beams, storage rings) and plasma techniques (afterglows), sometimes give strikingly differing results, in particular in the case of some polyatomic ions. For instance, the afterglow recombination coefficient for H$_{3}^{+}$ is twice that found in storage rings. Water-cluster ions H$^{+}$(H$_{2}$O)$_{n}$ for n=4 seem to recombine about eight times faster in afterglows! Neither of the methods necessarily gives ``wrong'' results but it is questionable if recombination in the plasma environment is truly a binary process or if third-body interactions play a role. Plasma modelers also face a problem: Should they prefer the recombination coefficients obtained by single-collision methods or those measured in plasmas that are closer to the intended application? Which recombination coefficient applies in the D-region of the ionosphere that is dominated by water clusters? I will critically examine possible three-body mechanisms (l-mixing of Rydberg electrons, complex stabilization by ambient molecules) and estimate the magnitude of their contributions. It appears that some proposed mechanisms seriously overestimate third-body effects unless complex lifetimes are much longer than is indicated by available theory. [Preview Abstract] |
Friday, October 17, 2008 10:45AM - 11:00AM |
WF2.00003: C$_2^-$ Formation By Resonant Dissociative Electron Attachment to Acetylene S.T. Chourou, A.E. Orel Experimental work on dissociative electron attachment (DEA) of acetylene shows a peak in the cross section at around $8.1\ eV$ corresponding to the formation of C$_2^-$ anions. It has been further predicted that these anion fragments result from the decay of a series of Feshbach resonant states with the configurations $^2(\pi_u, 3s^2 )$, $^2(\pi_u, 3s3p\sigma )$, $^2(\pi_u, 3s3p\pi )$ and $^2(\pi_u, 3p^2 )$ between 7 and 9.5 $eV$. In this work, we perform electron scattering calculations using the Complex Kohn Variational Method to determine the positions and autoionization widths of the Feshbach resonances. We iterate this process for relevant geometries of the molecule to construct the multidimentional complex potential energy surfaces. In order to study the dissociation dynamics leading to the (C$_2^-$ + H + H) and (C$_2^-$ + H$_2$) rearrangements, we treat the system in 4D taking into account the stretching and bending of the two C-H bonds of C$_2$H$_2$ in an appropriate coordinate system. By computing the flux of the wavepacket into the decoupled asymptotic regions associated with these two rearrangements, we deduce the DEA cross section and compare it to available experimental results. [Preview Abstract] |
Friday, October 17, 2008 11:00AM - 11:15AM |
WF2.00004: Plasma decay in Air and N$_{2}$:O$_{2}$:CO$_{2}$ mixtures at elevated gas temperatures Nickolay Aleksandrov, Svetlana Kindusheva, Ilya Kosarev, Andrei Starikovskii Plasma decay after a high-voltage nanosecond discharge has been studied experimentally and numerically behind an incident and reflected shock wave in high temperature (900 -- 3000 K) air and N$_{2}$:O$_{2}$:CO$_{2}$ mixtures for pressures between 0.1 and 2 atm. Time-resolved electron density history was measured by a microwave interferometer for initial electron densities in the range (1-3)x10$^{12}$ cm$^{-3}$. It was shown that the electron density varies in the air afterglow in the ``recombination manner'', 1/$n_{e}(t)$ = 1/$n_{e}$(0) + \textit{$\alpha $}$_{eff}t$, where \textit{$\alpha $}$_{eff}$ is the effective electron-ion recombination coefficient. A numerical simulation was carried out to describe the temporal evolution of the densities of charged particles under the conditions considered. A good agreement was obtained between the calculated and the measured electron density histories in the air afterglow when taking into account electron attachment to O$_{2}$ to form O$_{2}^{-}$ ions and electron detachment from them, as well as electron-ion and ion-ion recombination. In CO$_{2}$-containing mixtures, it was necessary to consider the formation of complex negative and positive ions. These ions were formed in three-body reactions; therefore, the rate of plasma decay increased with gas density. [Preview Abstract] |
Friday, October 17, 2008 11:15AM - 11:30AM |
WF2.00005: Dissociative recombination of CF$^+$ Valery Ngassam, A.E. Orel We present results from our recent studies of the dissociative recombination of the CF$^+$ ion. Extensive calculations of energy positions and autoionization widths for the doubly excited states of CF between the first and second ionization thresholds have been obtained from electron scattering calculations using the complex Kohn variational method, followed by calculations of the dissociative recombination process using the time dependent wave packet method. All dissociative states in each molecular symmetry, were included. The resonances leading to dissociation into the product channels will be discussed and the calculated cross sections will be reported and compared to available experiment. [Preview Abstract] |
Friday, October 17, 2008 11:30AM - 11:45AM |
WF2.00006: Electron attachment to SF$_{6}$ at high temperatures T.M. Miller, J.F. Friedman, A.A. Viggiano, J. Troe We have recently reported flowing-afterglow Langmuir-probe experiments on electron attachment to SF$_{6}$, thermal electron detachment from SF$_{6}^{-}$, and the pressure dependence of the processes involved, in the temperature range 300-670 K, including theoretical analysis of the possible outcomes of the electron-SF$_{6}$ interaction, with modeling of the data. One significant result of that work was the finding that the electron affinity of SF$_{6}$ is 1.20 $\pm $ 0.05 eV.\footnote{A. A. Viggiano et al., J. Chem. Phys. 127, 244303, (2007).} We have now extended the temperature range up to 1300 K. The electron attachment rate constant at 700 K is 1.7 x 10$^{-7}$ cm$^{3}$ s$^{-1}$ (yielding SF$_{5}^{-}$ and SF$_{6}^{-}$ product), and the thermal detachment rate constant for SF$_{6}^{-}$ is 580 s$^{-1}$. F$^{-}$ becomes a major ion product at 1000 K and above. We suspect that in this temperature range the SF$_{6}$ molecules are decomposing, because the SF$_{5}^{-}$ ion product disappears above 1100 K, and only the F$^{-}$ ion product remains. Further work must be carried out to determine the origin of the F$^{-}$, whether from decomposition or a surface-ionization effect. [Preview Abstract] |
Friday, October 17, 2008 11:45AM - 12:00PM |
WF2.00007: Thermal electron attachment to O$_{2}$, NO, N$_{2}$O, and Nucleobases Edward C. Chen, Edward Chen New electron affinities and activation energies for thermal electron attachment for O$_{2}$, NO, N$_{2}$O and the nucleic acids are presented. These are (in eV): O$_{2}$, 1.07(1)/1.05(1); NO, 0.91(1); Guanine(G),1.645(5); Adenine(A), 1.095(5); Cytosine(C);1.041(5);Uracil,(U) 1.000(5); and Thymine(T) 0.990(5) in agreement with literature values. The electron affinities for the nuclobases support that for Watson Crick AT, 1.40(10) eV and proposed mechanisms for electron conduction and radiation damage and repair and in DNA. Gas phase electron affinities from reduction potentials and voltage onsets for ESR spectra are: (in eV) [2,2] paracyclophane, -0.35(5); 3,3',5,5'-tetra-tbutylbiphenyl, -0.05(5); 4,4' di-tbutylbiphenyl, -0.02(5) 4,4-dimethylbiphenyl,-0.01(5); 2,3-dimethylnaphthalene,0.09(5); acenaphthaene,0.10(5) pyrimidine, 0.35(5); pyradazine, 0.49(5) pyrazine, 0.55(5); s-triazine, 0.64(5); as-triazine, 1.08(5); purine, 1.20(5); s-tetrazine, 1.84(5). Multiple excited state electron affinities including one for N$_{2}$O, 0.4(1) eV and activation energies are reported. These are examples of fundamental data that can be obtained from ion molecule reactions in plasmas. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700