Bulletin of the American Physical Society
70th Annual Gaseous Electronics Conference
Volume 62, Number 10
Monday–Friday, November 6–10, 2017; Pittsburgh, Pennsylvania
Session TR2: Dissociation, Recombination, and Attachment |
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Chair: Rainer Johnsen, University of Pittsburgh Room: Duquesne |
Thursday, November 9, 2017 4:00PM - 4:30PM |
TR2.00001: Electron/molecular cation collisions in low-temperature plasmas: from mechanisms to rate coefficients Invited Speaker: Ioan F. Schneider The kinetics of various cold ionized media is driven by electron-impact dissociative recombination, dissociative excitation and ro-vibrational (de)excitation of molecular cations [1]. These collisions are highly reactive, involve numerous super-excited molecular states undergoing predissociation and/or autoionization, and their measurement and modeling both result in cross sections desplaying a strong resonant character. Their theoretical study requires sophisticated methods capable to go far beyond the Born-Oppenheimer approximation, and to manage to describe a complex molecular dynamics relying on the superposition of many continua and infinite series of Rydberg states. We have used the Multichannel Quantum Defect Theory (MQDT) in order to compute cross sections and rate coefficients for the invoked processes for H$_2^+$, H$_3^+$, N$_2^+$ [2], BeH$^+$ [3], BF$^+$, NO$^+$, CO$^+$, SH$^+$, CH$^+$ and ArH$^+$, and we have compared them with the experimental ones - obtained in storage rings and flowing afterglows. [1] I. F. Schneider, O. Dulieu, and J. Robert (editors) 2015, Eur. Phys. J. Web of Conf., http://www.epjconferences.org/articles/epjconf/abs/2015/03/contents/contents.html, 84. [2] D. A. Little et al. 2014, Phys. Rev. A 90, 052705. [3] V. Laporta et al., 2017 Plasma Phys. Contr. Fusion 59, 045008. [Preview Abstract] |
Thursday, November 9, 2017 4:30PM - 5:00PM |
TR2.00002: Kinetics of transient species with cations and electrons Invited Speaker: Nicholas Shuman Weakly ionized plasma will generally contain some concentrations of transient species, e.g. small fluorocarbon radicals in a discharge through CF$_{\mathrm{4}}$. Experimental measurements of the kinetics of these species with electrons and with ions are scarce in the literature, in part due to the difficulty in producing and quantifying transient species. We have developed a technique, termed variable electron and neutral density attachment mass spectrometry (VENDAMS), employing a flowing afterglow-Langmuir probe apparatus that provides access to the kinetics of a wide range of radical or otherwise unstable species reacting with electrons or with cations. The kinetics of electron attachment to small fluorocarbon and hydrofluorocarbon radicals have been measured at thermal conditions from 300 -- 1000 K. The results are interpreted using a kinetic modeling approach rooted in statistical theory, which allows extrapolation of the results to conditions not accessible by the experiment, including to extreme temperatures, pressures, or non-thermal conditions. The ion-molecule kinetics of small hydrocarbon, fluorocarbon, and hydrofluorocarbon radicals with a number of cations were also studied under thermal conditions. Surprisingly, the radical species react less efficiently and with a lower likelihood of long-range charge transfer than similar reactions of stable, closed-shell species with the same cations. The VENDAMS technique is also used to study ion-ion mutual neutralization processes. The rate coefficients of mutual neutralization in systems involving greater than 3 atoms vary by no more than about a factor of 5. On the other hand, the rate coefficients of mutual neutralization of two atomic species can vary widely. In some systems the rate coefficients are of similar magnitude to those for polyatomic species, but in other cases at least 2 orders of magnitude smaller. A large number of measurements are distilled down to a simple parametrization to predict the rate coefficients of unstudied systems. [Preview Abstract] |
Thursday, November 9, 2017 5:00PM - 5:15PM |
TR2.00003: Development of DEA instrumentation for a comprehensive understanding of gas-phase molecular fragmentation Sylwia Ptasinska, Zhou Li, Aleksandar R. Milosavljevic, Ian Carmichael Electron attachment to a molecule triggers several dissociation pathways of transient molecular anions, each resulting in the formation of one negative ion and its counterpart. The counterpart can be a single neutral radical or several fragments. However, there are no studies that detect the neutrals formed from the dissociative electron attachment (DEA) process to molecules in the gas phase. In order to do this, we developed stepwise electron spectroscopy (SWES) [1]. We detected the neutrals produced upon DEA to CCl$_{\mathrm{4}}$ at \textasciitilde 0 eV by measuring the appearance energies of CCl$_{\mathrm{3}}$ radical as well as the other neutral species. In addition, we combined the experimental findings with high-level quantum chemical calculations to obtain a complete analysis of both the DEA to CCl$_{\mathrm{4}}$ and the subsequent electron-impact ionization of CCl$_{\mathrm{3}}^{\mathrm{\thinspace }}$radicals. The detection of neutral radicals can be essential from the point of view of radiation damage to DNA, particularly in the case of double strand breaks (DSBs) by low energy electrons [2]. [1] Z. Li et al., Phys. Rev. Lett. (2017) in press, [2] B. Boudaiffa et al., Science 287, 1658 (2000) [Preview Abstract] |
Thursday, November 9, 2017 5:15PM - 5:30PM |
TR2.00004: Contrast between electron attachment to CH$_{\mathrm{3}}$SCN and CH$_{\mathrm{3}}$NCS. Thomas M. Miller, Nicholas S. Shuman, Albert A. Viggiano We have made measurements of electron attachment to CH$_{\mathrm{3}}$SCN (methyl thiocyanate) and CH$_{\mathrm{3}}$NCS (methyl isothiocyanate) over the temperature range 300-1000 K in a flowing afterglow Langmuir probe apparatus. Both attachment processes yield mainly the SCN$^{\mathrm{-}}$ pseudohalide anion product. Both molecules are inefficient at attaching electrons, with the rate coefficient k$_{\mathrm{a}}$ for CH$_{\mathrm{3}}$SCN near 2 x 10$^{\mathrm{-10}}$ cm$^{\mathrm{3}}$ s$^{\mathrm{-1}}$ at 300 K and that for CH$_{\mathrm{3}}$NCS orders of magnitude lower and unmeasurable. Both rate coefficients increase strongly with temperature. The k$_{\mathrm{a}}$ for CH$_{\mathrm{3}}$SCN is 100 times larger at 1000 K, while k$_{\mathrm{a}}$ for CH$_{\mathrm{3}}$NCS reaches 4 x 10$^{\mathrm{-9}}$ cm$^{\mathrm{3}}$ s$^{\mathrm{-1}}$ at 1000 K. Calculations of potential energy surfaces imply that the electron attachment mechanisms are completely different. Attachment to CH$_{\mathrm{3}}$SCN requires vibrational excitation, but then dissociation of the parent anion readily follows. Formation of the transient CH$_{\mathrm{3}}$NCS$^{\mathrm{-}}$ anion appears to be more facile, but the dissociative surface has a rate-limiting barrier. The formation of a CN$^{\mathrm{-}}$ anion product is calculated to be 0.5 eV endothermic from CH$_{\mathrm{3}}$SCN or 0.7 eV from CH$_{\mathrm{3}}$NCS at 0 K. CN$^{\mathrm{-}}$ appears faintly in the product mass spectra for CH$_{\mathrm{3}}$NCS, which could be due to impurities, but appears more strongly for CH$_{\mathrm{3}}$SCN as temperature increases, which is less easily explained. We also have results for C$_{\mathrm{2}}$H$_{\mathrm{5}}$SCN, which attaches electrons similarly to CH$_{\mathrm{3}}$SCN at 300 K, but is 3 times less efficient at 1000 K. [Preview Abstract] |
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