Bulletin of the American Physical Society
67th Annual Gaseous Electronics Conference
Volume 59, Number 16
Sunday–Friday, November 2–7, 2014; Raleigh, North Carolina
Session KW3: Electron-Molecule Collisions and Related Processes I |
Hide Abstracts |
Chair: Leigh Hargreaves, California State University, Fullerton Room: State D |
Wednesday, November 5, 2014 1:30PM - 2:00PM |
KW3.00001: Electron attachment to fluorocarbon radicals Invited Speaker: Nicholas Shuman Most plasma environments contain populations of short-lived species such as radicals, the chemistry of which can have significant effects on the overall chemistry of the system. However, few experimental measurements of the kinetics of electron attachment to radicals exist due to the inherent difficulties of working with transient species. Calculations from first principles have been attempted, but are arduous and, because electron attachment is so sensitive to the specifics of the potential surface, their accuracy has not been established. Electron attachment to small fluorocarbon radicals is particularly important, as the data are needed for predictive modeling of plasma etching of semiconductor materials, a key process in the industrial fabrication of microelectronics. We have recently developed a novel flowing afterglow technique to measure several types of otherwise difficult to study plasma processes, including thermal electron attachment to radicals. Variable Electron and Neutral Density Attachment Mass Spectrometry (VENDAMS) exploits dissociative electron attachment in a weakly ionized plasma as a radical source. Here, we apply VENDAMS to a series of halofluorocarbon precursors in order to measure the kinetics of thermal electron attachment to fluorocarbon radicals. Results are presented for CF$_{\mathrm{2}}$, CF$_{\mathrm{3}}$, C$_{\mathrm{2}}$F$_{\mathrm{5}}$, C$_{\mathrm{2}}$F$_{\mathrm{3}}$, 1-C$_{\mathrm{3}}$F$_{\mathrm{7}}$, 2-C$_{\mathrm{3}}$F$_{\mathrm{7}}$, and C$_{\mathrm{3}}$F$_{\mathrm{5}}$ from 300 K to 900 K. Both the magnitude and the temperature dependences of rate coefficients as well as product branching between associative and dissociative attachment are highly system specific; however, thermal attachment to all species is inefficient, never exceeding 5{\%} of the collision rate. The data are analyzed using a recently developed kinetic modeling approach, which uses extended Vogt-Wannier theory as a starting point, accounts for dynamic effects such as coupling between the electron and nuclear motions through empirically validated functional forms, and finally uses statistical theory to determine the fate of the highly excited anion intermediate formed during attachment. The kinetic modeling, along with complimentary data from electron beam measurements, is used to extrapolate the electron attachment rate coefficients to temperature and pressure regimes inaccessible to the experiment, including to non-thermal plasma conditions most relevant to plasma etching. [Preview Abstract] |
Wednesday, November 5, 2014 2:00PM - 2:15PM |
KW3.00002: H$_{2}$-Assisted Ternary Recombination of H$_{3}^{+}$ with Electrons at 300 K Rainer Johnsen, Petr Dohnal, Peter Rubovic, Abel Kalosi, Michal Hejduk, Radek Plasil, Juraj Glosik Afterglow measurements in ionized He/Ar/H$_{2}$ gas mixtures at 300 K show that the recombination of H$_{3}^{+}$ ion with electrons is very strongly enhanced in the presence of molecular hydrogen. In the experiments the decay of H$_{3}^{+}$ ions was measured by near-infrared (NIR) absorption spectroscopy (SA-CRDS).\footnote{P. Macko et al, \textit{Int. J. Mass Spectrom.}~\textbf{233}, 299 (2004).} Rather surprisingly, the H$_{2}$-assisted three-body recombination coefficient ($K_{\mathrm{H2}} = $ (8.7 $+$/- 1.5) $\times$ 10$^{-23}$ cm$^{6}$s$^{-1})$ exceeds by more than two orders of magnitude the corresponding He-assisted coefficient ($K_{\mathrm{He}} = $ (3.3 $+$/- 0.7) $\times$ 10$^{-25}$ cm$^{6}$s$^{-1})$ that we measured earlier.\footnote{R. Johnsen et al, \textit{J. Phys. Chem. A }\textbf{11}7, 9477 (2013).} Formation of faster recombining H$_{5}^{+}$ cluster ions does not play a significant role at temperature near 300 K. The ternary processes are found to saturate at high He and H$_{2}$ densities, suggesting that recombination proceeds by a two-step process, electron capture (with a rate coefficient $\alpha_{\mathrm{F}} = $ (1.5 $+$/- 0.1) $\times$ 10$^{-7}$ cm$^{3}$s$^{-1})$ into a long-lived Rydberg state with an excited core, followed by collisional stabilization. While these findings provide a plausible explanation for some of the discrepancies between earlier afterglow measurements of H$_{3}^{+}$ recombination, the exact nature of these long-lived complexes, and their collisional interactions remain to be elucidated. [Preview Abstract] |
Wednesday, November 5, 2014 2:15PM - 2:30PM |
KW3.00003: Kinetics of ion-ion mutual neutralization Thomas M. Miller, Justin P. Wiens, Nicholas S. Shuman, Albert A. Viggiano We have measured rate coefficients for 87 mutual neutralization reactions between thermal energy anions and cations, a number of them as a function of temperature. In addition, in two cases we have observed a transfer ionization channel in which there is enough energy for the anion reactant to be doubly ionized, yielding a cation product rather than neutralization. We will summarize these results and note correlations, namely: (1) binary neutralization rate coefficients are primarily a function of the chemical nature of the system for atom-atom ionic pairs (with a wide range of rate coefficients), but quickly become dominated by physical aspects (i.e., relative velocity) as the number of atoms in the system increases. (2) Rate coefficients for atom-atom ionic pairs are well fit at 300 K by k $=$ 3 x 10$^{-4}$ R$_{x}^{-3.15}$, where R$_{x}$ is the curve crossing radius given by R$_{x} \quad =$ 27.2/$\Delta $E, with $\Delta $E being the electron transfer energy released in the reaction. ($\Delta $E in eV, R$_{x}$ in Bohr, and k in cm$^{3}$/s.) (3) Rate coefficients for systems of more than 4 or 5 atoms are well described by k $=$ 2.7 x 10$^{-7}$ (T/300)$^{-0.9} \quad \mu ^{-0.5}$. (T in K, and the reduced mass $\mu $ in amu.) (4) Triatomic systems have rate coefficients smaller than given by the expression in (3). [Preview Abstract] |
Wednesday, November 5, 2014 2:30PM - 3:00PM |
KW3.00004: Electron scattering measurements from molecules of technological relevance Invited Speaker: Darryl Jones Biomass represents a significant opportunity to provide renewable and sustainable biofuels [1]. Non-thermal atmospheric pressure plasmas provide an opportunity to efficiently breakdown the naturally-resilient biomass into its useful subunits [2]. Free electrons produced in the plasma may assist in this process by inducing fragmentation though dissociative excitation, ionization or attachment processes [3]. To assist in understanding and refining this process, we have performed electron energy loss experiments from phenol (C$_{6}$H$_{5}$OH), a key structural building block of biomass. This enables a quantitative assessment of the excited electronic states of phenol. Differential cross sections for the electron-driven excitation of phenol have also been obtained for incident electron energies in the 20-250eV range and over 3-90$^{\circ}$ scattering angles. \\[4pt] [1] A. J. Ragauskas \textit{et al}, Science \textbf{311}, 484 (2006).\\[0pt] [2] M. Benoit \textit{et al}, Angew. Chem. Int. Ed. \textbf{50}, 8964 (2011).\\[0pt] [3] E.M. de Oliveira \textit{et al}, Phys. Rev. A. \textbf{86}, 020701(R) (2012). [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