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 LW3: Electron-Molecule Collisions and Related Processes II |
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Chair: Don Madison, Missouri S&T Room: State D |
Wednesday, November 5, 2014 3:30PM - 4:00PM |
LW3.00001: Tailoring Bond Cleavage in Gas-Phase Biomolecules by Low Energy Electrons Invited Speaker: Sylwia Ptasinska The high energy quanta of impinging radiation can generate a large number (about 5x10$^{4}$) of secondary electrons per 1 MeV of energy deposited. When ejected in condensed phase water, the kinetic energy distribution of these free or quasi-free electrons is peaked below 10 eV. Low energy electrons also dominate in the secondary emission from biomolecular targets exposed to different energies of primary radiation. Due to the complexity of the radiation-induced processes in the condensed-phase environment, mechanisms of secondary electrons induced damage in biomolecules (BM) still need to be investigated. However, based on results from theory and different experiments accumulated within the last decade, it is now possible to determine the fundamental mechanisms that are involved in many chemical reactions induced in isolated gas-phase biomolecules by low energy electrons. The central finding of earlier research was the discovery of the bond- and site- selectivity in the dissociative electron attachment (DEA) process to biomolecules. It has been demonstrated that by tuning the energy of the incoming electron we can gain control over the location of the bond cleavage. These studies showed the selectivity in single bond cleavage reactions leading to the formation of the dehydrogenated closed shell anion (BM-H)$^{-}$ or the complementary reaction leading to H$^{-}$. The loss of a hydrogen atom or an anion is fast compared with ring cleavage and the excision of heavier fragments and, hence, this reaction can compete efficiently with autodetachment. Moreover, site selectivity has been also observed in the metastable anion formation via the DEA process. Such delayed fragmentation was studied recently for the dehydrogenated closed-shell anion conversion into NCO$^{-}$ upon DEA proceeded a few $\mu$ sec after electron attachment, indicating a rather slow unimolecular decomposition. Interestingly, site selectivity was observed in the prompt as well as the metastable NCO$^{-}$ formation in DEA. [Preview Abstract] |
Wednesday, November 5, 2014 4:00PM - 4:15PM |
LW3.00002: Vibrational change by electron collision Steven Guberman The vibrational change in molecular ions due to collisions with electrons can be a fast process. This has not been generally recognized until relatively recently. For a high rate constant, the process requires a resonance state intermediate that is either vibrationally quasidiscrete or continuous. These highly excited states are common at energies just above the neutral ionization potential. The ab initio calculation of vibrational change rate constants is reported for N2$+$, i.e. N2$+$(v) $+$ e $\to $ N2$+$(v') $+$ e. The calculations utilize accurate potential curves, electronic widths and the MQDT approach for the calculation of cross sections and rate constants. The rate constants are found to be comparable to those for dissociative recombination. Rate constants over a wide electron temperature range for the lowest 5 ion vibrational levels will be reported. [Preview Abstract] |
Wednesday, November 5, 2014 4:15PM - 4:30PM |
LW3.00003: On Helium Anions in Helium Droplets: Interpreting Recent Experiments Andreas Mauracher, Stefan E. Huber Helium droplets provide an ideal environment to study elementary processes in atomic systems at very low temperatures. Here, we discuss properties of charged and neutral, atomic and molecular helium species formed in helium droplets upon electron impact. By studying their interaction with atomic ground state helium we find that He, $He_{2} $ and excited (metastable) $He^{\ast -}$ are well bound within the helium droplet. In comparison, $He^{\ast }$, $He_{2}^{\ast }$ and $He_{2}^{\ast -}$ are found to be squeezed out due to energetic reasons. We also present the formation pathways of atomic and molecular helium anions in helium droplets. Transition barriers in the energetic lowest $He^{\ast -}$ - He interaction potentials prevent molecule formation at the extremely low temperatures in helium droplets. In contrast, some excited states allow a barrier-free formation of molecular helium (anions). With these theoretical results at hand we can interpret recent experiments in which the resonant formation of atomic and molecular helium anions was observed. Furthermore, we give an outlook on the implications of the presence of these anionic species in doped helium droplets with regard to charge transfer reactions. [Preview Abstract] |
Wednesday, November 5, 2014 4:30PM - 5:00PM |
LW3.00004: Dynamical Studies of Resonant Electron-Molecule Collisions Invited Speaker: Daniel Slaughter A unique capability of low-energy electrons is to break molecular bonds through low-energy resonant processes. We report a combined experimental and theoretical study on the dynamics following dissociative electron attachment (DEA) at low collision energies that induce ring-breaking in uracil. The experiments employ a DEA reaction microscope [1], consisting of a 3D momentum-imaging negative ion spectrometer, a pulsed low-energy electron gun and an effusive gas target. Building further upon a recently-established technique [2-5], fragment ion kinetic energy and angular distributions resulting from DEA are measured and compared with ab initio scattering calculations to reveal key aspects of the dynamics of the transient anion system. Recent experiments on other related systems will also be presented. \\[4pt] [1] Adaniya et al. Rev. Sci. Inst. 83 023106 (2012)\\[0pt] [2] Slaughter et al. Physical Review A 87 052711 (2013)\\[0pt] [3] Moradmand et al. Physical Review A 88 032703 (2013)\\[0pt] [4] Haxton et al. Physical Review A 84 030701 (2011)\\[0pt] [5] Adaniya et al. Physical Review Letters 103 233201 (2009) [Preview Abstract] |
Wednesday, November 5, 2014 5:00PM - 5:15PM |
LW3.00005: Elastic electron scattering from carbon dioxide Allan Stauffer, Tapasi Das, Rajesh Srivastava We have derived a method to obtain the spherically symmetric part of the static interaction between an electron and an arbitrary molecule represented by Gaussian wave functions [1]. Adding polarization-correlation and local exchange potentials provides a total potential that represents electrons scattering from the molecule averaged over all spatial orientations. We will present results for electron scattering from the linear molecule CO$_{2}$ using such a potential. Since this molecule has no permanent dipole moment, we expect our method to produce accurate results for elastic scattering. We will compare our results with existing experimental and theoretical data for this process to assess the accuracy of the method. \\[4pt] [1] Tapasi Das, A D Stauffer and Rajesh Srivastava, Eur. Phys. J. D \textbf{68}, 4, 102 (2014) [Preview Abstract] |
Wednesday, November 5, 2014 5:15PM - 5:30PM |
LW3.00006: Cross sections and products of electron ionization of m-xylene, p-xylene and o-xylene Charles Jiao, Steven Adams Xylenes are contained in many jet fuels and are one of the components in surrogate mixtures for JP-8. In this study using Fourier-transform mass spectrometry to measure the electron ionization cross sections of m-xylene, p-xylene and o-xylene, it is found that the total cross sections of the three xylene isomers are approximately equal at low energies (\textless 25 eV), and become slightly different at higher energies, reaching maxima of 2.24, 2.10 and 2.05x10$^{\mathrm{-15}}$ cm$^{\mathrm{2}}$, respectively, at 80 eV. The electron ionization on these xylenes produces similar products, mainly the parent ion C$_{\mathrm{8}}$H$_{\mathrm{10}}^{\mathrm{+}}$ and fragment species including (C$_{\mathrm{8}}$H$_{\mathrm{9}}^{\mathrm{+}}$ $+$ H), (C$_{\mathrm{8}}$H$_{\mathrm{7}}^{\mathrm{+}} \quad +$ H $+$ H$_{\mathrm{2}})$, (C$_{\mathrm{7}}$H$_{\mathrm{7}}^{\mathrm{+}} \quad +$ CH$_{\mathrm{3}})$, (C$_{\mathrm{6}}$H$_{\mathrm{7}}^{\mathrm{+}} \quad +$ C$_{\mathrm{2}}$H$_{\mathrm{3}})$, (C$_{\mathrm{6}}$H$_{\mathrm{6}}^{\mathrm{+}} \quad +$ C$_{\mathrm{2}}$H$_{\mathrm{4}})$, and (C$_{\mathrm{6}}$H$_{\mathrm{5}}^{\mathrm{+}} \quad +$ C$_{\mathrm{2}}$H$_{\mathrm{5}})$. The results indicate that the major by-products of the electron ionization of xylenes are CH$_{\mathrm{3}}$ and H. The latter is believed to play an important role in fuel ignition because it is involved in both chain branching and chain breaking steps, and it triggers the fuel oxidation. [Preview Abstract] |
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