Bulletin of the American Physical Society
43rd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 57, Number 5
Monday–Friday, June 4–8, 2012; Orange County, California
Session N3: Electron Interactions with Atoms and Molecules |
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Chair: Don Madison, Missouri University of Science and Technology Room: Grand Ballroom E |
Thursday, June 7, 2012 10:30AM - 10:42AM |
N3.00001: Radiative electron attachment to molecules of astrophysical interest. Benchmark study of CN$^-$. Viatcheslav Kokoouline, Nicolas Douguet, Olivier Dulieu, Maurice Raoult, Ann E. Orel We develop a first-principles approach to study the process of radiative electron attachment (REA) to linear molecules of astrophysical interest $Mol +e^- \to Mol^- + \hbar\omega$. ($Mol^-$ = C$_n$H$^-$, C$_n$N$^-$). The approach is based on accurate ab initio calculations of electronic bound and continuum states of the negative ion. The electronic continuum states are obtained with the complex-Kohn variational method. A preliminary calculation for the formation of the simplest observed ion, CN$^-$, by REA gave a low rate coefficient. We will present also a preliminary result for the C$_4$H$^-$ formation by REA. For this molecule, the REA rate coefficient is expected to be somewhat larger due to the Renner-Teller non-adiabatic coupling that should enhance electron capture. The goal of this study is to answer the question if negative molecular ions C$_n$H$^-$ and C$_n$N$^-$ recently observed in the interstellar space could indeed be formed by REA as previously suggested. [Preview Abstract] |
Thursday, June 7, 2012 10:42AM - 10:54AM |
N3.00002: Low-energy electron scattering from water vapor Kevin Ralphs, Gabriela Serna, Leigh Hargreaves, Murtadha A. Khakoo, Carl Winstead, B. Vincent McKoy Absolute differential and integral cross section data for elastic scattering and electronic excitation of water molecules by low-energy electron impact are presented. Cross sections were obtained for the elastic and lowest lying 6 states, which dissociate to form the hydroxyl radical. The investigation is focused on OH due to its importance in living systems. The energy range of the measurements was between 1 -- 100eV for the elastic data and 9 -- 20eV for the excitation data. The absolute scale of the excitation data was established by comparison with the elastic scattering results of Khakoo et al. [1]. The present elastic data were compared to earlier data from our laboratory [1], to the data of [2] and others, to investigate a notable difference between the two sets of data. The excitation data are compared to earlier data from Thorn et al. [3] and further extend their data to near threshold energies. Important agreements and differences are observed between the two data sets and are discussed. \\[4pt] [1] M. A. Khakoo et al., Phys. Rev. A, \textbf{78}, 052710 (2008) \\[0pt] [2] H. Cho et al., J. Phys. B: At. Mol. Phys., \textbf{37} 625 (2004) \\[0pt] [3] P. A. Thorn et al., J. Phys. B.: At. Mol. Phys. \textbf{40} 697 (2007) [Preview Abstract] |
Thursday, June 7, 2012 10:54AM - 11:06AM |
N3.00003: Mechanisms of dissociative recombination of N$_2$H$^+$ and HCO$^+$ Samantha Fonseca dos Santos, Nicolas Douguet, Ann Orel, Viatcheslav Kokoouline N$_2$H$^+$ and HCO$^+$ are among the first molecular ions observed in the interstellar medium. They are both formed by fast proton transfer mechanisms and are destroyed either by taking part on the molecular synthesis of more complex molecules or by dissociative recombination (DR). Given the astrophysical importance of these ions, their DR rates have been the subject of several different studies over the years. Recently, good agreement between theory and experiment have been achieved for HCO$^+$, however this is not the case for N$_2$H$^+$. This is mainly due to the fact that previous theories only considered the direct DR mechanism in their calculations. Applying a general simplified model based on multichannel quantum defect theory that accounts for all the main ingredients of indirect DR we calculate cross sections and DR rates for N$_2$H$^+$ as well as for HCO$^+$ using an improved coupling scheme. [Preview Abstract] |
Thursday, June 7, 2012 11:06AM - 11:18AM |
N3.00004: Refutation of a propensity rule in low-energy electron scattering by neon atoms Leigh Hargreaves, Colin Campbell, Murtadha A. Khakoo, Oleg Zatsarinny, Klaus Bartschat Since the work of Kohmoto and Fano [1] there has been considerable interest in the sign of the `orientation' parameter L$_{perp}$, which describes the angular momentum of an excited electronic state perpendicular to the scattering plane imparted by the projectile electron. In a polarization-correlation experiment, L$_{perp}$ can be related to the measurable Stokes parameter P$_{3}$ (L$_{perp}$ = -P$_{3})$, which describes the circular polarization of the photon emitted perpendicular to the scattering plane. Of particular interest is the empirical observation that, for S to P transitions, P$_{3}$ universally trends to negative values for small scattering angles, regardless of the target or incident energy. A number of studies (e.g. [1-3]) have therefore considered the generality of this `propensity rule' and its theoretical basis. Here, a recent joint experimental and theoretical study of electronic excitation of the resonant transition in neon by 25eV electrons is presented. In both the theoretical and experimental data it is observed that P$_{3}$ is positive at small scattering angles, demonstrating a refutation of this propensity rule, in disagreement with the classical arguments of Kohomoto and Fano [1]. \\[4pt] [1] M. Kohmoto and U. Fano, J. Phys. B \textbf{14}, L447 (1981)\\[0pt] [2] D. H. Madison and K. H. Winters, Phys. Rev. Lett. \textbf{47}, 1885 (1981)\\[0pt] [3] K. Bartschat, N. Andersen and D. Loveall, Phys. Rev. Lett. \textbf{83}, 5254 (1999) [Preview Abstract] |
Thursday, June 7, 2012 11:18AM - 11:30AM |
N3.00005: Dynamics of Dissociative Electron Attachment to Methanol Probed By 3-Dimensional Momentum Imaging Dan Slaughter, Hidehito Adaniya, Thorsten Weber, Ali Belkacem Dissociation of a stable molecule by low-energy electrons via a resonant transient negative ion species, in the dissociative electron attachment (DEA) process, plays an important role in phenomena such as atmospheric and interstellar chemistry and radiation damage in biological systems by low-energy secondary electrons. DEA is also a useful tool in materials processing and can be used to control chemical reactions through enhancement of specific dissociation pathways. We present new data on the dynamics of DEA to methanol, which we have measured using a 4pi negative ion momentum spectrometer. The data reveal that the dynamics of DEA to this fundamental polyatomic molecule can be understood in terms of a few similarities with equivalent processes involving the much better-understood water anion [Haxton et al. Phys. Rev. A 84, 030701 (2011)] suggesting the possibility of successfully extending our experimental approach to study DEA dynamics in more complex systems. [Preview Abstract] |
Thursday, June 7, 2012 11:30AM - 11:42AM |
N3.00006: Theoretical and Experimental Fully Differential Cross sections for 54 eV electron-impact ionization of oriented H2 Adam Upshaw, Don Madison, James Colgan, Xueguang Ren, Arne Senftleben, Thomas Pflueger, Alexander Dorn, Joachin Ullrich Experimental fully differential cross sections are measured for 54 eV electron impact ionization of oriented H2. One final state electron is measured in the scattering plane at an angle of 50 degrees from the incident beam direction and the other final state electron is measured in a plane perpendicular to the incident beam. Both electrons have an energy of 18 eV. The experimental results will be compared with TDCC (time dependent close coupling) results and M3DW (molecular 3-body distorted wave) results. The importance of different scattering mechanisms will be discussed. [Preview Abstract] |
Thursday, June 7, 2012 11:42AM - 11:54AM |
N3.00007: Deep minimum in the triple differential cross sections for the electron-impact ionization of He James Colgan, James Feagin, Michael Pindzola We further explore the unusual deep minimum found in the triple differential cross sections for the electron-impact ionization of helium. This feature has been observed experimentally more than 15 years ago [1] and confirmed via close-coupling calculations [2]. A recent study [3] identified the minimum with a vortex in the two-electron continuum, and an analytic expansion of the electron pair about the vortex has recently been derived [4]. The imaging theorem [3] is invoked to compute the TDCS from the radial wavefunctions propagated in time via solution of the time-dependent Schr\"odinger equation for the electron helium system. This allows us to more easily visualize the portion of the wavefunction that contributes to the TDCS at the specific ejected electron angles where the deep minimum is observed. Interesting features in the radial wavefunction as a function of time are found, which appear to be consistent with the prediction [3] that a vortex in the two-electron wavefunction is responsible for the observed deep minimum. [1] A. J. Murray and F. H. Read, J. Phys. B {\bf 26}, L359 (1993). [2] J. Colgan et al, J. Phys. B. {\bf 42}, 171001 2009. [3] J. H. Macek, et al, Phys. Rev. Letts. {\bf 104}, 033201 (2010). [4] J. M. F Feagin, J. Phys. B {\bf 44}, 011001 (2011). [Preview Abstract] |
Thursday, June 7, 2012 11:54AM - 12:06PM |
N3.00008: Ultrafast Electron Pulse (e,2e) Processes Hua-Chieh Shao, Anthony Starace, Lars Madsen Techniques for producing ultrafast electron pulses have been proposed\footnote{P.~Baum and A.H.~Zewail, Proc.~Natl.~Acad.~Sci.~U.S.A.~\textbf{104}, 18409 (2007); S.A.~Hilbert, C.~Ulterwaal, B.~Barwick, H.~Betalaan, and A.H.~Zewail, \textit{ibid}.~\textbf{106}, 10558 (2009).} and prospects for using such pulses to image electron dynamics in the H atom and the hydrogen molecular ion have been theoretically demonstrated.\footnote{H.-C.~Shao and A.F.~Starace, Phys.~Rev.~Lett.~\textbf{105}, 263201 (2010).} The (e,2e) process provides a means to directly image the momentum distribution of the target.\footnote{M.A.~Coplan, J.H.~Moore, and J.P.~Doering, Rev.~Mod.~Phys.~\textbf{66}, 985 (1994).} We explore here the possibility of observing the time dependence of a coherent superposition of target orbitals by means of the (e,2e) process with ultrafast incident electron pulses. Using scattering theory for a longitudinally coherent beam,\footnote{F.~Robicheaux, Phys.~Rev.~A \textbf{62}, 062706 (2000).} we find that the momentum distribution of a coherent state of the H atom can be retrieved. [Preview Abstract] |
Thursday, June 7, 2012 12:06PM - 12:18PM |
N3.00009: Experimental and theoretical investigation of the triple differential cross section for electron impact ionization of molecules of biological interest -- thymine, pyrimidine, and tetrahydrofurfuryl alcohol Hari Chaluvadi, D.H. Madison, J.D. Builth-Williams, S.M. Bellm, D.B. Jones, M.J. Brunger, C.G. Ning, B. Lohmann Cross-section data for electron impact induced ionization of bio-molecules are important for modeling the deposition of energy within a biological medium and also for gaining knowledge of electron driven processes at the molecular level. Triply differential cross sections have been measured and calculated for the electron impact ionization of the inner valence 2a and 14a orbitals of thymine, the outer valence 7b$_{2}$ and 10a$_{1}$ orbitals of pyrimidine, and the HOMO of tetrahydrofurfuryl alcohol. The experimental measurements will be compared with theoretical M3DW (molecular 3-body distorted wave) model calculations. [Preview Abstract] |
Thursday, June 7, 2012 12:18PM - 12:30PM |
N3.00010: Nonperturbative Treatment of Electron-Impact Ionization of Ar(3p) Klaus Bartschat, Oleg Zatsarinny We present triple-differential cross sections for electron-impact ionization of a $3p$ electron in Ar. Results from a fully non\-perturbative close-coupling formalism using our $B$-Spline $R$-matrix with Pseudo-States (BSRMPS) approach~[1] are compared with those from a hybrid distorted-wave plus $R$-matrix expansion~[2] as well as recent experimental data~[3]. We find overall good agreement between the two sets of entirely independent theoretical predictions, but serious discrepancies with the published experimental data. A detailed investigation of the dependence of the results on the fixed detection angle of the ``scattered projectile'', i.e., the faster of the two outgoing electrons, suggests that obtaining reliable results, both experimentally and theoretically, is highly challenging in the regime where the largest discrepancies occur. Consequently, care should be taken before much weight is put on the remaining deviations between experiment and theory. Further independent tests seem highly desirable.\\[4pt] [1] O.~Zatsarinny and K.~Bartschat, Phys.~Rev.~Lett.~{\bf 107} (2011) 023203.\\[0pt] [2] K.~Bartschat and O.~K.~Vorov, Phys.~Rev.~A~{\bf 72} (2005) 022728.\\[0pt] [3] X.~Ren, A.~Senftleben, T.~Pfl\"uger, A.~Dorn, K.~Bartschat, and J.~Ullrich, Phys.~Rev.~A~{\bf 83} (2011) 052714. [Preview Abstract] |
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