Bulletin of the American Physical Society
2006 37th Meeting of the Division of Atomic, Molecular and Optical Physics
Tuesday–Saturday, May 16–20, 2006; Knoxville, TN
Session Z2: Gaseous Electronics Conference Session |
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Chair: Rainer Johnsen, University of Pittsburgh Room: Knoxville Convention Center 301AB |
Saturday, May 20, 2006 9:00AM - 9:36AM |
Z2.00001: Site Selective Bond Cleavage Upon Dissociative Electron Attachment - A Tool to Control Chemical Reactions. Invited Speaker: Paul Scheier Free electron attachment to gas phase nucleobases (NB) and these molecules embedded in superfluid He droplets and Ne clusters is studied experimentally. For most of the biomolecules studied so far, the dominant reaction channel is e$^{-}$ + NB $\to $ (NB$^{-})^{\# } \quad \to $ (NB-H)$^{-}$ + H. (1) For the DNA bases adenine (A) and thymine (T) the attachment cross section in the electron energy range between 1 and 3 eV reveals several narrow resonances. By using partially deuterated and methylated NB molecules it is possible to assign these resonances to the loss of hydrogen from a specific nitrogen site [1]. The complementary reaction channel of (1) is the formation of H$^{-}$. For the formation of H$^{-}$ from A and T the attachment cross section shows several resonances in the energy range between 5 and 12 eV. Experiments with partly deuterated T and methylated NB show that the different peaks in the H$^{-}$ ion yield can be associated to the loss from the different molecular sites [2,3]. The energy dependence for H$^{-}$ abstraction from the carbon sites shows a remarkable resemblance to the energy dependence of strand breaks observed in plasmid DNA [4] suggesting that this reaction may be an important initial step towards strand breaks. Free electron attachment to NB embedded in superfluid He droplets exhibits a novel two-step process for electron energies higher than 5~eV. From an initially formed H$^{-}$ an electron is transferred to the opposite neutral radical and forms the (NB-H)$^{-}$. References [1] S. Ptasi\~nska et al., \textit{Angew. Chem. Int. Ed.} \textbf{44} (2005) 6941 [2] S. Ptasi\~nska et al., \textit{Angew. Chem. Int. Ed.} \textbf{44} (2005) 1647 [3] S. Ptasi\~nska et al., \textit{Phys. Rev. Lett.} \textbf{95} (2005) 093201 [4] B. Boudaiffa et al., \textit{Science} \textbf{278} (2000) 1658 [Preview Abstract] |
Saturday, May 20, 2006 9:36AM - 10:12AM |
Z2.00002: Electron-impact ionization and dissociative ionization of biomolecules Invited Speaker: Oxidative damages by ionizing radiation are the source of radiation-induced damages to human health. It is recognized that secondary electrons play a role in the damage process, particularly important is the damage of DNA by electrons, potentially leading to mutagenesis. The damage can be direct, by creating a DNA lesion, or indirect, by producing radicals that attack the DNA. Molecular-level study of electron interaction with DNA provides information on the damage pathways and dominant mechanisms. This investigation focuses on ionization and dissociative ionization (DI) of DNA fragments by electron-impact. For ionization we use the improved binary-encounter dipole (iBED) model [W.M. Huo, Phys. Rev. A64, 042719-1 (2001)]. For DI it is assumed that electron motion is much faster than nuclear motion, allowing DI to be treated as a two-step process and the DI cross section given by the product of the ionization cross section and dissociation probability. The ionization study covers DNA bases, sugar phosphate backbone, and nucleotides. An additivity principle is observed. For example, the sum of the ionization cross sections of the separate deoxyribose and phosphate fragments is in close agreement with the C$_{3}$'- and C$_{5}$'-deoxyribose-phospate cross sections, differing by less than 5{\%}. The result implies that certain properties of the DNA, like the total ionization cross section, are localized properties and an additivity principle may apply. This allows us to obtain properties of a larger molecular system built up from the results of smaller subsystem fragments. The DI of guanine and cytosine has been studied. For guanine, a proton is produced from the channel where the ionized electron originates from a molecular orbital with significant charge density along the N(1)-H bond. The interaction of the proton with cytosine was also studied. [Preview Abstract] |
Saturday, May 20, 2006 10:12AM - 10:48AM |
Z2.00003: Resonant Dissociative Recombination Invited Speaker: In the collision of electrons with molecules and molecular ions, excitation and dissociation are dominated by resonant processes, where the electron becomes temporarily trapped, changing the forces felt by the nuclei. We will outline our method for treating these collision processes, where one or more resonant states exist. We separate the problem into two steps. First, the resonance parameters are obtained from accurate electron scattering calculations using the Complex Kohn variational method. Then these parameters are used as input to the dynamics calculations. We will illustrate the method with the study of dissociative recombination for the He$_{2}^{+}$, Ne$_{2}^{+}$, and Ar$_{2}^{+}$ molecular ions following collision with low energy electrons. Dissociative recombination of the rare gases are important processes in the ionosphere as well as laboratory plasmas and gaseous discharges. Comparison will be made to the available cross sections and rate coefficients. In collaboration with V. Ngassam and J. Royal. Work supported by the NSF PHY-02-44911, The Center for Biophotonics, an NSF Science and Technology Center PHY 0120999. and the NATO Science Program PST.GLG.9794033 [Preview Abstract] |
Saturday, May 20, 2006 10:48AM - 11:24AM |
Z2.00004: Low-Energy Electron Interactions with Complex Targets. Invited Speaker: We have examined low-energy electron collisions with complex targets such as pristine nanoscale ice films and water/DNA interfaces by monitoring the reactive scattering processes leading to the formation of H (H$^{+})$, H$_{2}$ (H$_{2}^{+})$, O $^{3}$P$_{J}$, O $^{1}$D, OH$^{+}$, H$^{+}$(H$_{2}$O)$_{n}$ and DNA fragments. This work has shown that temperature-induced changes in the yields are due to subtle geometric and electronic structure changes brought about by changes in the interfacial hydrogen bonding structure. We have then exploited the fact that the two-hole localized states governing cation production and excitations involving a$_{1}$ levels are good probes of subtle structural changes in the water network. Since low-energy electrons can cause lethal damage to DNA, understanding the role of water and the DNA constituents in the damage event has recently received wide-spread attention. We have modified our multiple scattering ``path approach'' used to describe diffraction effects in stimulated desorption to calculate the diffraction and incident electron intensity at particular sections within a DNA double-strand. This approach assumes hypothetical electron scattering paths inside the target and calculates the interference of all elastically scattered components with the initial incoming wave. Constructive interference at zones localized within the DNA may locally enhance the dissociation probability. [Preview Abstract] |
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