Bulletin of the American Physical Society
69th Annual Gaseous Electronics Conference
Volume 61, Number 9
Monday–Friday, October 10–14, 2016; Bochum, Germany
Session UF1: Electron-Impact CollisionsFocus
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Chair: Bruno de Harak, Illinois Wesleyan University Room: 1 |
Friday, October 14, 2016 8:30AM - 9:00AM |
UF1.00001: Electron Impact Ionization and Fragmentation Dynamics of Small Atomic and Molecular Clusters Invited Speaker: Alexander Dorn New ionization and fragmentation reactions emerge if target atoms or molecules are embedded in an environment as it is the case in small clusters or in the condensed phase. These can be intermolecular energy and charge transfer processes or a completely modified fragmentation behavior of the molecular ions. Here we study low energy electron impact induced ionization with a multi-electron and ion imaging spectrometer (reaction microscope) and a supersonic gas jet target which can produce small clusters of various target species. Interatomic reactions are studied for the model system of weakly bound Ar$_{\mathrm{2}}$ dimers. Here, the coincident detection of three electrons and two ions gives detailed insight in interatomic Coulombic decay and radiative charge transfer processes. Such processes were also found in bio-relevant systems like water clusters. We studied pure and water-mixed clusters of tetrahydrofuran (C$_{\mathrm{4}}$H$_{\mathrm{8}}$O, THF) which is the simplest analog of deoxyribose in the DNA backbone. One observation is that ionization of the outermost valence orbital for the monomer leads to stable THF ions. In contrast if THF is bound to another THF or a water molecule the molecular ring breaks. In addition we identify intermolecular Coulombic decay induced by energy transfer from a water molecule ionized in the inner valence shell to the neighboring THF molecule. [Preview Abstract] |
Friday, October 14, 2016 9:00AM - 9:15AM |
UF1.00002: Inner shell excitation of Cu, Ag and Au Allan Stauffer, Robert McEachran The ground states of Cu, Ag and Au have the configuration nd$^{\mathrm{10}}$(n$+$1)s with n $=$ 3, 4 and 5. The lowest excited manifold for Cu and Au has the configuration nd$^{\mathrm{9}}$(n$+$1)s$^{\mathrm{2}}$ which is well separated from the next excited manifold nd$^{\mathrm{10}}$(n$+$1)p. However, for Ag, the lowest 4d$^{\mathrm{9}}$5s$^{\mathrm{2}}$ level with J $=$ 5/2 lies between the two levels of the 4d$^{\mathrm{10}}$5p manifold. In [1] we compared our Relativistic Distorted Wave calculations for the excitation of the 4d$^{\mathrm{10}}$5p manifold with experimental measurements which would have included a contribution from the 4d$^{\mathrm{9}}$5s$^{\mathrm{2}}$ J $=$ 5/2 level. While we do not expect the cross section for this forbidden transition to be large compared to the optical allowed transitions of the P levels, we decided to investigate excitation of these inner shell levels, in part because they are the lowest excited levels in Cu and Au, We will discuss the theoretical expressions for these excitations as well as give numerical results of our cross section calculations. [1] S. D. To\v{s}i\'{c}, V. Pej\v{c}ev, D. \v{S}evi\'{c}, R. P. McEachran, A. D. Stauffer, and B. P. Marinkovi\'{c}, Phys. Rev. A \textbf{91}, 052703 (2015) [Preview Abstract] |
Friday, October 14, 2016 9:15AM - 9:30AM |
UF1.00003: Differential cross sections for atomic ionization determined through the Bohm's velocity field. Juan M Randazzo, Lorenzo Ugo Ancarani, Flavio D Colavecchia Differential cross sections for atomic ionization are usually evaluated via the scattering amplitude defined as the transition matrix element between the initial and final states of the collision. R. Peterkop proposed an alternative approach -- known as flux formula - based on the relation linking the cross section to the ratio between the incident electronic flux and the emitted post-collisional one, through the asymptotic outgoing behavior of the scattering wave function. The flux formula was seen to fail for very unequal energy sharing when evaluating Single Differential Cross Sections (SDCSs) for the s-wave electron-hydrogen problem [1]. The procedure was thereafter abandoned. However, an alternative way of defining the electrons' local momenta by using the Bohm's velocity field was recently proposed [2], and it was found that SDCS results with a new definition of the energy fraction are well behaved on the whole range. In this contribution, we apply the modified quantum flux approach with local momenta to the electron impact ionization of hydrogen by considering the problem in its whole dimensionality, i.e., not only the s-wave contribution. We compare triple differential cross section results with other theoretical and experimental data, and this for several incident energies. [1] Baertschy M et al (1999) Phys. Rev. A 60, R13. [2] Randazzo J M and Ancarani L U (2015) Phys. Rev. A 82, 062706. [Preview Abstract] |
Friday, October 14, 2016 9:30AM - 9:45AM |
UF1.00004: Tracing the Origin of Orientation Effects in Excitation-Ionization Collisions A.L. Harris, T. Saxton The ionization of atoms and molecules is of great importance in many areas of physics, chemistry, and biology. Recently, interest has increased in collisions involving oriented target atoms or molecules. Specifically, several groups have investigated the effects of initial- and final-state target orientation on fully differential cross sections (FDCS). These results have shown that the shape of the FDCS for collisions with oriented atoms or ions can change significantly with the orientation direction of the target. We investigate some possible causes of these orientation effects using our 4-Body Distorted Wave model for both fully and double differential cross sections. [Preview Abstract] |
Friday, October 14, 2016 9:45AM - 10:00AM |
UF1.00005: Dissociative recombination of HCl$^{\mathrm{+}}$, H$_{\mathrm{2}}$Cl$^{\mathrm{+}}$, DCl$^{\mathrm{+}}$, and D$_{\mathrm{2}}$Cl$^{\mathrm{+}}$ (300-500 K), and the astrophysical relevance. T. M. Miller, J. P. Wiens, N. S. Shuman, A. A. Viggiano A review by Neufeld and Wolfire$^{\mathrm{1}}$ pointed out the unique chemistry of chlorine in the interstellar medium (ISM), including (a) Cl is the only species in the ISM with an IE less than that of H atom, which allows Cl$^{\mathrm{+}}$ to survive among an abundance of H atoms; (b) those Cl$^{\mathrm{+}}$ can react with H$_{\mathrm{2}}$ to form HCl$^{\mathrm{+}}$ exothermically; and (c) HCl$^{\mathrm{+}}$ can in turn react with another H$_{\mathrm{2}}$ to form H$_{\mathrm{2}}$Cl$^{\mathrm{+}}$. Only in the past 6 years have HCl$^{\mathrm{+}}$ and H$_{\mathrm{2}}$Cl$^{\mathrm{+}}$ been observed in the ISM. Modeling the true quantities of chlorinated species in the ISM requires knowing dissociative recombination (DR) kinetics for HCl$^{\mathrm{+}}$ and H$_{\mathrm{2}}$Cl$^{\mathrm{+}}$. We have used a flowing afterglow apparatus to measure DR rate coefficients at 300-500 K for HCl$^{\mathrm{+}}$, H$_{\mathrm{2}}$Cl$^{\mathrm{+}}$, DCl$^{\mathrm{+}}$, and D$_{\mathrm{2}}$Cl$^{\mathrm{+}}$. For 300 K, we find 7.7 x 10$^{\mathrm{-8}}$ cm$^{\mathrm{3}}$/s (HCl$^{\mathrm{+}})$, 2.6 x 10$^{\mathrm{-7}}$ cm3/s (H$_{\mathrm{2}}$Cl$^{\mathrm{+}})$, and 1.1 x 10$^{\mathrm{-7}}$ cm$^{\mathrm{3}}$/s (D$_{\mathrm{2}}$Cl$^{\mathrm{+}})$, each with $\sim$ 35{\%} accuracy. The DR rate coefficient for DCl$+$ is too slow for us to measure, especially in the face of dealing with mixed H/D species formed in apparatus feedlines when introducing DCl. Novotn\'{y}, et al.$^{\mathrm{2}}$ have carried out storage ring measurements in Heidelberg on this problem and will soon report new results over a wide electron energy range and including neutral product information. \\ \\$1.$ D. A. Neufeld and M. G. Wolfire, Astrophys. J. \textbf{706}, 1594 (2009).\newline 2. O. Novotn\'{y}, et al., Astrophys. J. \textbf{777}, 54 (2013). [Preview Abstract] |
Friday, October 14, 2016 10:00AM - 10:15AM |
UF1.00006: Multiple scattering in laser assisted free-free scattering experiments B.A. deHarak, M. R. McGill, S. Kim, C. M. Weaver, B. N. Kim, N.L.S. Martin We present the results of a series of Monte Carlo simulations of the laser assisted free-free experiments reported by Wallbank and Holmes$^{\mathrm{1}}$. Our simulations make use of the cross sections calculated by Fursa and Bray$^{\mathrm{2}}$, and the Kroll-Watson approximation$^{\mathrm{3}}$ to account for the effect of the laser field on the scattering process. The target density for these simulations is based on the experimental conditions reported by Wallbank and Holmes$^{\mathrm{4}}$. We find that our results are in reasonable agreement with the experimental data.\\ \\$1.$ B. Wallbank and J. K. Holmes, Phys. Rev. A 48, R2515 (1993). \newline 2. D. V. Fursa and I. Bray, Phys. Rev. A 52, 1279 (1995). \newline 3. N. M. Kroll and K. M. Watson, Phys. Rev. A 8, 804 (1973). \newline4. B. Wallbank and J. K. Holmes, Can. J. Phys. 79, 1237 (2001). [Preview Abstract] |
Friday, October 14, 2016 10:15AM - 10:30AM |
UF1.00007: Ionization Cross Sections for Electron Collision from PCl$_{\mathrm{5}}$. Satyendra Pal Phosphorous pentachloride PCl$_{\mathrm{5}}$ and its free radicals are widely used in plasma, plasma-assisted etching and deposition of phosphorous layers in the fabrication of microelectronic components and other high technological devices. Keeping the wide interest of the molecule, in the present work, we report the calculations for differential cross sections as a function of secondary and or ejected electron energy in the ionization of PCl$_{\mathrm{5\thinspace }}$by electron collision corresponding into the production of various cations singly charged ions through direct and dissociative ionization processes at a fixed incident electron energy of 100 eV. The modified Jain-Khare semi-empirical formalism [1-2] based on oscillator strength has been employed for evaluation of cross sections. The corresponding derived partial integral cross sections in terms of the partial ionization cross sections for these cations in the energy range varying from ionization threshold to 1000 eV, revealed a reasonably good agreement with the available data. In addition to the differential and integral ionization cross sections, we have also calculated the ionization rate coefficients using the evaluated partial ionization cross sections and the Maxwell-Boltzmann distribution as a function of electron energy. [Preview Abstract] |
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