Bulletin of the American Physical Society
41st Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 55, Number 5
Tuesday–Saturday, May 25–29, 2010; Houston, Texas
Session L5: Collisions Involving Charged Particles |
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Chair: Gregory Childers, California State University, Fullerton Room: Arboretum I-III |
Thursday, May 27, 2010 2:00PM - 2:12PM |
L5.00001: Electron-impact ionization-excitation of helium in the quasi-photon regime Jean Marcel Ngoko Djiokap, Emmanuel Foumouo, Moise Godfroy Kwato Njock, Xavier Urbain, Bernard Piraux The triply differential cross section (TDCS) for ionization and excitation of helium (leaving the residual ion in the n=2 excited states) is evaluated for the kinematics considered experimentally by Dupr\'{e} \emph{et al.} [J. Phys. B \textbf{25}, 259 (1992)]. The interaction of the incident electron with the target is described in first order, while that of the ejected electron with the residual ion is treated accurately within the Jacobi matrix method formalism. In the quasi-photon limit and for small ejected electron energies, the presence of series of doubly excited states below the n=3 single ionization threshold in helium makes the TDCS extremely sensitive to both the energy and angle of the ejected electron. Comparison of our theoretical results (convoluted both in energy and angle) with the experimental data demonstrates the importance of an accurate description of the He spectrum. A possible two-step mechanism involving single ionization of the target followed by excitation of the core electron is proposed in order to explain the remaining discrepancies. [Preview Abstract] |
Thursday, May 27, 2010 2:12PM - 2:24PM |
L5.00002: Dissociative Electron Attachment to Polyatomics Slim Chourou, Ann Orel Previous work on the dissociative electron attachment (DEA) to acetylene, hydrogen cyanide and its isomer and cyano-acetylene shows that the dissociation process for these systems may exhibit intrinsically polyatomic effects. We present a comparative summary of the study of these species believed to play a role in the chemistry of interstellar media and to present key elements in the prebiotic synthesis in early Earth. Our treatment was carried out in the low energy range (0-6 eV for HCCH and HCN/HNC and 0-12 eV for HCCCN) using a suitable coordinate system that allows taking into account distortions in the symmetry of the polyatomic target molecule. The Complex Kohn Variational Method was used to determine the resonance energies and the autoionization widths; thus allowing the construction of the multidimensional complex potential energy surfaces of the temporary polyatomic anion. The dissociation dynamics calculation is then performed using the Multi-Configuration Time-Dependent Hartree approach to compute the DEA cross sections. [Preview Abstract] |
Thursday, May 27, 2010 2:24PM - 2:36PM |
L5.00003: Inclusion of two vibrational modes in dissociative electron attachment and vibrational excitation of CF$_3$Cl revisited Michal Tarana, Karel Houfek, Ilya I. Fabrikant We present a study of multimode effects in vibrational excitation of CF$_3$Cl molecule by electron impact as well as in the dissociative electron attachment. We use a time-independent version of the local complex potential theory. Symmetric stretch C-Cl vibrations $\nu_3$ and symmetric deformation (or so-called ``umbrella'') vibrations $\nu_2$ are included. The approach used in the present study was described previously. The complex potential energy surfaces used in the present calculations were obtained using the ab initio R-matrix method. In our previous study of the dissociative electron attachment we found an increase of the total cross section as compared to one-dimensional calculations. This was explained by an increase of the anion survival probability due to model-like width function used. Our present calculations use the ab initio width and lead to smaller survival probability. [Preview Abstract] |
Thursday, May 27, 2010 2:36PM - 2:48PM |
L5.00004: Imaging of the dynamics on the 2B2 resonance in dissociative electron attachement to water H. Adaniya, T. Osipov, A. Belkacem We used a COLTRIMS-like apparatus to study experimentally dissociative electron attachment to gas phase water molecules. The momentum imaging technique yields some very unique insight into the dissociation dynamics involving the 2B2 resonance. Dynamics involving transition through a conical intersection connecting the 2B2 and 2A1 surfaces is clearly observed in the momentum sphere of the H- channel. The transition through the conical intersection is found to depend exquisitely on the electron energy. The angular distribution of H- exhibits a very complicated pattern and comparison to theory sheds some unique light on the dynamics through the conical intersection. [Preview Abstract] |
Thursday, May 27, 2010 2:48PM - 3:00PM |
L5.00005: Dissociation of CO molecular ions produced in collisions with 2.5 MeV/u Xe(34+) Vladimir Horvat, Rand Watson The breakup of CO molecular ions following multiple electron removal by 2.5 MeV/u Xe(34+) ions has been re-examined by means of recoil-ion momentum spectroscopy (RIMS) using a position- sensitive 80 mm diam microchannel plate detector backed by a hexagonal delay-line anode. Fragment pairs originating from molecular ions with charge up to 8e were isolated and analyzed in order to determine the distribution of total recoil-ion kinetic energy released in the breakup (KER) and the angular distribution of the ion momenta with respect to the beam direction. The results are significantly improved compared to those reported previously in terms of accuracy, resolution and overall counting statistics. The KER distributions were determined with a resolution of 1 eV (FWHM) or better. While most of the KER distributions displayed only a single broad asymmetric peak, a few of them showed a combination of well- resolved narrow peaks and partially overlapping broader peaks. The angular distributions were found to be essentially isotropic, except for those ion pairs originating from highly charged molecular ions, which indicated a slight preference for emission in the beam direction. [Preview Abstract] |
Thursday, May 27, 2010 3:00PM - 3:12PM |
L5.00006: Absolute Merged Beams Charge Exchange Cross Sections for H- like and Fully Striped Ions on Atomic Hydrogen I.N. Draganic, D. McCammon, P.C. Stancil, C.C. Havener Absolute total cross sections for single electron capture by hydrogen-like ions of carbon, nitrogen, oxygen and fully-stripped oxygen ions from atomic hydrogen are being measured in the relative collision energy range of 0.01-20 keV/amu. Measurements are performed using a merged-beams technique with intensive highly charged ion beams extracted from an all-permanent-magnet ECR ion source on a 250 kV platform. At the higher energies the measurements are in good agreement with previous H-oven measurements (Meyer et al. PRA 32, 3310(1985)) but unexpectedly decrease toward lower energies. The experimental results will be discussed and compared with different theoretical calculations. [Preview Abstract] |
Thursday, May 27, 2010 3:12PM - 3:24PM |
L5.00007: Asymmetry of electron cusps in the strong perturbation domain Siegbert Hagmann, Chunlei Liao, Bertold Kraessig One of the most remarkable features in the doubly differential cross sections for electron emission in ion-atom collisions is the prominent electron capture to continuum (ECC) cusp. For small perturbations q$_{proj}$/v$_{proj }<<$1 the skewness of the ECC towards lower lab velocities served as evidence for second Born terms in the electron capture amplitude. A tacit assumption in these considerations has always been the one-active-electron model. We have now shown that for strong perturbations, 0.35AMeV I$^{23+}$ + Ar, the dominant contribution to the ECC cusp arises from simultaneous capture of several electrons into bound states changing its nature into a multiple transfer ionization (mTI) cusp. A remarkable observation is the strong dependence of the asymmetry on the number of electrons transferred with the asymmetry even switching from negative at pure ECC to strongly positive, i.e. skewness towards higher laboratory velocities, in the multiple TI region. [Preview Abstract] |
Thursday, May 27, 2010 3:24PM - 3:36PM |
L5.00008: Capture and loss correlated to x-ray emission in O$^{5+}$ on Ar collisions T. Elkafrawy, A. Kayani, J.A. Tanis K x rays emitted from projectile and target atoms correlated to projectile capture and loss charge states were investigated for 18 and 25 MeV O$^{5+}$ on Ar collisions. Single capture coincidences are of interest as they translate to a target electron transferred to the projectile ion causing excitation followed by x-ray emission. Such a process is the signature for resonant charge transfer and excitation followed by x-ray emission (RTEX). We plan to cover the energy range for such RTEX cross sections. Heavy targets like Ar are not commonly used so we seek to test RTE theory for such targets. It is also planned to run for O$^{7+}$ to increase the capture probability. We are planning to measure single loss coincidences as well so we can study projectile ionization associated with projectile and target K-x-ray emission. In the case of Li-like O, K emission correlated with O$^{6+}$ can be obtained by K-shell ionization accompanied by 2s $\to $ 2p (or higher) excitation followed by a 2p $\to $1s decay, or the same emission line can be obtained from K-shell excitation to 2p (or higher) accompanied by ionization of the 2s electron. On the other hand, Ar K emission can be obtained by K-shell ionization followed by 2p $\to $1s or 3p $\to $1s decay, or possibly by K-shell excitation to 3d or higher levels. [Preview Abstract] |
Thursday, May 27, 2010 3:36PM - 3:48PM |
L5.00009: Magnetic-Dipole Lines in Highly-Charged Ions of Tungsten J. Reader, Yu. Ralchenko, I.N. Draganic, J.D. Gillaspy, J.N. Tan, J.M. Pomeroy, S.M. Brewer We observed spectra of highly-charged ions of tungsten in the region 10-20 nm with the NIST electron beam ion trap (EBIT). Electron beam energies varied from 4.5 to 7.0 keV. Ionization stages were distinguished by studying intensities of observed lines at the different beam energies. Using an extensive collisional-radiative model, we identifies about 40 new magnetic-dipole (M1) lines, mainly within the ground state configurations 3d$^n$ (n=1-9) for charge states W$^{47+}$ to W$^{55+}$. The results complement our two previous studies of W in this region, carried out at lower and higher beam energies [1,2].The intensities of the M1 lines are highly sensitive to electron densities in various plasmas. We will discuss their application to density diagnostics in hot plasmas such as will be found in the ITER tokamak. \\[4pt] [1] Yu. Ralchenko et al, J. Phys. B: At. Mol. Opt. Phys. {\bf 40}, 3861 (2007).\\[0pt] [2] Yu. Ralchenko et al, J. Phys. B: At. Mol. Opt. Phys. {\bf 41}, 021003 (2008). [Preview Abstract] |
Thursday, May 27, 2010 3:48PM - 4:00PM |
L5.00010: High Energy and Temperature Features in Photoionization and Electron-Ion Recombination of Fe~XVII Sultana Nahar, A. Pradhan, W. Eissner A comprehensive study of high-accuracy photoionization cross sections is carried out using the relativistic Breit-Pauli R-matrix method for (h$\nu$~+~Fe~XVII $\rightarrow$ Fe~XVIII + e). Owing to its importance in high-temperature plasmas, the calculations cover a large energy range to include core excitations of $n=2$ and $n=3$ complexes. This allows opening up the myriad of photoexciation-of-core (PEC) resonances belonging to the $n$ = 3 levels not heretofore considered. The cross sections reveal extended PEC resonances enhancing their effective values by orders of magnitude above the background. The close coupling wave function expansion of 60 core levels correspond to a wide energy separation of $\sim$65 Ry between the $n=2$ and $n=3$ complexes. Yet, the high-lying $n$ = 3 levels introduce resonant structures extending from much lower to higher energies, and are far larger than those due to n=2 excitations. Comparisons with the Opacity Project data, and other calculations, show that the currently available cross sections severely underestimated. The total electron-ion recombination rate coefficients obtained using these cross sections reveal a dominant broad peak at high temperature. [Preview Abstract] |
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