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 O1: Poster Session II |
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Room: Knoxville Convention Center Ballroom AB |
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O1.00001: ATOMIC, MOLECULAR AND CHARGED PARTICLE COLLISIONS AND SPECIAL TOPICS |
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O1.00002: New Collisional Ionization Equilibrium Calculations for Optically Thin Plasmas P. Bryans, W. Mitthumsiri, D.W. Savin, N.R. Badnell, T.W. Gorczyca, J.M. Laming Reliably interpreting spectra from electron-ionized cosmic plasmas requires accurate ionization balance calculations for the plasma in question. However, much of the atomic data needed for these calculations have not been generated using modern theoretical methods and their reliability are often highly suspect. We have carried out state-of-the-art calculations of dielectronic recombination (DR) rate coefficients for the hydrogenic through Na-like ions of all elements from He to Zn. We have also carried out state-of-the-art radiative recombination (RR) rate coefficient calculations for the bare through Na-like ions of all elements from H to Zn. Using our data and the recommended electron impact ionization data of [1], we present improved collisional ionization equilibrium calculations. We compare our calculated fractional ionic abundances using these data with those presented by [1] for all elements from H to Ni, and with the fractional abundances derived from the modern DR and RR calculations of [2] for Mg, Si, S, Ar, Ca, Fe, and Ni. \\ $[1]$ Mazzotta, P. et al., 1998, A\&AS, 133, 403 \\ $[2]$ Gu, M. F., 2003a, ApJ, 590, 1131; 2003b, 589, 1085; 2004, 153, 389 [Preview Abstract] |
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O1.00003: Dielectronic Recombination of Mg$^{2+}$: Importance of Relaxation and Term-Dependence J. Fu, D. Nikolic, T.W. Gorczyca, D.W. Savin, N.R. Badnell Discrepancies between two recent theoretical results [1,2] for the dielectronic recombination (DR) of low-charged Ne-like ions have been found and are attributed to orbital relaxation and term-dependence. For Ne-like Mg$^{2+}$, these effects cause differences in the maximum DR rate coefficients by more than 50\% between similar calculations employing orbitals determined from different optimization procedures. We resolve this discrepancy by utilizing a separate basis of non-orthogonal orbitals for the $2s^22p^6$ ground and $2l\rightarrow 3l^\prime$ excited states, and present new, more accurate DR rate coefficients for low-charged Ne-like ions.\newline $[1]$ O. Zatsarinny, et al., A\&A {\bf 426}, 699 (2004).\newline $[2]$ M. F. Gu, ApJ {\bf 590}, 1131 (2003). [Preview Abstract] |
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O1.00004: Dielectronic Recombination of Argon-Like Scandium: A Revisited Theoretical Investigation D. Nikolic, J. Fu, T.W. Gorczyca, D.W. Savin, N.R. Badnell As an improvement to an earlier study [1], we have calculated dielectronic recombination rate coefficient spectra for Sc$^{3+}$ ions as a test case toward the assembly of a database [2] for the Ar-like isoelectronic sequence required for modeling of dynamic finite-density plasmas [3]. Our theoretical spectra contain dominant $\Delta N=0$ and $\Delta N=1$ core excitation channels and exhibit nearly all features found in a recent ion storage ring experiment [4]. In order to compare Maxwellian-averaged rate coefficients, which are of main interest to the astrophysics community, we have developed an iterative deconvolution procedure that enables us to extract the cross section from storage ring data. After folding the resultant cross section with a Maxwellian electron velocity distribution, theoretical and experimental rate coefficient spectra agree to better than $\sim$18\%, subject to field re-ionization effects via high Rydberg states in Sc$^{2+}$ ions.\newline $[1]$ T. W. Gorczyca, et al., Phys. Rev. A {\bf 56}, 4742 (1997).\newline $[2]$ http://homepages.wmich.edu/$\sim$gorczyca/drdata/.\newline $[3]$ N. R. Badnell, et al., A\&A {\bf 406}, 1151 (2003).\newline $[4]$ S. Schippers, et al., Phys. Rev. A {\bf 65}, 042723 (2002). [Preview Abstract] |
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O1.00005: Dielectronic Recombination In Active Galactic Nuclei D. Luki\' c, D.W. Savin, M. Schnell, C. Brandau, E. Schmidt, S. Schippers, A. M\"{u}ller, M. Lestinsky, F. Sprenger, A. Wolf, Z. Altun, N.R. Badnell Recent X-ray satelitte observations of active galactic nuclei point out shortcomings in our understanding of low temperature dielectronic recombination (DR) for iron M- shell ions. In order to resolve this issue and to provide reliable iron M-shell DR data for modeling astrophysical plasmas, we are carrying out a series of laboratory measurements using the heavy-ion Test Storage Ring at the Max- Plank-Institute for Nuclear Physics in Heidelberg, Germany. Storage rings are currently the only laboratory method capable of studying low temperature DR. We use our results to produce experimentally- derived DR rate coefficients. We are also providing our data to atomic theorist to benchmark their DR calculations. Here we will report our recent DR results for selected Fe M-shell ions. At temperatures where these ions are predicted to form in photoionized gas, we find a significant discrepancy between our experimental results and previously recommended DR rate coefficients. [Preview Abstract] |
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O1.00006: Three-Body Recombination in One-Dimensional Systems Nirav Mehta, Chris Greene, Brett Esry Few-body processes in one dimension are of critical importance in the understanding of quantum gases in highly confined trap geometries. We consider three- particle systems for a variety of 2-body interactions using the adiabatic hyperspherical formalism. In particular, we numerically demonstrate that the three-body recombination probability vanishes for zero- range interactions, verifying an old result found by McGuire (1964). We further consider the recombination probability for a variety of finite range interactions and study the dependence on the one- dimensional scattering length. Finally, we present some preliminary results for four-body systems. [Preview Abstract] |
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O1.00007: Computation of Sturmian States for H$_2^+$-like Ions in a Scaled Coordinate Representation. V.M. Chupryna, A.N. Saveliev, S. Yu. Ovchinnikov, J.H. Macek Computing Sturmian states is an important step in calculations of cross-sections in atomic physics. The dynamical equations in a scaled representation include the two-center Coulomb potential, an isotropic harmonic oscillator potential, and a rotation operator [1]. We obtain Sturmian states using solutions of a singular 3-D homogeneous Fredholm integral equation of the second kind. Numerical results will be presented. \newline \newline [1] S.Yu. Ovchinnikov, G.N. Ogurtsov, J.H. Macek, Yu.S. Gordeev, Physics Reports, {\bf 389}, 119 (2004) [Preview Abstract] |
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O1.00008: Search for High-Frequency Structures in Electron Emission from H$_{2}$ by 2 MeV/u C$^{6+}$ Impact Daniel Garvin, Diane Strohschein, Jamie Baran, John Tanis Electron emission from H$_{2}$ by fast ion impact has been shown to produce interference effects in the spectra of ejected electrons.$^{1}$ This interference is analogous to Young's double-slit experiment. However, unlike Young's experiment, intramolecular scattering can give rise to secondary interferences with 2-3 times higher frequencies.$^{1}$ Recently, still higher-frequency structures were observed for p+H$_{2}$ collisions, and it was suggested that this might occur when the impinging particle is identical with the H$_{2}$ target nuclei.$^{2}$ To test this hypothesis, electron emission was measured for several observation angles using C$^{6+}$ projectiles instead of H$^{+}$. This work was done using the WMU tandem Van de Graaff. If no high-frequency structure occurs for C$^{6+}$, then the identical particle explanation emerges as a stronger possibility; if the structure persists, then another explanation must be sought. Preliminary examination of the data suggests high-frequency structures, but further detailed analysis is needed. $^{1}$N. Stolterfoht \textit{et al}., Phys. Rev. Lett. \textbf{87}, 023201 (2001); and \textbf{69}, 012701 (2004). $^{2}$S. Hossain \textit{et al}., Phys. Rev. A\textbf{72}, 010701 (2005). [Preview Abstract] |
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O1.00009: Dissociative Ionization of C$_{2}$H$_{2}$ by 1MeV/u C$^{5+}$ projectiles J.C. Thompson, Allen Landers, E.J. Clothiaux, J.B. Williams, J.M. Sanders We have measured the multiple breakup channels resulting from the interaction between a 1MeV/u C$^{5+}$ projectile ion and acetylene C$_{2}$H$_{2}$ target using Cold Target Recoil-Ion Momentum Spectroscopy (COLTRIMS). The many observed fragment ion-pairs include [H$^{+}$, H$^{+}$], [H$^{+}$, C$^{2+}$], [H$^{+}$, C$^{+}$], [H$^{+}$, C$_{2}^{+}$], [H$^{+}$, C$_{2}$H$^{+}$], [C$^{+}$, C$^{2+}$], [C$^{2+}$, C$^{3+}$], and [C$^{+}$, C$^{+}$]. Preliminary examination of the two-body [H$^{+}$, C$_{2}$H$^{+}$] breakup channel shows an isotropic fragment-momentum distribution, indicating that this dissociation is equally probable for all orientations of the intermolecular axis relative to the projectile beam within the axial recoil approximation. Further investigations, including determination of branching ratios and additional momentum calculations for the other breakup channels are underway. [Preview Abstract] |
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O1.00010: Dissociative Ionization of CO$_{2}$ by 1MeV/u C$^{5+}$ projectiles A.L. Landers, J.C. Thompson, E.J. Clothiaux, J.B. Williams, J.M. Sanders We have measured the dissociative ionization of carbon dioxide (CO$_{2})$ by 1MeV/u C$^{5+}$ projectiles using a Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS). By far the dominant fragment ion pair produced in these collisions is the two body breakup [O$^{+}$, CO$^{+}$]. We have momentum analyzed this channel and find that although the dissociation is forward/backward symmetric, production of the ion-pair is clearly more likely for ions oriented perpendicular to the direction of incoming projectile. Additional exploration of the correlation between the kinetic energy release and molecule orientation is in progress. [Preview Abstract] |
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O1.00011: Fragmentation of Hydrocarbon Molecules following Ionization by 1 MeV/u C$^{5+}$ Projectiles J.M. Sanders, J.B. Williams, J.C. Thompson, E.J. Clothiaux, A.L. Landers Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) has been used to identify fragmentation channels for breakup of CH$_4$, C$_2$H$_2$, C$_2$H$_4$, C$_2$H$_6$, C$_3$H$_6$, and C$_3$H$_8$ following ionization by 1 MeV/u C$^{5+}$ projectiles. Those channels for which the molecule dissociates into at least two ion fragments are identified through an ion time-of-flight coincidence map. Relative probabilities for each of these dissociative reaction channels are then determined. [Preview Abstract] |
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O1.00012: Angular distribution in the dissociation of H$_2$O by swift heavy ions R. Cabrera-Trujillo, N. Stolterfoht, Y. {\"O}hrn, E. Deumens, J.R. Sabin In this work, we present calculations of the angular distribution of the products of the dissociation of water molecules when bombarded with He$^{q+}$ for projectile energies between 1 and 5 keV. Here $q=0,1,2$ is the charge of the incoming ion. Our theoretical results are based on the Electron-Nuclear Dynamics formalism (END)\footnote{ E. Deumens, A. Diz, R. Longo, and Y. {\"O}hrn, Rev. Mod. Phys. {\bf 66}, 917 (1994).} We present results for the dissociation cross section, charge transfer cross section, the stopping cross section (nuclear and electronic) for the projectiles, and the angular distribution of He$^{q+}$, H, OH, and O. [Preview Abstract] |
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O1.00013: Translational spectroscopy of the CO$^{+ }$ fragments from collision-induced dissociation of the molecular ions with He at 6 keV Carmen Cisneros, Ignacio Alvarez, Alfonso Guerrero, Gladys Garcia, Enrique Mejia-Ospino We measured the kinetic energy distribution of C$^{+}$ and O$^{+}$ fragments arising from 6 keV collision induced dissociation of CO$^{+}$ molecular ion in collision with He. With the appropriate transformation from the laboratory energy distribution to the center of mass of the molecular ion it was obtained the spectrum of the energy liberated and shared by the molecular fragments. The process gives rise to fragments with energies as large as $\sim $ 9.9 eV. In the spectrum some of fragment energies observed are in good agreement with the results of Krishnamurthi et al (1) and Caraby (2) et al. Additionally, we have observed fragments with energies corresponding to other available transitions not been observed before. 1. - V. Krishnamurthi et al. Phys. Rev. A. \textbf{44}, 5460 (1991) 2. - C. Caraby, et al. Eur. Phys. J. D. \textbf{2}, 53 (1998) [Preview Abstract] |
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O1.00014: Isotopic effects in collision induced dissociation of HD$^{+}$ on atomic targets. Nora G. Johnson, A.M. Sayler, J.W. Maseberg, M.A. Smith, K.D. Carnes, I. Ben-Itzhak Isotopic effects in collision induced dissociation of molecules and molecular ions have been studied for many years yielding conflicting reports on branching ratios. Of particular interest is HD$^{+}$ dissociating into H$^{+}$ + D or H + D$^{+}$ due to the simplicity of the molecular ion. Previous studies have resulted in all possible conclusions: some say the H$^{+}$ + D channel dominates, some say the H + D$^{+}$ channel dominates, while others say there is no isotopic preference. In our experiment, the fragments were measured in coincidence by a 3D momentum imaging system. A weak longitudinal electric field following a field free collision region was utilized for distinguishing the dissociation channels of interest in order to resolve the discrepancy of this particular branching ratio. In addition, two dissociation mechanisms were identified, namely electronic excitation in soft collisions and vibrational excitation in hard collisions. [Preview Abstract] |
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O1.00015: Rotational Quenching of CO by collision with H$_2$ Benhui Yang, P. Stancil, N. Balakrishnan, R. Forrey Collisions of CO with H$_2$ have been the subject of numerous experimental and theoretical investigations, due in part, to their high abundance in a wide range of astronomical environments. We present quantum mechanical scattering calculations for the rotational relaxation of CO in collisions with para- and ortho-H$_2$ using the quantum close-coupling and coupled states approaches. State-to-state cross sections for the pure rotational quenching of the $j$=1, 2, $\cdots$, 10 levels of CO were computed using the new H$_2$-CO potential surface of Jankowski and Szalewicz (JS, 2005). The results are compared to previous calculations and to cross sections obtained with the 1998 potential of JS. Rate coefficients for astrophysically relevant temperatures, as well as in the ultracold regime, are also obtained. We acknowledge support from NASA grant NNG04GM59G. [Preview Abstract] |
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O1.00016: Three-Body Scattering via the Faddeev Scheme in Configuration Space. Nolan Samboy, George Rawitscher, Robin C\^ot\'e, Walter Gloeckle We are adapting Faddeev Equations to three-body atomic and molecular systems by reformulating them in the configuration space using integral equations with Green’s functions. The non-additive forces between the three bodies, as well as transitions between rearrangement channels, are included rigorously. The method includes all partial waves, but we will present only the $s$-wave pieces relevant to ultracold collisions. This formulation appears to be a valuable alternative to current approaches based on solving the differential Schroedinger Equation. [Preview Abstract] |
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O1.00017: Experimental Study of Hyperfine State Changing Collisions in the Cs(6\textit{P}$_{1/2}$) State using Argon Perturbers Laurie Morgus, Tyler Morgus, John Huennekens A two-step excitation experiment has been employed to measure the collisional rate coefficient and to study the velocity distribution of Cs(6$P_{1/2})$ atoms that have undergone a single hyperfine state changing collision with Ar. First, a single-mode, cw Ti:Sapphire laser is tuned to line center of the 6$S_{1/2}(F$ = 4) $\to $ 6$P_{1/2}(F'$ = 3 or 4) transition. Then, the frequency of a single-mode cw dye laser is scanned over the various 6$P_{1/2} \quad \to $ 8$S_{1/2}$ hyperfine transitions to measure relative populations in the 6$P_{1/2}$ hyperfine levels. Absorption of the probe laser is monitored by detecting 8$S_{1/2} \quad \to $ 6$P_{3/2}$ fluorescence. The experiment is conducted at room temperature, where the Cs density is low ($n \quad \sim $ 3.4 $\times $ 10$^{10}$ atoms cm$^{-3})$,and thus Cs-Cs collisions are negligible. The Ar pressure is varied from 0 -- 3 Torr, and Cs-Ar collisions cause transfer of population from the directly excited 6$P_{1/2}(F')$ level to the other 6$P_{1/2}$ hyperfine level. The data are analyzed in the single collision, strong pump, and weak probe limits to yield the rate coefficient for Cs(6$P_{1/2})$-Ar hyperfine state changing collisions. In addition, the one-dimensional velocity changing collision kernel for Cs(6$P_{1/2})$ atoms prepared with $v_{z}$ = 0 that undergo $F'$ = 3 $\leftrightarrow \quad F'$ = 4 hyperfine state changing collisions is reported, as is the 6$P_{1/2} \quad \to $ 8$S_{1/2}$ argon pressure broadening rate. [Preview Abstract] |
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O1.00018: Velocity dependence of the Cs($5D$) + Cs($6P$) $\rightarrow$ Cs($7D$) + Cs($6S$) energy pooling process S. Sweeney, J. Huennekens, A. Marks We describe recent progress on an experiment using molecular photodissociation to study the velocity dependences of atomic collision processes in a vapor. Fast Cs($5D$) atoms with well defined velocities are created via the molecular photodissociation process Cs$_{2}$ + $h\nu \rightarrow$ Cs($6S$) + Cs($5D$) + $\Delta E$. The speed of the $5D$ atoms is controlled by the pulsed photodissociation laser frequency, and the speed distribution is measured as a function of time by scanning a cw probe laser over the $5D \rightarrow 5F$ Doppler-broadened line shape. As time progresses, the initial non-thermal $5D$ velocity distribution relaxes toward the thermal Maxwell-Boltzmann distribution, and the measured line shapes are used to determine thermalization rates. In our recent work, we add to the vapor a large thermal population of Cs($6P$) atoms (created using a cw laser), and measure the $7D$ fluorescence (due to $5D + 6P \rightarrow 7D + 6S$ energy pooling) in the early time before the fast $5D$ atoms thermalize. We obtain a relative measure of the energy pooling cross section by dividing the $7D$ signal by measures of the steady-state $6P$ density and the transient $5D$ density. By then varying the photodissociation laser frequency we can map the relative velocity dependence of the energy pooling cross section. Our preliminary results indicate that a significant velocity dependence does exist, but further work is required to verify these results. [Preview Abstract] |
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O1.00019: Prospects for sympathetic cooling of $OH$ in a cold $Rb$ enviroment. Manuel Lara, John L. Bohn, Daniel E. Potter, Pavel Soldan, Jeremy M. Hutson We present collision cross-sections for elastic and spin-changing inelastic proc esses in the Rb+OH($^2 \Pi$) system with emphasis on the implications for sympathetic cooling of OH molecule s within an enviroment of ultracold Rb atoms. We employ a system of coupled diabatic potenti al energy surfaces, built from accurate {\em ab initio} electronic structure calculations. To perform scattering calculations, we expand the problem in a set of channels suitable for accounting for $M$ levels in the presence of electic and/or magnetic fields. Because of the large number of scattering channels involved, we find it necessary to make suitable approximatio ns. To account for the hyperfine structure of the collision partners we develop a ` `corrected'' frame-transformation that includes the most of the hyperfine interaction in the short-range region. By contrast, the long range channels are selected by an energy criterion . [Preview Abstract] |
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O1.00020: Scattering lengths and low-energy cross sections for Na-He collisions Chih-Yuan Lin, Phillip C. Stancil, H.-Peter Liebermann, Peter Funke, Robert J. Buenker With the {\it ab initio} adiabatic potentials and nonadiabatic radial and rotational couplings obtained from multireference single- and double-excitation configuration interaction approach, we carried out scattering calculations by the quantum- mechanical molecular-orbital close-coupling method. The low- energy collisions of elastic and inelastic processes have been investigated. Scattering lengths as a function of the reduced mass and collision energy are obtained and their relation to bound states are illustrated. Elastic and inelastic cross sections for energies between 0.1 neV and 10 eV are also presented. [Preview Abstract] |
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O1.00021: Near-threshold electron-impact excitation of the 3$^3$S and 3$^1$S states of Helium. M. Stepanovic, M. Minic, J. Jureta, J. Kurepa, D. Cvejanovic, S. Cvejanovic, Oleg Zatsarinny, Klaus Bartschat A joint experimental and theoretical study of near-threshold electron-impact excitation of the 3$^3$S and 3$^1$S states in helium is reported. A high-resolution electron spectrometer was used to study integral cross sections in the energy region of the $n=3-5$ negative-ion resonances. Photons were detected from the decay of the two states and the observed intensities were normalised to theoretical predictions from a new $B$-spline $R$-matrix (close-coupling) method, which allows for non-orthogonal orbitals to improve the target description. Remarkable agreement between experiment and theory is demonstrated in both the overall energy dependence of the cross section and the fine details of a wealth of resonance structures. A detailed list of the resonances in the individual partial waves, along with their widths, will be presented. [Preview Abstract] |
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O1.00022: Fluorescence Polarization of Helium Negative Ion Resonances Excited by Polarized Electron Impact J.W. Maseberg, T.J. Gay The helium triply-excited negative ion $2s^22p\,^2\!P$ and $2s2p^2\,^2\!D$ resonance states have been previously investigated by numerous researchers. Observation of the resonant channels in the He $3\,^3\!D\rightarrow2\,^3\!P$ 587.5 nm transition is possible because of cascade contributions. Previous measurements of the intensity and linear polarization fraction of this line due to excitation by unpolarized electrons are available [1-3]. We report the integrated Stokes parameters P$_1$, P$_2$, and P$_3$ for excitation by spin-polarized electrons. Our null result for P$_2$ indicates that even though these resonances are long lived ($\sim$200 ps), magnetic forces acting on the temporarily-captured electron are negligible. Values of P$_3$ show no statistically-significant variation from their asymptotic non-resonant levels. [1] A. Defrance, J. Phys. B \textbf{13}, 1229 (1980). [2] H. Batelaan \textit{et al}., J. Phys. B \textbf{24}, 5151 (1991). [3] D. Cvejanovi\'{c} \textit{et al}., J. Phys. B \textbf{33}, 2265 (2000). [Preview Abstract] |
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O1.00023: Electron impact excitation of atomic oxygen. Charles Malone, Paul Johnson, William McConkey, Joseph Ajello, Isik Kanik A composite beam of atomic and molecular oxygen was generated using a microwave cavity and special gas handling surfaces to minimize O recombination. Further, a magnetically constrained electron beam of well-known energy was used to excite the oxygen gas target. The resulting fluorescence was probed using a 0.2m vacuum monochromator and a CsI-coated channel electron multiplier. Spectral scans (intensity as a function of wavelength with fixed impact energy) and excitation function scans (intensity as a function of impact energy for a wavelength interval) were used to quantify the VUV emissions of atomic oxygen. The monitored dissociation fraction and known 130.4nm atomic oxygen emission cross section [Johnson et al., Can. J. Phys. 83, 589-616 (2005)] were used for normalization of the measured excitation functions. Resulting VUV emission cross sections of atomic oxygen via electron impact will be presented. [Preview Abstract] |
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O1.00024: Electron impact excitation of oxygen to a metastable state Hari P. Saha, Swaraj Tayal The recently extended MCHF method for multi-open channels$^{1}$ is applied to the excitation of oxygen from the ground 2p$^{4}$ ($^{3}$P) to the metastable 2p$^{3}$3s ($^{5} $S) state. This method solves coupled integro-differential equations for the excited bound channels and the number of open channels to obtain linearly independent solutions. The electron correlation and polarization effects which are very important, have been taken into account completely ab-initio by the extended MCHF method. The R-matrix method is also used to calculate the same in order to compare with the MCHF results. We will present the comparison of both MCHF and R-matrix results with the available experimental and other theoretical calculations. These results will be useful for astrophysical applications. \\ $^{1}$ H.P. Saha and D.J. Murray, J.Phys. B \textbf{38} 3015 (2005). [Preview Abstract] |
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O1.00025: Excitation Cross Sections, Collision Strengths and Line Ratios of O~II Maximiliano Montenegro, Sultana Nahar, Anil Pradhan, Werner Eissner The forbidden [O~II] lines are one of primary density diagnostics in astrophysical sources. We have carried out relativistic Breit-Pauli R-matrix (BPRM) calculations for the non-dipole transitions of interest. The coupled channel target is represented by a 16-level wavefunction expansion dominated by configurations $2s^22p^3$, $2s2p^4$, $2s^22p^23s$. Resonances are delineated in detail to ascertain the effect on Maxwellian averaged collision strengths. Results for the forbidden transitions among the lowest 5 levels of the ground configuration $2s^22p^3(^4S^o_{3/2},^2D^o_{5/2,3/2}$, $^2P^o_{3/2,1/2})$ show that the fine structure collision strengths do not significantly depart from values obtained from a purely $LS -> LSJ$ transformation. Present results differ significantly with similar BPRM calculations by McLaughlin and Bell (1998), but are essentially in agreement earlier results of Pradhan (1976). The astrophysically important line ratio I(3729)/I(3726) derived from the present results is in agreement with the high and low-density limits expected on physical grounds, and with recent observations. The present excitation cross sections for the $^4S^o-^2D^o$ transition shows excellent agreement with measurements from a merged beam experiment (Zuo et al 1995). [Preview Abstract] |
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O1.00026: Excitation of the 2p$^5$3p states of neon by incident electrons Albert Crowe, Gary Leighton A polarized photon-scattered electron correlation study of excitation of the individual 2p$^5$3p states of neon and their decay to the 2p$^5$3s states provides valuable information on a range of physical effects influencing the alignment and orientation of the excited states. These include exitation of a well LS-coupled state through electron exchange and the excitation of states with different J values, but where L and S are not good quantum numbers. Here we concentrate on excitation of the 3p[5/2]$_3$ state with the emission of a 640.2 nm photon in decay to the 3s[3/2]$_2$ state. All four differential Stokes parameters necessary to describe this pure $^3$D$_3$ excited state will be presented for an incident electron energy of 50 eV and for scattering angles up to 120$^\circ$. The data will also be shown in terms of parameters describing the excited state charge cloud and compared with available theory and a previous experiment over a more limited angular scattering range. [Preview Abstract] |
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O1.00027: Fine Structure effective collision strengths for the electron impact excitation of S~V Claire Hudson, Kenneth Bell A 14-state $R$-matrix calculation has been carried out to determine fine structure effective collision strengths for the electron impact excitation of S~v. The target states are represented by configuration interaction wavefunctions and consist of the 14 lowest $LS$ states, having configurations (2p$^6$)3s$^2$, 3s3p, 3p$^2$, 3s3d, 3s4s, 3p3d. These target states give rise to 26 fine structure levels and 325 possible transitions. The fine structure collision strengths have been obtained by transforming to a $jj$-coupling scheme using the JAJOM program of Saraph [1] and have been determined using a sufficiently fine energy mesh which properly delineates the resonance structure. The effective collision strengths were calculated by averaging the electron collision strengths over a Maxwellian distribution of velocities. The non-zero effective collision strengths for transitions between both the $LS$ states and the fine structure levels have been tabulated for electron temperatures ($T_{\rm e}$) in the range $\log_{10} T_{\rm e} ({\rm K}) = 4.0 - 6.0$. Comparisons are made with the earlier 8-state $R$-matrix calculation of Dufton \& Kingston [2] and distorted-wave evaluations of Christensen et al [3] and Pradhan [4]. [1] Comp. Phys. Commun., 15, 247 (1978); [2] J. Phys. B, 17, 3321 (1984); [3] Phys. Rev. A, 34, 4704 (1986); [4] Atomic Data \& Nuclear Data Tables, 40, 335 (1988). [Preview Abstract] |
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O1.00028: Peculiarities in optical excitation functions of Zn and Cd atoms. H. Bogachov, E. Remeta, V. Borovik, O. Zatsarinny We report on the results of a thorough study of the excitation functions for the spectral lines of the Zn I (n=5-7) and Cd I (n=6,7) principal series. Experimental setup similar to that in [1] was used in the present study with electron beam energy spread of $\sim $0.7 eV. The measured excitation functions have a number of features (mostly maxima) in the autoionizing region which are similar for Zn and Cd but less clear in the Cd case. The first features (shoulders), located near the ionization threshold just below lowest nd$^{10}$np$^{2}$ autoionizing terms (n=4 for Zn, n=5 for Cd), could be the evidence of an effective Zn$^{-}$ (Cd$^{-})$ np$^{3}$ state formation. The most prominent features were observed at $\sim $12 eV and $\sim $16 eV in Zn and at $\sim $13 eV in Cd. They are located at the energies just above the parent terms 3d$^{9}$4s$^{2}$4p and 3d$^{9}$4s$^{2}$5p in Zn and 4d$^{9}$5s$^{2}$5p in Cd, with the specific energy shifts which can be attributed to the post-collision interaction population mechanism. [1] G. G. Bogachev and E. Yu. Remeta, Optics and Spectroscopy \textbf{86}, 647 (1999). [Preview Abstract] |
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O1.00029: PCI effects in electron-impact excitation of the (3p$^{5}$4s$^{2})^{2}$P$_{J}$ levels in potassium. V. Borovik, E. Remeta, A.A. Borovik, Oleg Zatsarinny, Klaus Bartschat We report preliminary results for the PCI lineshift in ejected-electron spectra corresponding to the decay of the (3p$^{5}$4s$^{2})^{2}$P$_{3/2,1/2}^{ }$ autoionizing levels in potassium atoms. The spectra were measured precisely for impact energies up to 7 eV above the corresponding excitation thresholds. The energy dependencies of the lineshift were fitted using a semi-classical approach [1], which ignores the potential influence of K$^{-}$ resonances. The autoionizing widths for the $^{2}$P$_{3/2 }$ and $^{2}$P$_{1/2 }$ levels obtained in this method were, respectively, 16 meV and 26 meV. These values exceed, by approximately a factor of three, previous experimental [2,3] results that were free of PCI effects. Hence this discrepancy may be due to the presence of the strong near-threshold K$^{-}$ resonances observed earlier in the excitation functions for the above levels [4]. [1] A. Niehaus, J. Phys. B \textbf{10} 1845 (1977). [2] R.D. Hudson and V.L. Carter, J. Opt. Soc. Amer. \textbf{57} 1471 (1967). [3] R.D. Driver, J. Phys. B \textbf{9} 143 (1976). [4] A.A. Borovik, A.N. Grum-Grzhimailo, K. Bartschat and O. Zatsarinny, J. Phys. B \textbf{38} 1081 (2005). [Preview Abstract] |
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O1.00030: Spin-Orbit Activated Intra-Doublet Coupling in Generalized Oscillator Strengths of Cs and Ba M. Ya. Amusia, L.V. Chernysheva, Z. Felfli, A. Z. Msezane This paper investigates how the generalized oscillator strengths (GOS's) of the Cs and Ba $3d_{5/2}$ and $3d_{3/2}$ levels are affected by the intra-doublet correlations. The calculations are carried out using the Hartree-Fock approximation and a modified version of the Spin-Polarized Random Phase Approximation with Exchange which takes into account multi-electron correlations and relaxation effects of the excited electrons due to the creation of the $3d$-vacancy. Our GOS's for Cs and Ba, obtained for momentum transfer $q=0$ to 4~{\it a.u.} and energy transfer $\omega=0.01$ to 8~Ry, demonstrate the strong interaction between components of the spin-orbit doublet of the $3d$ electrons in Cs and Ba. This leads to the appearance of an additional maximum in the GOS for the $3d_{5/2}$, due to the action of the $3d_{3/2}$ electrons. The intra-doublet correlations are very important in the dipole, monopole and quadrupole transitions and have dramatic effects upon the GOS's for the $3d_{5/2}$ of Cs and Ba. [Preview Abstract] |
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O1.00031: B-Spline Breit-Pauli R-matrix calculations for electron collisions with Krypton and Xenon. Klaus Bartschat, Oleg Zatsarinny We have extended our previous work on electron collisions with Ne~[1] and Ar~[2] to the heavier noble gas targets Kr and Xe. In our $B$-Spline $R$-matrix method~[3,4], relativistic effects are accounted for through the most important terms of the Breit-Pauli hamiltonian in the inner region of the $R$-matrix box. Several sets of non-orthogonal valence orbitals were employed to account for the strong term dependence in the one-electron orbitals. Using non-orthogonal basis sets avoids the need for pseudo-orbitals to improve upon the target description and virtually eliminates pseudo-resonance problems. The agreement between our predictions and experiment~[5] is much better than obtained in previous calculations based on the standard $R$-matrix approach with strictly orthogonal orbitals, particularly in details such as resonance positions and widths. Consequently, the new results are expected to represent a significant improvement of the current database for electron collisions with heavy noble gases. \par\noindent [1] O. Zatsarinny and K. Bartschat, J. Phys. B {\bf 37} (2004) 2173. \par\noindent [2] O. Zatsarinny and K. Bartschat, J. Phys. B {\bf 37} (2004) 4693. \par\noindent [3] O. Zatsarinny and C. Froese Fischer, J. Phys. B {\bf 33} (2000) 313. \par\noindent [4] O. Zatsarinny, Comp. Phys. Commun. {\bf 174} (2006) 274. \par\noindent [5] S.J. Buckman and C.W. Clark, Rev. Mod. Phys. {\bf 66} (1994) 539. [Preview Abstract] |
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O1.00032: R-matrix Calculations Along the F-like Iso-electronic Sequence Michael Witthoeft, Allan Whiteford, Nigel Badnell With the increased availability of parallel computers in recent years, it has become feasible to perform large-scale R-matrix electron-impact excitation caclulations over iso-electronic sequences. We present a 195-level ICFT R-matrix calculation for the F-like sequence from Ne$^+$ to Kr$^{27+}$. The data from this, and future, sequences not only provide broad coverage of the periodic table for plasma modeling codes, but also allow for another means of determining the quality of the calculations as behaviors along the sequence are analyzed. [Preview Abstract] |
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O1.00033: Measurement of the Absolute Excitation Cross Section of the 3s$^{2} \quad ^{2}$P$^{o}$ - 3s$^{2}$$^{2}$P$_{3/2}$ transition in Fe$^{13+}$ Steven Smith, Ara Chutjian, Sabbir Hossain, Swaraj Tayal Measurements have been made of the electron excitation cross section for highly-charged positive ions (HCIs). These ions and charge states are important in interpreting data obtained from the HST, EUVE, FUSE, Chandra and Newton missions. Experimental cross sections are reported for the 3s$^{2} \quad ^{2}$P$^{o}$ -- 3s$^{2} \quad ^{2}$P$_{3/2}$ transition in \textit{Fe}$^{13+}$ at 2.34 eV. The JPL electron-cyclotron resonance ion source is utilized [1], along with the energy loss method, in a merged electron-ion beams geometry [2]. The center-of-mass interaction energies are in the range 1 eV (below threshold) to 6 eV. Clear resonance enhancement is observed in both experiment and theoretical results [3] near threshold for this $^{2}$P$^{o}$--$^{2}$P$_{3/2}$ transition. Sabbir Hossain acknowledges support through the NASA-NRC program. This work was carried out at JPL/Caltech and supported by NASA. [1] J. B. Greenwood, S.J. Smith, A. Chutjian, and E. Pollack, Phys. Rev. A \textbf{59} 1348 (1999). [2] A. Chutjian, Physica Scripta \textbf{T110}, 203 (2004). [3] P.J. Storey, H.E. Mason, H.E. Saraph, Astron. Astrophys,. \textbf{309}, 677 (1996). [Preview Abstract] |
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O1.00034: Relativistic $R$-matrix calculations of the electron-impact excitation of W$^{46+}$ Connor Ballance, Donald Griffin, Stuart Loch, Michael Pindzola, Nigel Badnell The current design plans for ITER call for tungsten to be employed for certain plasma facing components in the divertor region. Thus, accurate atomic collision data are needed for emission modeling of tungsten. Electron-impact excitation and radiative rates are of particular importance for Ni-like W, since this ion emits some of the most intense spectral lines of all ionization stages. We report on a fully relativistic 115-level $R$-matrix calculations of W$^{46+}$, which includes the effects of radiative damping. These calculations were performed using a newly developed parallel version of the Dirac-Fock atomic $R$-matrix codes (DARC) that have been modified to include radiation damping. Although radiation damping is very important in most highly ionized species, its effects are reduced in this case because of the closed-shell Ni-like ground state. The rates from these relativistic atomic calculations have been employed for collisional-radiative modeling of this ion. [Preview Abstract] |
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O1.00035: Electron Impact Ionization of Helium Eric Schow, Ken Hazlett, Cristina Medina, Gil Vitug, J. G. Childers, Murtadha A. Khakoo, I. Bray, D. V. Fursa, J. Colgan Recently completed measurements of the absolute singly- and doubly-differential cross sections for the electron impact ionization of helium at low incident energies will be presented. The measurements were taken using the moveable nozzle technique.\footnote{M.\ Hughes, K.\ E.\ James, Jr., J.\ G.Childers, and M.\ A.\ Khakoo, {\it Meas. Sci. Technol.} {\bf 14}, 841 (2003)} Data were taken at incident energies of 26~eV, 28~eV, 30~eV, 32~eV, 34~eV, 36~eV, and 40~eV. The results are compared to the theoretical convergent close-coupling and time-dependent close coupling calculations. Generally good agreement is observed, but areas of small but significant disagreement are noted. This work is funded in part by the National Science Foundation under grant \# NSF-RUI-PHY-0096808. [Preview Abstract] |
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O1.00036: Electron impact ionization of helium at 102 eV in coplanar and perpendicular geometries Zhangjin Chen, Don Madison The triple differential cross sections for electron impact ionization of helium at 102 eV are presented for the case of coplanar and perpendicular plane asymmetric geometry within the framework of second-order distorted wave theory. The closure approximation and the simplified Green's function approximation which are currently used in the evaluation of the second-order amplitudes are investigated and the accuracy of the approximations is examined. It is found that the second-order effects are significant for this incident energy and the simplified Green's function approximation is generally more valid than the closure approximation. [Preview Abstract] |
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O1.00037: Electron impact ionization from p-orbital targets Bidhan Saha, Arun K. Basak, M. Alfaz Uddin Electron impact ionization cross sections are evaluated using a modified version [1] of the BELL formula [2] for a wide range of isoelectronic targets, ranging from Li to Ne targets with both the open and closed shell configurations. In this report the MBELL parameters are generalized for treating the orbital quantum numbers \textit{nl} dependency; its accuracy has been tested by evaluating cross sections for a wider range of species and energies. Details will be presented at the meeting. [1] A. K. F. Haque, M. A. Uddin, A. K. Basak, K. R. Karim and B. C. Saha, Phys. Rev. A73, 012708 (2005). [2] K. L. Bell, H. B. Gilbody, J. G. Hughes, A. E. Kingston, and F. J. Smith, J. Phys. Chem. Ref. Data 12, 891 (1983). [Preview Abstract] |
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O1.00038: Single and multiple electron--impact ionization of Kr as a function of momentum transfer Oscar G. de Lucio, Jared Gavin, Robert D. DuBois Measurements of electron--impact ionization cross sections of Kr are being performed for an incident electron beam with energies between 100 eV and 500 eV, colliding with a Kr gas jet target. Scattered electrons are detected by means of a channeltron detector which can be positioned at angles ranging from 0\r{ } up to 60\r{ } with respect to the beam direction. An electrostatic spectrometer is used to determine the energy loss of the projectile. Ionized Kr is detected by a second channeltron detector. Signals coming from both detectors are measured in coincidence by means of a TDC. Producing time of flight spectra in which Kr$^{1+, 2+, 3+}$ can be distinguished. A pulsed extraction field for detecting the Kr ions has been used in order to reduce the background. Ionization yields as a function of momentum transfer will be presented. [Preview Abstract] |
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O1.00039: Electron Impact Ionization Cross Sections in Rb and Cs. T.J. Reddish, M. Lukomski, S. Sutton, W. Kedzierski, J.W. McConkey, K. Bartschat, P.L. Bartlett, A.T. Stelbovics, I. Bray We present a new atom trapping technique for determining absolute, total ionisation cross sections (TICS) out of an excited atom. The novel feature of this method is in utilizing Doppler cooling of neutral atoms to determine ionisation cross sections. This fluorescence-monitoring experiment, which is a variant of the `trap loss' technique, has enabled us to obtain the experimental electron impact ionisation cross sections out of the Cs 6$^{2}$P$_{3/2}$ excited state between 7 - 400 eV. New CCC, $R$-Matrix with Pseudo-States (RMPS), and Born approximation single ionisation cross sections (SICS) are also presented for both the ground and excited states of Cs and Rb, and compared with the available experimental data. The comparison of the results reveals the importance of the autoionisation and multiple ionisation contributions to the TICS. The autoionisation contribution appears to be substantial for ionisation out of the Cs 6$^{2}$P and Rb 5$^{2}$P excited states; $\sim $ 3-4 larger than the direct ionisation contribution predicted by CCC at $\sim $ 30-50 eV. This surprising result shows the importance of multi-electron processes in determining the ionisation cross sections of heavy alkali atoms. [Preview Abstract] |
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O1.00040: The role of metastable levels in Be-like ionization cross sections Stuart Loch, Connor Ballance, Michael Pindzola, Mike Fogle, Mark Bannister We present recent experimental measurements and theoretical calculations for the electron impact ionization cross sections of C$^{2+}$, N$^{3+}$ and O$^{4+}$, with a view to studying the role of metastable levels. The theoretical calculations consist of R-matrix with pseudostates results for both the ground 1s$^2$ 2s$^2$ $^1S$ and the metastable 1s$^2$ 2s2p $^3P$ terms. We compare this with distorted-wave calculations, to determine the point along the iso-electronic sequence at which non-perturbative methods are appropriate. The experimental measurements were taken at the Oak Ridge National Laboratory, using a crossed-beam technique. A gas attenuation method was used to experimentally measure the metastable fraction. We compare these measurements with the metastable fractions inferred from our theoretical calculations. [Preview Abstract] |
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O1.00041: Electron collisions with Fe-peak elements: Fe V B.M. McLaughlin, P.H. Norrington, A. Hibbert, M.P. Scott, P.G. Burke, V.M. Burke, C.J. Noble Lines of Fe$^+$ -- Fe$^{6+}$ ions appear in the UV spectra of O-type stars. Accurate assessments of the relevant abundances of heavy elements and their ions can be obtained from the comparison of the observed spectra with synthetic NLTE spectra, if the atomic data for electron and photon interaction processes are known with sufficient accuracy. Electron-impact excitation collision strengths for the Fe-peak element Fe$^{4+}$ are calculated in the close-coupling approximation using the $R$-matrix suite of codes PRMAT. Eighty $LS$ - coupled states arising from the $\rm 3d^4$, $\rm 3d^34s$ and $\rm 3d^34p$ configurations of Fe$^{4+}$ are retained in our calculations. Accurate multi-configuration interaction target and scattering wavefunctions are used with the aid of $\rm 3p^2 \rightarrow 3d^2$ two-electron promotions and a n=4 basis set. Effective collision strengths for optically forbidden transitions in the $\rm 3d^4$ manifold, which are extremely important in the analysis of lines in the Fe$^{4+}$ spectra, are obtained by averaging the electron collision strengths over a Maxwellian distribution for the electron temperature. The present results when compared to previous investigations for temperatures below 100,000 Kelvin show an enhancement of a factor of two. Further details will be presented at the meeting. [Preview Abstract] |
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O1.00042: Excitation of CO$^2$ by electron impact Y.-K. Kim, H. Kato, M. Hoshino, H. Tanaka The BE$f$ scaling\footnote{Y.-K. Kim, Phys. Rev. A {\bf 64}, 032713 (2001).} ---which converts plane-wave Born (PWB) cross sections for electron-impact excitations of atoms to accurate results at low incident electron energy $T$---is also found to provide results in excellent agreement with the new energy-loss experimental data for the A~$^1\Pi(v^{\prime}=n) \leftarrow$ X~$^1\Sigma(v^{\prime\prime}=0),\ n=$ 0--7 excitations of the CO molecule at $T=100$ eV. The new experimental data were measured at the Sophia Univ. using the same apparatus desribed in Green et al.\footnote{M. A. Green et al., J. Phys. B {\bf 35}, 567 (2002)} The unscaled PWB cross sections for the individual vibrational excitations were obtained by integrating the generalized oscillator strengths published by Chandranupong et al.\footnote{L. Chantranupong et al., Chem. Phys. {\bf 164}, 183 (1992).} The dipole $f$ values and excitation energies compiled by Berkowitz \footnote{J. Berkowitz, {\it Atomic and Molecular Photoabsorption}, Academic Press (2002)} are used to apply the BE$f$ scaling to the integrated (as opposed to angular distributions) PWB cross sections. Experiment at lower $T$ is in progress at Sophia Univ. to provide additional test of the BE$f$ scaling for molecules. [Preview Abstract] |
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O1.00043: Electron Impact Excitation of Several Rydberg-Valence Series in Molecular Nitrogen. Murtadha A. Khakoo, Shiyang Wang, Vaasu Swaminathan, Daniel Nuyujukian, Paul V. Johnson, Charles P. Malone, Isik Kanik Preliminary analysis of recent high-resolution electron impact differential cross-section measurements for e.g. the excitation of the higher-lying b,c and o $^{1}\Pi _{u}$ series of N$_{2}$ are presented. The data are taken at 17.5, 20, 30, 50 and 100eV incident energies and scattering angles from 2 to 130 degrees. These data show clear interference effects in the differential scattering amplitudes (cross-sections) produced by the coupling within these series and are the first time such observations have been observed for electron scattering in N$_{2}$. The results have strong implications for modeling the excitation of these important series in this fundamental gas target, of importance to astrophysical as well as atomic-molecular physics applications. [Preview Abstract] |
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O1.00044: Electron Impact Excitation of the lowest-lying A$^{1}$B$_{1}$ Electronic state of Water P.J.O. Teubner, P.A. Thorn, M.J. Brunger, L. Campbell, H. Kato, C. Makochekanwa, M. Hoshino, H. Tanaka We report differential and integral cross sections for excitation of the A$^{1}$B$_{1 }$electronic state of water. The energy range of these measurements is 15--50eV and, where possible, comparison is made to the results of available theory. We additionally report generalised oscillator strengths (at energies 30, 100 and 200eV) and a value of the optical oscillator strength (OOS) for this state. The present OOS is also compared to the results of earlier studies. [Preview Abstract] |
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O1.00045: Electron Impact Ionization of H$_2$ Kelly Kuper, Eric Schow, Ken Hazlett, J. G. Childers, Murtadha A. Khakoo Relative doubly-differential cross sections for the low energy electron impact ionization of H$_2$ have been measured at incident energies ranging from 17.6~eV to 40~eV and scattering angles of $20^\circ$ to $130^\circ$. The calibration of the electron analyzer during these measurements employed the recent doubly-differential cross section measurements of atomic hydrogen.\footnote{J.\ G.Childers, K.\ E.\ James, Jr., Igor Bray, M.\ Baertschy, and M.\ A.Khakoo, {\em Phys.\ Rev.\ A} {\bf 69}, 022709 (2004).} These measurements represent a new calibration standard useful in the determination of the transmission function of electron analyzers. This work is funded by the National Science Foundation under grant \# NSF-RUI-PHY-0096808. [Preview Abstract] |
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O1.00046: Electron-impact Ionization Rates of Niobium Oxides M. Raskovic, S. Popovic, L. Vuskovic Electron-impact ionization rates are calculated for NbO, NbO$_{2}$, Nb$_{2}$O$_{4}$ and Nb$_{2}$O$_{5}$. These data were previously unavailable, but are necessary for understanding ion-beam or plasma-sheath based removal processes of layers of Nb oxides accumulated on the surface of super-conductive radio-frequency cavities for particle accelerators. Presence of oxide layers on the surface reduces the Q factor of the cavities by up to a factor of 2. Electronic structures of niobium oxides were described by several empirical basis sets. MO parameters were calculated using several Hartree-Fock and density functional methods. The results were tested with available experimental results on ground-state frequencies and appearance potentials. Electron-impact ionization cross-section data are generated using the Binary-Encounter-Bethe approximation. Ionization rates are calculated for three different sets of electron energy distribution functions, representing typical conditions in the oxide removal processes. Dissociative ionization cross-sections for generation of oxide ion fragments are included in calculation together with their estimated accuracy. [Preview Abstract] |
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O1.00047: Electron impact dissociation of molecular ions E.M. Bahati, M.R. Fogle, C.R. Vane, M.E. Bannister, V. Zhaunerchyk, R.D. Thomas The ORNL Electron-Ion Crossed Beams Experiment is used to determine absolute cross sections for electron-impact dissociation of molecular ions. Investigations have been carried out on DCO$^{+}$, CH$_3^+ $, and N$_2 $D$^+$ cations, considered as amongst those species playing important roles in such diverse areas as laboratory plasmas, astrophysics, and thermonuclear fusion. The obtained results on dissociation of DCO$^{+}$ resulting in CO$^{+}$ fragments are discussed in light of the energy levels and photo-dissociation cross section of HCO$^{+ }$predicted by \textit{ab initio} multi-reference configuration interaction calculations [Koch et al, 1995 \textit{Ber. Bunsenges Phys. Chem.} \textbf{99} (3) 393]. Preliminary results on CH$_3^+ $ are compared to the measurements of Lecointre et al [\textit{XXIV ICPEAC, }Rosario, Argentina, 20 - 26 July 2005; (Poster Session)]. Results for dissociation of N$_2 $D$^+$ producing ND$^+$ are also presented. [Preview Abstract] |
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O1.00048: Collisions between Electrons and the Cl$_2^-$ Ion D. Pegg, G. Collins, K. Fritioff, J. Sandstr\"om, D. Hanstorp, R. Thomas, F. Hellberg, A. Ehlerding, M. Larsson, F. \"Osterdahl, A. K\"allberg, H. Danared Collisions between electrons and molecular negative ions result in the following fragmentation processes: detachment, dissociation and detachment plus dissociation. In some cases attachment also occurs, resulting in resonance structure associated with the transient formation of a doubly charged negative ion. We report on a measurement of the cross sections for detachment, dissociation and detachment plus dissociation for the e$^-$ - Cl$_2^-$ collision system over a collision energy range 0-200 eV. The merged electron-ion beam experiment was performed at the heavy ion magnetic storage ring, CRYRING, situated at the MSL in Stockholm [1]. Over the energy range studied, the dominant breakup mechanism is dissociation. A prominent structure was observed just above threshold in the Cl$^-$ + Cl + e$^-$ dissociation channel. It is proposed that the structure is a resonance associated with the production and rapid decay of an excited state of the doubly charged Cl$_2$$^{2-}$ ion.\newline [1] G.F.Collins et al, Phys. Rev. A 72, 042708 (2005). [Preview Abstract] |
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O1.00049: Electron Collisions with Formic Acid Violaine Vizcaino, James Sullivan, Stephen Buckman Formic acid (HCOOH) is the simplest of the organic acids and it is thought that it could play a key role in the formation of simple biomolecules such as glycine and acetic acid in the interstellar medium. We have studied elastic electron scattering from formic acid using a crossed-beam electron spectrometer. Absolute cross sections are obtained using the relative flow technique. Flow rates for HCOOH, and the reference gas He, are measured at a number of temperatures, including both room temperature and 70\r{ }C, in order to investigate the effects of molecular dimers, which are thought to dominate at room temperature. Measurements at energies in the range 1.8-50 eV will be presented at the meeting and compared with several recent theoretical calculations. [Preview Abstract] |
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O1.00050: Electron interactions with DNA components Stefano Tonzani, Chris H. Greene The growing interest in radiation damage to DNA by low energy electrons, has spurred our efforts to understand and explain the experimental data and more generally the various processes involved in such a complex environment. We have started by performing calculations for electron scattering from DNA bases (S. Tonzani and C. H. Greene, J. Chem. Phys., in press) and now we are looking at other components of the DNA filament, like the sugar and phosphate group. Methods such as finite element R-matrix using static exchange and the local density approximation are employed in an effort to give an accurate prediction of resonance positions and widths and also to discuss possible mechanisms for the processes observed in the experiments. [Preview Abstract] |
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O1.00051: Behavior of Regge Poles with Increasing Potential Strength R.H. Pratt, N.B. Avdonina We illustrate, using the example of a model but realistic potential, that the family of the Regge pole trajectories (RPT) moves to the right along the real l-axis with increasing atomic charge Z. Every time Z exceeds Z=Z$_{N}$, the Z at which the Nth bound s-state first appears at zero energy, one more trajectory joins the RPT family in the first quadrant of the complex l-plane, coming from the left side of the real l-axis. Since for atoms with Z$<$ Z$_{N }$a part of the RPT rise off the real l-axis near but before integer values of l we can expect that for such atoms there will be resonances in electron scattering cross sections . The number of resonances depends on N=n+l (n is the principal quantum number)$^{1}$. We have calculated energies and widths for these resonances in the model potential using a perturbation theory approach. [Preview Abstract] |
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O1.00052: Regge Oscillations in Electron-Atom Total Scattering Cross Sections? D. Sokolovski, S. Yu. Ovchinnikov, Z. Felfli, J. H. Macek, A. Z. Msezane In quantum scattering, the presence of a sufficiently narrow resonance allows the collision partners to form a long-lived intermediate complex which rotates as it decays to preserve the total angular momentum. Here we consider a system trapped in such a resonance state and allow it to decay at zero scattering angle, which through the optical theorem can be related to the total cross section (TCS). If the complex has a large angular life, it will return to forward scattering %$\theta=0^{\circ}$ many times. For the resonance to contribute to the TCS requires: (i) Several rotations of the complex (Regge trajectory stays close to real axis) and (ii) Coherent addition of forward scattering sub-amplitudes (real part of Regge pole is close to an integer). Our analysis is based on the recent complex angular momentum approach [1] used to explain low energy oscillations in proton-H collision. Specifically, we want to establish whether similar oscillations can also be observed in electron-atom scattering. To this end, we present a detailed analysis of Regge trajectories and their contributions to the TCS for the model Thomas-Fermi potential. \begin{enumerate} \bibitem{1} J. H. Macek {\it et al.}, Phys. Rev. Lett. {\bf 93}, 183203 (2004). \end{enumerate} [Preview Abstract] |
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O1.00053: Low energy positron scattering from helium atoms. James Sullivan, Gerard Atkinson, Stephen Buckman A new experiment has been constructed to conduct positron-based research. The Australian Positron Beamline Facility is a new positron beamline built on buffer gas trap technology, developed at UCSD by the Surko group [1]. This beamline is intended for use in both atomic and molecular physics and materials science. For atomic and molecular physics, specifically the study of low energy positron scattering, it makes use of the superior energy resolution offered by the Surko trap system as well as the analysis techniques developed to measure scattering in a strong (500 gauss) magnetic field. The design and construction of the beamline will be outlined as well as the operation of the trap and beam system. The first experiments on the facility will be for low energy positron scattering from helium, providing high quality data to test the best available theoretical calculations. Progress towards the first scattering measurements will be presented, along with the plans for future experimental endeavour. [1] Murphy and Surko, Phys. Rev. A \textbf{46}, 5696 (1992) [Preview Abstract] |
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O1.00054: Elastic scattering of slow positrons on atoms Miron Amusia, Nikolai Cherepkov, Larissa Chernysheva The results of calculations are presented of the elastic scattering cross section of positrons upon noble gas and alkali atoms. The calculations are performed in the one-electron Hartree-Fock approximation, with account of multi-electron correlations in the lowest order and in the frame of essentially refined so-called Random Phase Approximation with Exchange (RPAE). The modification that comes from considerable improving of our code proved to be prominent. Virtual positronium formation that was taken into account appeared to be very important, noticeably bigger than in the lowest approximation to RPAE. Arguments are presented that for alkali atoms the positron's polarization potential is repulsive. The results obtained in this paper proved to be in reasonable agreement with experiment and with some previously reported calculations. [Preview Abstract] |
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O1.00055: Formation of triplet positron-helium bound state by stripping of positronium atoms in collision with ground state helium Joseph Di Rienzi, Richard J. Drachman The system consisting of a positron and a helium atom in the triplet state e$^{+}$He($^{3}$S$^{e})$ was conjectured long ago to be stable [1]. Its stability has recently been established rigorously [2], and the values of the energies of dissociation into the ground states of Ps and He$^{+}$ have also been reported [3] and [4]. We have evaluated the cross-section for this system formed by radiative attachment of a positron in triplet He state and found it to be small [5]. The mechanism of production suggested here should result in a larger cross-section (of atomic size) which we are determining using the Born approximation with simplified initial and final wave functions. [1] R. J. Drachman, Y. K. Ho, and S. K. Houston, J. Phys B \textbf{9}, L199 (1976) [2] G.G. Ryzhikh and J. Mitroy, J. Phys. B \textbf{31} 3465 (1998) [3] A. M. Frolov, Phys. Rev. A. \textbf{71} 032506 (2005) [4] J. Mitroy, Phys. Rev. A. \textbf{72 }032503 (2005) [5] J. Di Rienzi and R. J. Drachman, Phys. Rev A \textbf{73}, 012703 (2006) [Preview Abstract] |
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O1.00056: Resonances with unnatural parities in the positron-hydrogen system Zong-Chao Yan, Yew Kam Ho There has been continuous interest in calculations of resonances in positron-hydrogen scattering [1]. Here we present a calculation of resonances with unnatural parities in the positron-hydrogen system. While such resonances can not be accessed by positron collisions with the ground state hydrogen atom, they can be reached by positron scattering with the excited hydrogen atoms. The method of complex-coordinate rotation [2] is used in our work. Elaborate and extensive Hylleraas bases [3] are used to take into account of the correlation effects. Energies and widths for the P$^e$, D$^o$, F$^e$, G$^o$, H$^e$, and I$^o$ resonance states below various excited positronium and hydrogen thresholds have been calculated. Results will be presented at the meeting.\ \\ \ \\ \noindent [1] Y. K. Ho and Z.-C. Yan, Phys. Rev. A {\bf 70}, 032716 (2004), and references therein.\ \\ \noindent [2] Y. K. Ho, Phys. Rept. {\bf 99}, 1 (1983), and references therein.\ \\ \noindent [3] Z.-C. Yan and G. W. F. Drake, Chem. Phys. Lett. {\bf 259}, 96 (1996). [Preview Abstract] |
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O1.00057: Doubly excited resonance states of the positronium negative ion embedded in Debye (screened Coulomb) plasmas S. Kar, Y.K. Ho The $^{1,3}P^{o }$ resonance states of positronium negative ion embedded in dense plasma environments are determined by calculating the density of resonance states using the stabilization method [1]. A screened Coulomb potential obtained from Debye model is used to represent the interaction between the charge particles. Correlated wave functions are used to represent correlation effect between the three charge particles. In an earlier work, we reported a calculation for doubly excited $2s^{2}$ $^{1}S^{e }$ resonance of the positronium negative ion embedded in Debye plasmas [2]. In the present work, we have calculated $^{1}P^{o}$ and $^{3}P^{o}$ resonances associated with, and below, the n=2 threshold of the positronium atom. The resonance energies and widths for various Debye lengths ranging from infinity to a small value will be reported in the meeting. The work is supported by the National Science Council of ROC. \textbf{References:} [1] V. A. Mandelshtam, T. R. Ravuri, and H. S. Taylor, \textit{Phys. Rev. Lett. }\textbf{70}, 1932 (1993). [2] S. Kar and Y. K. Ho, \textit{Phys. Rev. A}\textbf{ 71}, 052503 (2005). [Preview Abstract] |
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O1.00058: Triply Differential Single Ionization of Argon by Positron and Electron Impact. Jared Gavin, Oscar De Lucio, Robert DuBois Triply differential cross sections for single ionization of argon by 200 and 500 eV positrons and electrons were measured using coincidences between the scattered projectile, ejected electrons and recoil ions.~ Time of flight spectra allow us to distinguish between different degrees of ionization and to discriminate against background contributions.~ Our experimental apparatus allows us to acquire data with energy losses $<$ 30 eV and scattering angles between $\pm $ 3\r{ } and to simultaneously measure the binary and recoil lobes of momenta transfer.~ Comparison between the positron and electron TDCS data will be presented and discussed with respect to projectile charge effects. We are now working towards removing the effects of an electric field, which is used to extract the recoil ions, on the ejected electron trajectories. [Preview Abstract] |
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O1.00059: Non-resonant positron-on-molecule annihilation. J.A. Young, L.D. Barnes, C.M. Surko Positron-on-molecule annihilation rates for many atoms and molecules have been measured previously using thermal distributions of positrons; and more recently, as a function of positron energy, using a cold, trap-based positron beam [1]. Large vibrational Feshbach resonances (VFR) dominate the energy resolved annihilation spectra in hydrocarbons and are thought to be responsible for their large thermal annihilation rates. This paper reports measurements of so-called ``direct'' annihilation rates at energies above the vibrational modes but below positronium formation. The normalized rates, Z$_{eff}^{(d)}$, are generally small (compared to the VFR values) and approximately independent of energy. Trends as a function of target species are discussed. As Z$_{eff}^{(d)}$ is free of resonances, it could appropriately be compared to the predictions of fixed-nuclei calculations. A new experiment, to study annihilation as a function of target molecule temperature, will also be described. \newline ~[1] L. D. Barnes, \textit{et al.}, \textit{Phys. Rev. A} \textbf{67}, 032706 (2003). [Preview Abstract] |
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O1.00060: Positron-impact vibrational excitation cross sections and the Born dipole model J.P. Marler, C. M. Surko, G. F. Gribakin We describe \textit{in situ} measurements of the positron- and electron-impact cross sections for vibrational excitation of the infrared-active (IR) $\nu_3$ mode in CF$_4$ [1]. These cross sections are virtually identical and agree quantitatively with the predictions of the Born dipole model (BDM), which describes the effect of long range dipole coupling. We also compare the predictions of the BDM with the other positron-impact vibrational cross sections for IR modes measured to date (CO, CO$_2$, H$_2$, and CH$_4$) [2]. The BDM contributions to the measured cross sections vary widely. However, for all molecules except H$_2$ (for which the transition dipole moment is zero), the BDM model predicts the energy dependence of these cross sections quite well. The possible significance of these results will be discussed. \newline \newline [1] J.P. Marler and C.M. Surko, \textit{Phys. Rev. A.} \textbf{72}, 062702 (2005). \newline [2] J.P. Marler, G.F. Gribakin and C.M. Surko, \textit{Nuclear Instrum. and Meth. B}, in press (2006). [Preview Abstract] |
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O1.00061: Evidence of Positron Annihilation at Electronic Excitation Threshold for N$_{2}$ $^{\ast }$ E.G. Miller, J.J. Edwards, W.E. Kauppila, T.S. Stein, E. Surdutovich We are investigating Positronium (Ps) formation for $<$ 20 eV positrons interacting with N$_{2}$ in a gas scattering cell. The technique [1] of Ps annihilation ratio spectroscopy (PsARS) is used to obtain the ratios of coincidence signals for two of the three gamma rays (S$_{3\gamma })$ in the photon energy window 300 to 460 keV resulting from ortho-Ps decay to that for two 511 keV gamma rays (S$_{2\gamma })$ arising from para-Ps decay and other processes. By comparing these ratios of S$_{3 \gamma }$/S$_{2\gamma }$ for N$_{2}$ to those for Ar it is found that N$_{2}$ exhibits strikingly anomalous behavior near and below the Ps formation threshold. Typically, this ratio remains constant within 2 eV above the Ps threshold. For N$_{2} $, this ratio decreases to zero at the threshold and an S$_{2 \gamma }$ signal remains for an energy of $\sim $0.3 eV below. Since N$_{2}$ has an electronic excitation threshold for positron impact that opens up at $\sim $0.3 eV below the Ps threshold, the present results strongly suggest that the incident positron is electronically exciting N$_{2}$ and then binding to the excited N$_{2}$ in a temporary resonance-like state from which the bound positron annihilates with a molecular electron. $^{\ast }$Research supported by NSF Grant PHY 99-88093.\\ $[1]$ W.E. Kauppila, E.G. Miller, H. F.M. Mohamed, K. Pipinos, T. S. Stein, and E. Surdutovich, Phys. Rev. Lett. 93, 113401 (2004).\\ [Preview Abstract] |
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O1.00062: Observation of positronium formation with inner orbital electrons for O$_{2}$, CO$_{2}$, C$_{2}$H$_{2}$, and N$_{2}$O using PsARS J.J. Edwards, W.E. Kauppila, E.G. Miller, T.S. Stein, E. Surdutovich We are investigating ortho- and para-positronium (Ps) formation for positrons interacting with simple gas molecules in a gas scattering cell using Ps annihilation ratio spectroscopy (PsARS)[1]. These measurements involve the detection of two gamma rays in coincidence for energy windows (a) centered at 511 keV resulting from the decay of short-lived (0.1 ns) para-Ps and the destruction of longer-lived (0.1 $\mu $s) ortho-Ps at the scattering cell walls, and (b) from 300 to 460 keV resulting from the three gamma decay of ortho-Ps. By taking the ratios of these signals versus positron impact energy we find we are able to compare the resulting curves with that obtained for argon as a reference. These comparisons reveal departures from the argon curve at well defined energies and are interpreted as Ps formation with inner orbital electrons. The fractions of the overall Ps formation cross-section due to inner orbital electrons are estimated to be as high as 30{\%} for O$_{2}$, 25{\%} for CO$_{2}$, 15{\%} for N$_{2}$O and 5{\%} for C$_{2}$H$_{2}$ near their respective threshold energies. [1] W.E. Kauppila, E.G. Miller, H.F.M. Mohamed, K. Pipinos, T.S. Stein, E. Surdutovich, Phys. Rev. Lett. 93, 113401 (2004). [Preview Abstract] |
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O1.00063: Towards cross sections for positrons scattered by uracil molecules E. Surdutovich, W.E. Kauppila, E.G. Miller, T.S. Stein We report measurements of relative total and positronium (Ps) formation cross sections as well as results from using the Ps annihilation ratio spectroscopy (PsARS) method~[1] for low energy ($<30$eV) positrons ($e^+$s) scattered by uracil (an RNA nucleobase) molecules. In our experiments, $e^+$ beam passes through the scattering cell with heated uracil powder. These measurements are the first step in an experimental study of positron interactions with biological molecules. The interest in $e^+$ interactions with such molecules is boosted by possible relevance to positron emission tomography (PET), an important diagnostic tool in medicine. It has been recognized that Ps formation is a ``subtle aspect of PET''[2] and it has been found that the Ps yield in water and organic solvents ranges from 38 to 70\%[3]. A study of $e^+$ scattering on uracil molecules is a beginning in a detailed study of a variety of possible interactions of $e^+$s with a live organism. Our preliminary results indicate large relative importance of Ps formation compared with other scattering channels at energies close to the threshold of Ps formation. [1]W.E. Kauppila et al., Phys. Rev. Lett. 93, 113401 (2004). [2]M.D. Harpen, Med. Phys. 31 (1), 57-61 (2004). [3]P. Castellaz et al., J. Nucl. Radiochem. Sci. 3, R1-R7 (2002). [Preview Abstract] |
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O1.00064: Open Quantum System Approach with Sources and Sinks M. Seliger, S. Yoshida, J. Burgdorfer, C.O. Reinhold, D.R. Schultz, T. Minami, M.S. Pindzola, E. Lamour, J.P. Rozet, D. Vernhet We study the time evolution of electronic states of hydrogenic projectile ions penetrating solids. Multiple scattering inside the solid as well as radiative processes represent strong environmental coupling which can both transiently generate coherences as well as lead to decoherence for longer times. This electronic system is not only open to exchange energy with the environment but also open to probability flux. We therefore have developed a new open quantum system approach [1] that allows for both sources (capture) and sinks (ionization). We calculate the build-up of transient coherences created in electron capture by an initially bare argon projectile in transport through an amorphous carbon foil at 13.6 MeV/amu and its decoherence due to the interaction with the foil. Our results are in close agreement with experimental data. Experiments for thin foils are shown to provide a very stringent test of ion-atom collisions theories. [1] M. Seliger et al Phys. Rev. A 71, 062901 (2005); Submitted to Phys. Rev. A (2006). [Preview Abstract] |
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O1.00065: Time ordering and time correlation in atomic collisions and in quibts J.H. McGuire, L. Kaplan, Kh. Shakov, A. Chalastaras, A.M. Smith, A. Godunov, H. Schmidt-Boecking, D. Uskov Time ordering may be defined by first defining the limit of no time ordering in terems of the time average of the external interaction, $V(t)$. Time correlation is defined in terms of a similar limit called the independent time approximation. Several cases are discussed including weakly perturbed atomic collisions and strongly perturbed qubits. Experimental evidence for time correlation has not yet been distinguished from evidence for time ordering. [Preview Abstract] |
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O1.00066: Relative Intensities of L x-rays from Proton Bombardment of Selected Rare Earth Elements. Sam Cipolla Multiple ionization effects are investigated for 75-300 keV proton bombardment of selected thick elemental rare earth targets. The relative intensities of x-rays emissions resulting from vacancy production in the L3 sub-shell are analyzed to deduce the presence of multiple ionization of the M- and N-shells. [Preview Abstract] |
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O1.00067: Ionization and charge transfer in collisions of singly charged intermediate-energy heavy ions with gas targets. J.L. Shinpaugh, N.L. Evans, E.L.B. Justiniano, L.H. Toburen, R.D. DuBois Absolute total and partial cross sections for charge transfer and ionization are presented for collisions of C$^+$, F$^+$, Cl$^+$, and Cl$^{++}$ projectile ions with H$_2$, He, Ar, Ne, N$_2$, H$_2$O, and CH$_4$ gas targets in the energy range of 25 - 200 keV/u. The cross sections were determined by measuring post-collision projectile ion charge states in coincidence with target recoil ion charge states at the 2-MV tandem Van de Graaff accelerator facility at East Carolina University. Experimental data for collision processes for low-charge-state heavy ions incident on many-electron targets in this intermediate-energy range are particularly scarce. The present results are compared to available experimental data, to the recommended (theoretical) cross sections compiled by Janev, Phaneuf, and Hunter,\footnote{R.K. Janev, R.A. Phaneuf, and H.T. Hunter, Atom.~Data and Nucl.~Data Tables $\underline{40}$, 249 (1988).} and to scaling laws from Santos and DuBois.\footnote{A.C.F. Santos and R.D. DuBois, Phys. Rev. A. $\underline{69}$, 42709 (2004).} For electron capture for singly charged ions colliding with 10-electron targets (Ne and CH$_4$), the cross sections are observed to follow a single curve, independent of projectile Z, over several orders of magnitude of velocity. [Preview Abstract] |
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O1.00068: CTMC Calculations of Double Ionization of Oriented D$_{2}$ by 1MeV/u F$^{8+}$ projectiles K. Cornelius, A.L. Landers We have used a Classical Trajectory Monte Carlo calculation to explore the orientation dependence of double ionization by 1MeV/u F$^{8+}$ ions. Previous measurements$^{1}$ of this system show a strong double ionization preference for deuterium molecules oriented perpendicular to the ion beam. The authors of [1] used a simple quantum mechanical model to qualitatively explain this behavior. In this classical calculation, we find the exact opposite result: the double-ionization probability is much stronger for molecules oriented along the beam axis. What is particularly striking is the magnitude of the discrepancy between the classical calculation and the experimental result, indicating a strongly quantum mechanical effect. $^{1}$A. L. Landers, \textit{et al}. Phys. Rev. A \textbf{70}, 042702 (2004). [Preview Abstract] |
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O1.00069: Total cross sections for transfer-ionization in fast ion-helium collisions Alex Godunov, Jim McGuire, V. Schipakov, James Walters, Colm T. Whelan The effects of electron correlation and second order terms on theoretical total cross sections of transfer ionization in collisions of the helium atom with fast H$^+$, He$^{2+}$, and Li$^{3+}$ ions are studied and reported. The total cross sections are calculated using highly correlated wave functions with expansion of the transition amplitude in the Born series through the second order. The allowance for electron correlation directly determines how closely theoretical calculations agree with experimental data. Second-order calculations with uncorrelated functions differ considerably from experimental data. The expansion of the transition amplitude in the Born series through the second order, and allowance for the ``off-shell'' terms and electron correlation result in calculations that are in sensible agreement with experimental data, including the $v^{-b}$ dependence of the total cross sections. [Preview Abstract] |
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O1.00070: Enhancement of the 1s2s2p $^{4}$P State Associated with Electron Transfer in 1-2 MeV/u C$^{4,5+}$ + He and Ne Collisions D. Strohschein, D. Garvin , J. Baran, J.A. Tanis Auger electron emission spectra produced in collisions of 0.5, 1.5, and 2 MeV/u C$^{4,5+}$ with He and Ne targets were investigated. The measurements were conducted at Western Michigan University using the tandem Van de Graaff accelerator. Auger electrons emitted at zero degrees were detected using a high-resolution parallel-plate electron spectrometer located downstream from the target cell. Intensities of the autoionizing metastable (1s2s2p) $^{4}$P state were found to be strongly enhanced relative to the similarly configured (1s2s2p) $^{2}$P$_{-}$ and (1s2s2p) $^{2}$P$_{+}$ states in single and double electron transfer to C$^{4+}$(1s2s $^{3}$S) and C$^{5+}$(1s$^{2})$, respectively, in agreement with earlier results for $\sim $1 MeV/u F$^{7,8+}$ ions$^{1}$. For single transfer the 1s2s2p intensities produced in the mixed-state beam C$^{4+}$(1s$^{2}$+1s2s $^{3}$S) were compared with the intensities for the ground-state beam C$^{4+}$(1s$^{2})$ to extract the contribution due solely to the 1s2s $^{3}$S metastable beam. The role of Pauli exchange$^{1}$ in leading to enhancement of the $^{4}$P state is considered along with other mechanisms. $^{1}$J.A. Tanis \textit{et al}., Phys. Rev. Lett. \textbf{92}, 133201 (2004). [Preview Abstract] |
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O1.00071: Mutual Ionization in 200 keV H$^{-}$ + He Collisions Michael Schulz, Thomas Ferger, Daniel Fischer, Robert Moshammer, Alexander Voitkiv, Bennaceur Najjari, Joachim Ullrich We have performed a kinematically complete experiment on mutual ionization in 200 keV H$^{-}$ + He collisions by measuring the momentum analyzed recoil ions and both ejected electrons (from the projectile and the target) in coincidence with the neutralized projectiles. Comparison of the data to our calculations, based on various theoretical models, show that mutual ionization proceeds predominantly through the interaction between both electrons. The post-collision interaction between the outgoing ejected electrons as well as higher order processes involving the interaction between the core of both collision partners are also important. [Preview Abstract] |
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O1.00072: Angular Distributions and Dalitz plots for C$^{6+}$ ionization of He Sebastian Otranto, Ronald Olson, Juan Fiol Single ionization fully differential cross sections for 2 MeV/amu C$^{6+}$ + He collisions are presented and analyzed using the classical trajectory Monte Carlo (CTMC) and Continuum Distorted Wave (CDW) models. The present theoretical results are compared with recent experimental data of Fischer \textit{et al} [1]. The published experimental conditions are considered in the theoretical models. The inclusion of the thermal motion of the target atom leads to an improved description of the forward electron emission [2]. Moreover, we present cross sections in the plane perpendicular to that of the collision, for which experimental angular distributions have not been yet reported. Dalitz plots for single ionization fully differential cross sections in ion-atom collisions are presented and are used to help elucidate the collision dynamics. [1] D. Fischer, R. Moshammer, M. Schulz, A. Voitkiv and J. Ullrich, J. Phys. B: At. Mol. Opt. Phys. 36, 3555 (2003). [2] R. E. Olson and J. Fiol, Phys. Rev. Lett. 95, 263203 (2005). [Preview Abstract] |
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O1.00073: Electron stripping at MeV/u energies for U $^{28+ }$and Xe$^{18+ }$ ions . Robert Dubois, Oscar de Lucio, Mark Thomason Beam lifetimes were measured for 10 MeV/u to75 MeV/u Xe$^{18+ }$ and 10 MeV/u to 50 MeV/u U $^{28+}$. Lifetimes resulting from interactions with background gases at base pressures in the GSI ESR storage ring and for molecular hydrogen and molecular nitrogen were measured. For H$_{2}$ and N$_{2 }$, ion beams where injected in the ESR and a fast valve was opened to admit the supersonic jet from the ESR gas target. The N$_{2}$ results imply that the stripping cross section has approximately a v$^{-1}$ dependence, whereas the H$_{2 }$results imply nearly a v$^{-2}$ dependence. [Preview Abstract] |
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O1.00074: Absolute Charge Exchange Cross Sections for C$^{3,4,5,6+}$, N$^{4,5+}$, O$^{5,6,7+ }$, and Ne$^{7,8+ }$Collisions with H$_{2}$O, CH$_{4}$, CO, and CO$_{2}$ Richard Mawhorter, Nada Djuric, Sabbir Hossain, John MacAskill, Steven J. Smith, Ara Chutjian The success of the Deep Impact and Stardust missions is advancing our understanding of the structure and composition of comets. This information drives and enhances ongoing cometary EUV and X-ray studies. In turn, our experimental absolute charge exchange cross sections for collisions of highly-charged solar wind ions with cometary gases are necessary for detailed modeling and analysis of these studies. For systems studied earlier, these data agree with the previous measurements made with smaller apertures [1], demonstrating reproducibility and complete angular collection. The ion beam accelerating potential was typically 7 kV, which yields ion velocities consistent with the fast component of the solar wind. Slow solar wind velocity data for O$^{6+}$ on CO and CO$_{2}$ will also be presented. This work was carried out at JPL/Caltech, and was supported through contract with NASA. N. Djuric and S. Hossain also acknowledge support through the NASA-NRC program. [1] J.B. Greenwood, et al., Phys. Rev A \textbf{63}, 062707 (2001). [Preview Abstract] |
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O1.00075: Visible spectroscopy of collisions of solar-wind ions on gas targets of astrophysical interest Kenneth Miller, Christopher Verzani, Anne Wrigley, Phillip Gee, Quentin Kessel, Winthrop Smith In 1996 the ROSAT satellite discovered x-ray emission from comets. It has been established that the primary mechanism for the emission of cometary x-rays is charge exchange during collisions of highly-charged solar-wind ions with neutral gases of cometary atmospheres. The x-ray spectra from collisions of O$^{q+}$, C$^{q+}$, Ne$^{q+}$, and S$^{q+}$ with CO were investigated at Jet Propulsion Laboratory. We present a complimentary investigation of visible spectra from these collisions. Our 2 MV Van de Graaff accelerator was used to produce ions of O$^{q+}$, C$^{q+}$, Ne$^{q+}$, and S$^{q+}$ (q = 1 to a max of 5 for O) to collide with neutral targets of CO and H$_{2}$O. The research at the University of Connecticut has been sponsored by NASA EPSCoR Grant NCC5-601. [Preview Abstract] |
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O1.00076: Contributors to the solar-wind-induced spectrum of cometary x-rays: S and Ne L x-rays, Fe M x-rays and others C. J. Verzani, K. A. Miller, Q. Kessel, W. W. Smith, S.J. Smith, S. Hossain, A. Chutjian Cravens explained the presence of well resolved x-ray lines from comets as being due to the transfer of electrons from cometary gases (CO, H$_{2}$O, etc.) to create excited states of highly-charged C and O ions from the solar wind (1).~ However, these x rays, with energies in the 250 to 700 eV range, represent only a small fraction of the x rays observed compared with those found in the 50 - 250 eV range (2). Spectra obtained, using solar wind-type ions such as Ne$^{7+,8+}$, S$^{9+}$ and Fe$^{11+}$ from the JPL ECR ion source, suggest that soft x-ray lines from charge-transfer processes terminating on the n=2, and n=3 levels of solar wind ions may contribute significantly to cometary x-ray spectra in this range. \newline 1. T.E. Cravens, Science 297, 1042 (2002). \newline 2. K. Dennerl, J. Englhauser, Joachim Trumper, Science 277,1625 (1997). [Preview Abstract] |
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O1.00077: X-ray Emission Cross Sections following Charge Exchange by Multiply-Charged Ions of Astrophysical Interest. Sebastian Otranto, Ronald Olson, Peter Beiersdorfer State selective \textit{nl}-electron capture cross sections calculated using the classical trajectory Monte Carlo (CTMC) model are presented for highly charged ions with $Z = $6-10 colliding with atoms and molecules. The energy dependence of the $l$-level populations is investigated. The calculated cross sections are compared with measurements made by Greenwood \textit{et al} [1], using O$^{8+}$ and Ne$^{10+}$ on various targets at 3 keV/amu, and with recent x-ray emission cross sections measured with the EBIT machine at LLNL using O$^{8+}$ and Ne$^{9+,10+}$ on different targets at 10 eV/amu. We use the calculated cross sections to present an ab initio determination of the soft x-ray spectrum of comet C/Linear 1999 S4 that was observed on the Chandra X-ray Observatory [2]. [1] J. B. Greenwood, I. D. Williams, S. J. Smith and A. Chutjian, Phys. Rev. A 63, 062707 (2001). [2] C. M. Lisse, D. J. Christian, K. Dennerl, K. J. Meech, R. Petre, H. A. Weaver and S. J. Wolk, Science 292, 1343 (2001). [Preview Abstract] |
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O1.00078: Low Energy Charge Exchange Using the Upgraded ORNL Ion-Atom Merged-Beams Apparatus Charles Havener, Riad Rejoub The Oak Ridge National Laboratory (ORNL) Ion-Atom Merged-Beams Apparatus is used to measure absolute charge exchange cross sections for multicharged-ions on neutral atoms from kev/u down to meV/u collision energies. The apparatus has recently been upgraded and moved to accept beams from an all-permanent magnet ECR ion source mounted on a 20-250 kV High Voltage platform. Upgrades include a dual rotating-wire scanner which provides more accurate beam profiles and a shortened merge-path to increase the angular acceptance of the apparatus. A high transmission beam line from the HV platform and spherical sector electrostatic mergers provide an observed factor of four decrease in the angular divergence of the ion beam. This translates into a significant improvement in the collision energy uncertainty and allows access to lower energies with higher resolution. The higher velocity ion beams permit charge exchange measurements with heavier ions and measurements with both H and D at eV/u energies and below to directly observe the isotope effect. Measurements with N2+ + H are in progress and are compared to previous measurements with D. [Preview Abstract] |
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O1.00079: Proposed Development of a Novel Approach for Studying Anion-Neutral Reactions D.W. Savin, W. Mitthumsiri, M. Schnell, B. Seredyuk, C.C. Havener, M.E. Bannister, A. Dorn We propose a new method for studying anion-neutral reactions. We will use fast, merged anion-neutral beams and detect the charged end products. This novel approach will utilize the ion- atom merged-beams apparatus, a unique state-of-the-art facility located at Oak Ridge National Laboratory, and expand its capabilities by adding a detection system to collect detached product electrons. Future extensions will allow us to detect product anions. Laboratory beam energies will be in the keV range; but because the beams run co-linear, center-of-mass energies from the meV to keV range can be achieved. Proof-of- principle measurements will be carried out using the associative detachment process ${\mathrm H}^- + {\mathrm H} \to {\mathrm H}_2^- \to {\mathrm H}_2 + {\mathrm e}^-$. Published values for this reaction range over a factor of 5. The situation is little better for the time-reverse dissociative attachment process ${\mathrm H}_2 + {\mathrm e}^- \to {\mathrm H}_2^- \to {\mathrm H}^- + {\mathrm H}$. Our proposed research will help to resolve this issue. Future possible research directions include adding a cold molecular anion source in order to study reactions such as ${\mathrm X} + {\mathrm Y} {\mathrm Z}^-$. This will allow investigations into a wide range of anion-neutral reactions. [Preview Abstract] |
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O1.00080: Reactions of Protons and Hydrogen at Electron-volt Energies. A.G. Calamai, S.M. Brewer, J.C. Gleeson, B.W. Myer, L.M. Reynolds, R.J. Stamilio, A.N. Daw, M. Schnell, D.W. Savin We combined a simple time-of-flight (TOF) mass spectrometer in tandem with a cylindrical rf ion trap to monitor the temporal evolution of an ion population as it reacts with a neutral gas buffer following an electron impact interval which is used to create the ions. At precisely determined times following electron impact, the stored ions are extracted from the trap into an 82-cm flight path terminated by an active-film detector. As charge-exchange (CX) reactions typically dominate ion losses from the trap for neutral gas pressures $\ge $ 10$^{-9}$ Torr, an analysis of the TOF spectra as a function of the extraction time and neutral gas pressure will yield the relevant CX collision-rate coefficient. Initial measurements are being carried out using H$^{+}$ and H$_{2}$. A discussion of the apparatus, data associated with the reaction, and systematic issues associated with these measurements will be presented. [Preview Abstract] |
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O1.00081: An updated single electron capture cross section scaling rule for A$^{q+}$ + H$_2$ collisions Kevin Cornelius An updated high energy universal scaling rule for single electron capture cross sections is presented for A$^{q+}$ + H$_2$ collisions. The new scaling rule is built empirically on experimental data using a wide range of projectile ions and collision energies. The inclusion of such a broad range of values resulted in a model that more accurately predicts the single capture cross sections for a wide range of collision systems. The results of the current scaling rule are compared to three previous scaling rules to illustrate how using limited data in the development a universal scaling rule effects a models general predictive power. The reduction effects of all four scaling rules on both experimental and theoretical data are examined. [Preview Abstract] |
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O1.00082: Vibrationally-resolved charge transfer between O$^+$ ions and H$_2$ molecules L.B. Zhao, J.G. Wang, P.C. Stancil, Y. Li, R.J. Buenker, M. Kimura The non-dissociative charge transfer process O$^+$ + H$_2$ (v) $\to$ O + H$_2^+$ (v') has been investigated using the quantal molecular-orbital coupled-channel method. Vibrationally-resolved total and electronic state-selective cross sections based on the infinite order sudden approximation (IOSA) and the electronic approximation (EA) will be presented for relative collision energies from 0.1 to 1000 eV/u. The IOSA and EA results will be compared to the limited previous theoretical work and to the wealth of experimental data for both ground and metastable incident oxygen ions. [Preview Abstract] |
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O1.00083: Lattice method approach to $\bar{p}p$ formation in $\bar{p}$+H collisions: Inclusion of classical trajectories R. Cabrera-Trujillo, B. D. Esry In this work, we present calculations of protonium formation using a lattice approach for the electronic wavefunction and a classical treatment for the nuclear degrees of freedom. We implement our model in the laboratory reference frame as well as in the reference frame where the proton is fixed. Here the non-inertial terms are taken into account by means of a gauge transformation. We present preliminary results for the protonium formation cross section and the stopping cross section (nuclear and electronic) for low projectile energies in the few eV range. [Preview Abstract] |
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O1.00084: Numerical study of charge transfer in ion-ion collisions using the lattice time-dependent Schr\"odinger equation method T. Minami, M.S. Pindzola, T.-G. Lee, D.R. Schultz Br\"auning, et.al. (J. Phys. B, \textbf{38}, p2311) recently measured cross sections of charge transfer in collisions of He$^{2+}$ with Li$^{2+}$. At the same time, they also performed calculations of the cross sections of this collision system and of collisions of H$^{+}$ with He$^{+}$ using the basis generator method (BGM). For the former system, BGM seems to overestimate the cross sections compared to their experiment, while for the latter system, BGM provides excellent agreement with past experiments. Since both are ion-ion collision with asymmetrical nuclear charges involving one electron, BGM agreeing one system but the other points to a situation requiring further study. Motivated by this question, we have performed atomic orbital coupled channel (AOCC) calculations for both systems. When we use our most complete basis set for the calculations, we obtain results close to BGM on both systems. However, results from AOCC do not readily converge with regard to the size of the basis sets and therefore results were not conclusive. We also have performed the lattice time-dependent Schr\"odinger equation (LTDSE) calculations. Preliminary results for both systems by LTDSE are close to BGM. In this presentation, we compare BGM, AOCC and LTDSE with the experiment. This research used resources of NCCS at ORNL and NERSC, and was supported by DOE OFES. [Preview Abstract] |
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O1.00085: Hyperspherical close-coupling calculations for electron capture cross sections in low energy Ne$^{10+}$+ H(1s) collisions Patricia Barragan, Anh-Thu Le, C.D. Lin We present total and partial electron capture cross sections for Ne$^{10+}$ + H(1s) collisions at energies from 0.01 eV to 1 keV using the hyperspherical close-coupling method (HSCC). Good agreements with the previous calculations by the classical trajectory Monte-Carlo (CTMC) method are found for total capture cross section, but not for partial cross sections, especially below about 200 eV/amu. We found that the total cross section is mainly due to the population of the n=7 channels, and only at energies above 50 eV/amu n=5, 6 channels begin to contribute to the total cross section. [Preview Abstract] |
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O1.00086: Electron Transfer, Ionization, and Excitation in Collisions between $\alpha$ Particles and H($1s$) Atoms. Thomas Winter At the last DAMOP meeting,\footnote{T. G. Winter, Bull. Am. Phys. Soc. {\bf 50}, 82 (2005).} the author reported Sturmian calculations on electron transfer in 8-200 keV $\alpha$-H collisions, both checking numerically and extending to somewhat lower energies and substantially larger bases the calculations carried out a quarter of a century ago, which were limited to 19-24 states.\footnote {T. G. Winter, Phys. Rev. A {\bf 25}, 697 (1982).} The importance of revisiting earlier coupled-state calculations- --limited by the computers then available---was pointed out in a recent review.\footnote{ T. G. Winter, Adv. At. Mol. Opt. Phys. {\bf 52}, 391 (2005).} Present larger basis calculations should be able to extend the range of energies while treating ionization as well as capture and excitation to lower individual states up to $n=3$, albeit with much less accuracy than the summed cross section. Further, rough error bars may be placed on the cross sections by comparing values with different bases. The present calculations are with up to about 80 states on a 3.3 GHz IBM ThinkPad, taking on the order of one hour per integrated cross section. [Preview Abstract] |
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O1.00087: Regge Oscillations in Ion-Atom collisions S. Yu. Ovchinnikov, J.H. Macek, P.S. Krstic Regge oscillations associated with quantum corrections to the integral cross sections computed using the impact parameter method were previously discovered in H$^+$ + H cross sections \cite{MKO04}. In the present work we argue that these oscillations are a general feature of slow, sub-eV ion-atom collisions. This is illustrated by proton-noble gas elastic cross sections computed using {\it ab-initio} adiabatic molecular energy curves. A detailed analysis of the H$+$ + He collisions is given. This research has been supported by the Chemical Science, Geosciences and Biosciences Division, Office of Basic Energy Science, Office of Science, under Grant No. DE-FG02-02ER15283 and by Offices of Basic Energy and Fusion Energy Sciences, U.S. Department of Energy, through Oak Ridge National Laboratory, managed by UT-Battelle, LLC under contract DE-AC05-00OR22725. \begin{thebibliography}{99} \bibitem{MKO04} J. H. Macek, P. S. Krstic, and S. Yu. Ovchinnikov, Phys. Rev. Lett. \textbf{93}, 183203 (2004). \end{thebibliography} [Preview Abstract] |
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O1.00088: Circular Dichroism in Laser-Assisted Proton -- Hydrogen Collisions Thomas Niederhausen, Uwe Thumm We theoretically investigate the effects of a strong circularly polarized laser field on the electron dynamics in proton--hydrogen collisions. We compare results for two classical trajectory models that include either two or all three degrees of freedom of the electron and assess dimensionality effects. In the 2D model the electron is restricted to move in the scattering plane that also contains the laser electric field~[1]. We study the dependence of the electron capture and ionization probability on laser (intensity, laser phase at collision time) and projectile parameters (impact parameter). We provide new 3D ab-initio results for capture and ionization, exhibiting a strong dependence on the initial laser phase and the impact parameter. We find that charge-resonant enhanced ionization at larger impact parameters for the collision reaction is strongly phase dependent.\newline [1] T.~Niederhausen, B.~Feuerstein, and U.~Thumm, Phys. Rev. A \textbf{70}, 023408 (2004). [Preview Abstract] |
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O1.00089: Laser assisted charge transfer in He$^{++}$+H collisions Fatima Anis, V. Roudnev, R. Cabrera-Trujillo, B. D. Esry We present a study of He$^{++}+$H collision in the presence of linearly polarized laser field. We performed three dimensional calculations in the semi-classical impact parameter approximation. A remarkable enhancement of four to five fold in the capture cross section is seen even for a moderate intensity of $3.5\times10^{12}$W/cm$^2$. Our results are for linearly polarized light both perpendicular and parallel to the collision plane. We focus on the parameters easily accessible in the laboratory and discuss the possibility of performing such an experiment. [Preview Abstract] |
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O1.00090: COLTRIMS Study of Transfer-Excitation in Proton-Helium Collisions A. Hasan, B. Tooke, M. Schulz We have carried out a cold-target recoil-ion momentum spectroscopy (COLTRIMS) measurement for p + He collisions to study the capture of one and simultaneous excitation of the second target electron (transfer excitation, TE) as a function of the projectile scattering angle and velocity. The measurement was carried out at projectile energies of 25, 50, and 75 keV. The neutralized projectiles were measured in coincidence with the recoil ions. We obtained the energy loss of the projectile from the longitudinal component of the recoil-ion momentum and the scattering angle from its transverse component. The energy loss, in turn, was used to identify and distinguish TE from single capture. Absolute differential cross sections for TE as well as for state-selective single capture were obtained. Furthermore, cross section ratios of transfer excitation to single electron capture were analyzed. The role of correlated versus uncorrelated TE mechanisms will be discussed. [Preview Abstract] |
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O1.00091: Angular Distributions of Ions Transmitted by a Nanocapillary Array H.F. Krause, C.R. Vane, F.W. Meyer, H.M. Christen Angular distributions of incident 10--20 keV/q multi-charged ions of Ar and Ne (q=3,7) transmitted in an aluminum oxide nanocapillary array were studied. The diameter and length of the pores are 100 nm and 60 micron, respectively. Charge-state selected angular distributions were obtained. The principal transmitted q-state (incident q-state) is about seven orders of magnitude smaller than the surface porosity ($\sim $30{\%}). The angular distributions, composed of many narrow peaks, can be steered in the longitudinal direction of the nanopores within $\sim \quad \pm $ 1 degrees by rotating the sample with respect to the incident beam. Target tilting experiments prove that the angular structure arises when ions bounce at ultra-low grazing angles in very large impact parameter ($>$10 nm) Coulomb collisions with electrically charged nanopore walls.$\backslash $This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy under Contract No.DE-AC05-00OR22725 with UT-Battelle, LLC. [Preview Abstract] |
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O1.00092: Studies of Xenon Rydberg Atoms at Au(111) and Heavily-Doped Si(100) Surfaces Hardin Dunham, Jim Lancaster, F. Barry Dunning, Stephen Wethekam The ionization of xenon atoms excited to the lowest states in the $n $= 17 and $n $= 20 Stark manifolds at a flat Au(111) surface is being studied. The data show that, despite the strong perturbations in the energies and structure of the atomic states that occur as the surface is approached, the experimental data can be well fit by assuming that the ionization rate on average increases exponentially as the surface is approached, and the inferred mean ionization distances are in good agreement with theoretical predictions. Under appropriate conditions, each incident atom can be detected as an ion. Similar studies are underway using heavily-doped n- and p-type Si(100) surfaces. Marked differences in behavior to the Au(111) surface are observed and further work is being undertaken in an attempt to identify the reasons for this. Possible candidates include reduced image charge effects and local electric fields at the surface. [Preview Abstract] |
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O1.00093: A study of Rb hyperfine polarization at submicron distance from glass surfaces K. Zhao, Z. Wu We study {\it regionally specific} hyperfine polarization $\langle\mbox{\boldmath $S\cdot I$}\rangle$ of Rb atoms at submicron distance from coated and uncoated Pyrex glass surfaces in optical pumping cells. This is in contrast to the previous hyperfine polarization studies, where the quantity measured is the bulk hyperfine polarization, which depends on surface interactions {\it averaged over the entire cell surfaces}. We probe the hyperfine polarization of the Rb atoms in the vicinity of cell surfaces using the evanescent wave of a weak laser beam. We find that the polarization in the vicinity of uncoated surfaces is significantly lower than that in the bulk. The polarization decreases rapidly with decreasing distance from the surface. By contrast, the polarization in the vicinity of a silicone coated Pyrex glass surface is independent of the distance from the cell surface and is equal to the bulk polarization. Regionally specific measurement of the hyperfine polarization as a function of the penetration depth of the evanescent wave allows us to deduce the hyperfine polarization, its normal gradient, as well as the normal gradient coefficient $\mu_{\mbox {\boldmath \tiny{$S\cdot I$}}}$ at the cell surface. We also demonstrate that hyperfine polarization at submicron distance from the cell surface can be used as a novel way to quantify and map the regional quality of the coatings inside optical pumping cells. [Preview Abstract] |
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O1.00094: Molecular Dynamics Simulation of Chemical Sputtering S.J. Stuart, P.S. Krstic, C.O. Reinhold We study chemical sputtering by D and D$_{2}(\upsilon )$ at deuterated amorphous carbon surface. The dynamics of the surface characteristics is a function of the initial surface and the cumulative effect of the projectiles. We study evolution of the spectrum of the sputtered particles, hydrocarbons, in search for the steady ``state'' sputtering yield. The comparison of the steady yields of atoms vs. molecular projectiles in various vibrationally excited states enables interpretation of the ORNL experimental results. [Preview Abstract] |
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O1.00095: Atomistic Modeling of Segregation and Alloying in Epitaxial Ag films on Al(100) Cary Pint, Guillermo Bozzolo, Richard Smith, Jorge Garces The Bozzolo-Ferrante-Smith (BFS) method for alloys is applied to the study of interfacial alloying and interdiffusion in epitaxial Ag layers on Al(100). For submonolayer coverage, Ag is seen to first grow epitaxially until bulk diffusion sets in at higher temperatures. The presence and magnitude of this diffusion energy barrier has significant consequences for Ag films on Al(100), where stable layers of Ag are formed on the Al(100) substrate at low temperatures. This is followed by a progressive segregation of the Ag atoms into the Al bulk upon heating, accompanied by the formation of a stable interfacial compound. This behavior becomes more pronounced with higher Ag coverage. A simple analysis of the energetics indicates interesting trends during this process- particularly a chemical energy transition that shifts to higher temperatures with increasing Ag coverage. The results from this simple modeling technique seem to be in agreement with the behavior of Ag/Al(100) established through previous experiment. [Preview Abstract] |
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O1.00096: Neutralization of H- on vicinal surfaces Boyan Obreshkov, Uwe Thumm Experiments [1,2] and theoretical calculations [3] for the neutralization of H- ions on metallic surfaces show a dependence on the surface morphology. The availability of nanostructured metallic surfaces motivates the study of the effects of the surface structure on the neutralization probability of H-. We discuss the results from a theoretical study of the electron charge transfer rate and neutralization probability of H- ions colliding with metal surfaces, based on a generalized Thomas- Fermi approach for the description of the equilibrium electronic structure of the surface [4]. The ion-surface scattering calculations were performed for vicinal surfaces with different step densities at ion collision energies of 1 keV. For the studied range of collision parameters and surface morphologies, our numerical results for the neutralization probability for ``step up'' and ``step down'' scattering are significantly different, and the anion is more efficiently neutralized if the out asymptote of its scattering trajectory crosses vicinal structures in ``step down'' direction. \newline [1] E.Sanchez, L.Guillemot, V.A.Esaulov, Phys. Rev. Lett. 83, 28 (1999). \newline [2] T.Hecht et al., Phys. Rev. Lett. 84, 2517 (2000). \newline [3] H.Chakaraborty, T.Niederhausen and U.Thumm, Phys. Rev. A 70, 052903 (2004); 69, 052901 (2004). \newline [4] E.Zaremba and H.C.Tso, Phys. Rev. B 59, 2079 (1999). [Preview Abstract] |
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O1.00097: First Application of Engineered Very-High-$n$ Polarized Rydberg States Jeff Mestayer, Wei Zhao, Jim Lancaster, Barry Dunning, Carlos Reinhold, Shuhei Yoshida, Joachim Burgdorfer Direct excitation of very-high-$n$ quasi-one-dimensional (quasi-1D) Stark states in a weak dc field is problematic because stray fields and effective laser linewidths lead to creation of a range of states with no preferred orientation. We show, however, that very-high-$n$ ($n \quad \sim $ 600) quasi-1D atoms can be engineered from lower-n quasi-1D atoms using a tailored sequence of half cycle pulses (HCPs). We exploit this to realize the high-scaled-frequency regime of the ``kicked Rydberg atom'' by subjecting the engineered atoms to a periodic train of HCPs, reaching scaled frequencies as high as $\nu _{0} \quad \equiv \quad \nu _{T}$/$\nu _{n}$ $\sim $ 15, where $\nu _{T}$ is the frequency of the train and $\nu _{n}$ is the classical electron orbital frequency. Pronounced non-monotonic structure in the survival probability is observed as N, the number of HCPs in the train, is increased, the survival probability actually increasing with N over certain ranges of N. This behavior is very sensitive to the polarization of the Rydberg states and is explained using classical simulations. Research supported by NSF, DoE, the R. A. Welch foundation and the FWF (Austria). [Preview Abstract] |
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O1.00098: Collisional Properties of Dipole-Bound Negative Ions Y. Liu, M. Cannon, L. Suess, F.B. Dunning A Penning ion trap is being used to study low-energy collisions between dipole-bound CH$_{3}$CN$^{-}$ ions and a variety of attaching targets and polar molecules. In dipole bound negative ions the extra electron is weakly bound by the dipole potential of the neutral molecule in a diffuse orbital localized near the positive end of the dipole. In consequence, it is reasonable to expect that such species will be quite reactive. Charge transfer is observed in collisions with CH$_{3}$NO$_{2}$ and CS$_{2}$ leading to the formation of long-lived ($\tau \sim $ 1 ms) CH$_{3}$NO$_{2}^{-}$ and CS$_{2}^{-}$ ions. Destruction is seen in collisions with polar targets as a result of rotational energy transfer. The measured reaction rates are large ($\sim $ 10$^{-7}$ cm$^{3}$ s$^{-1})$ and different reaction models are being explored. The data demonstrate the rich variety of processes that can occur in collisions involving dipole-bound anions. [Preview Abstract] |
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O1.00099: Temperature Dependent Studies of Ion Lifetimes M. Cannon, Y. Liu, L. Suess, F.B. Dunning The lifetimes of SF$_{6}^{-}$ ions produced in K(np)/ SF$_{6}$ collisions at high n , n$\ge $ 30 are being investigated over a temperature range of 300K $\sim $600K. At room temperature, it is observed that collisions lead predominantly to the formation of long-lived ($\tau \ge $ 1ms) SF$_{6}^{-}$ ions. The mean ion lifetime, however, decreases as the target gas temperature is increased and the data provide evidence of creation of short-lived ions having a range of lifetimes that extends below one microsecond. Such behavior is consistent with simple statistical theory. Limited SF$_{5}^{-}$ production is also observed at the higher temperatures. Measurements are being extended to a range of other attaching targets to further examine the role of internal energy in governing electron capture processes. [Preview Abstract] |
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O1.00100: Phase-dependent electron dynamics of a two-level atom driven by a resonant microwave field H. Maeda, J. Nunkaew, T.F. Gallagher When exposed to a weak radiation field an atom undergoes the Rabi oscillation if the radation frequency is resonant with one of atomic electric dipole transitions. A probability of finding the atom in a ground/excited state oscillates at the Rabi frequency, and the oscillation accompanies absorbing/emitting the energy from/to the radiation field, depending on the relative phase between the field and a dipole moment induced in the atom. Here we have demonstrated these features by detecting phase-dependent motion of the electron along the field axis (z) in a Rydberg atom which is driven by a linearly polarized resonant microwave (MW) field. In the experiment electron motion of Yb and Li Rydberg atoms has been monitored using a sub-ps half-cycle pulse (HCP), which works as a time-resolved detector of the electron’s momentum. The HCP is polarized along the polarization axis of the MW field and synchronized to it, so that it can ionze the atoms when the z component of the electron’s momentum exceeds a value determined by the HCP amplitude as a function of the the MW phase. We have also studied the phase dependence and change in amplitude of the induced dipole moment around an avoided crossing between neighboring Rydberg states, showing that at the crossing the electron motion is in fact phase-locked to the MW field and the induced dipole moment is maximum. This work has been supported by the NSF. [Preview Abstract] |
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O1.00101: Intuitive and counter-intuitive multiphoton adiabatic rapid pasasage H. Maeda, J.H. Gurian, D.V.L. Norum, T.F. Gallagher In coherent population transfer via a sequence of adiabatic rapid passages through overlapping single-photon resonances, the oscillating dipole remains phase-locked to the field as the frequency of the radiation field is chirped [1]. The process is intuitive, and the atomic frequency follows that of the chirped radiation field [1]. Efficient counter-intuitive population transfer can also occur in which the atomic frequency changes in the direction opposite the change in the radiation frequency [2,3]. In such cases the sequence of single-photon transitions of the intuitive transfer is replaced by a single multiphoton transition, enormously reducing the range of the chirp required to effect the population transfer. Such multi-photon adiabatic rapid passage occurs in both an intuitive and a counter-intuitive manner. We present results of such population transfers among the Rydberg states of atomic Li around $n=70\sim90$ using a frequency chirped microwave pulse. This work has been supported by the NSF. [1] H. Maeda, D.V.L. Norum, and T.F. Gallagher, Science {\bf 307}, 1757 (2005). [2]N.V. Vitanov {\it et al.}, Annu. Rev. Phys. Chem. {\bf 52},763 (2001); N.V. Vitanov {\it et al.}, Adv. At. Mol. Opt. Phys. {\bf 46}, 57 (2001).[3] B. Broers, H.B. van Linden van den Heuvell, and L. D. Noordam, Phys. Rev. Lett. {\bf 69}, 2062 (1992). [Preview Abstract] |
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O1.00102: Applications of Ponderomotive Potentials to Rydberg Atoms B. Knuffman, G. Raithel The ponderomotive potential is a time-averaged potential experienced by free electrons in a light field and is proportional to the intensity of that field. A Rydberg atom is essentially composed of a quasi-free electron that is weakly bound to an ionic core. This being the case, Rydberg atoms can be manipulated via the ponderomotive interaction between the quasi-free Rydberg electron and laser fields. In this poster, we present experimental plans for several applications of ponderomotive potentials to Rydberg atoms. For optical standing waves, the ponderomotive potential is spatially periodic and may be used to form an optical lattice for Rydberg atoms. In addition to trapping Rydberg atoms, driving transitions between Rydberg states may be achieved by amplitude- or phase-modulating the potential experienced by the Rydberg atoms. Using this method, coupling Rydberg states should be possible in a spatially-selective way without the direct application of microwaves to the Rydberg-atom sample. This technique could provide future opportunities for Rydberg spectroscopy, including high-precision measurements of the Rydberg constant. [Preview Abstract] |
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O1.00103: Long-range Rydberg-Rydberg interactions and molecular resonances Jovica Stanojevic, Robin C\"ot\'e We present a detailed theoretical treatment to describe the lineshape of molecular resonances in a cold dense gas of rubidium Rydberg atoms. We calculate molecular potentials in Hund's case (c) by diagonalization of an interaction matrix and show how the strong $\ell$-mixing due to long-range Rydberg-Rydberg interactions can lead to resonances in excitation spectra. Such resonances were first reported in S.M. Farooqi {\it et al.}, Phys. Rev. Lett. {\bf 91} 183002, where single UV photon excitations from the $5s$ ground state to $np$ Rydberg states occurred at energies corresponding to normally forbidden transitions or very far detuned from the atomic energies. Here, we focus our attention on resonances at energies corresponding to excited atom pairs $(n\!-\!1)p_{3/2}+(n\!+\!1)p_{3/2}$. We find a very good agreement between our theoretical and experimental lineshapes. [Preview Abstract] |
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O1.00104: Blockade and antiblockade of Rydberg excitations in ultracold gases Cenap Ates, Thomas Pohl, Thomas Pattard, Jan Michael Rost Experimental studies of the impact of strong interatomic interactions on the Rydberg excitation dynamics in ultracold gases have recently led to a verification of the interaction-induced suppression of excitation compared to a non-interacting reference system. This blockade effect has been demonstrated directly via the fraction of atoms that are excited by a laser pulse, and indirectly via the sub-Poissonian counting statistics of the number of excited atoms. For a theoretical treatment, we have developed a microscopic approach based on a description of the single-atom dynamics within a rate equation approximation and a Monte Carlo treatment of the many-particle system. This method is particularly well suited for a study of the statistical properties of the excitation process. Moreover, we show that under certain circumstances a reversal of the blockade effect is possible, leading to an enhancement of excitation rather than a suppression. [Preview Abstract] |
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O1.00105: Semiclassical and quantum methods for atomic spectra Chris Schleif, John Delos We use quantum and semiclassical methods to study the spectra of atoms in electric and magnetic fields. For Hydrogen in crossed fields we are examining a predicted phenomenon called monodromy. For Argon in an electric field we are examining closed orbits and the recurrence spectra. [Preview Abstract] |
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O1.00106: Binding energy of e$^+$Li using the Peach model potential. Janine Shertzer, Sandra Ward The $l-$independent, parametric model potential developed by Peach$^1$ for describing the electron interaction with the alkali ion core yields energy levels that are in excellent agreement with experiment. Because of its relative simplicity, this model potential is an attractive choice for studying e$^+$- Li collisions;$^{2,3}$ the e$^+$-ion core interaction is obtained by changing the sign of the static term in the interaction. In order to test the usefulness of the potential for describing the physics of an effective three-body system, we calculated the binding energy of e$^+$Li. This is a stringent test, because the system is very weakly bound. Our results are in excellent agreement with previous calculations,$^4$ including those using the exact four-body Hamiltonian.$^5$ This work was funded by NSF under collaborative Grant PHYS-0440714 (JS) and PHYS-0440565 (SJW). $^1$G. Peach, H.E. Saraph and M.J. Seaton, J. Phys. B \textbf{21}, 3669 (1988). $^2$M.S.T. Watts and J.W. Humberston, J. Phys. B \textbf{25, }L491 (1992). $^3$S. J. Ward and J. Shertzer, Phys. Rev. A \textbf{68, }032720 (2003). $^4$J. Mitroy, M.W.J. Bromley, and G.G. Ryzhikh, J. Phys. B \textbf{35}, R81 (2002). $^5$Massimo Mella, Gabriele Morosi, and Dario Bressanini, J. Chem. Phys. \textbf{111}, 108 (1999). [Preview Abstract] |
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O1.00107: Reactive collisions of atomic antihydrogen with H, He$^+$, He, H$_2^+$, and H$_2$ James S. Cohen The fermion molecular dynamics (FMD) method has been used to determine the rearrangement and destruction cross sections for collisions of antihydrogen ($\bar{H}$) with H, He$^+$, He, H$_2^+$, and H$_2$ at collision energies above 0.1 au. The results for the H and He$^+$ targets satisfactorily merge with previous calculations done for lower collision energies. There are no previous calculations for the other targets. Despite the absence of a critical distance, the destruction cross section for collisions of $\bar{H}$ with He is found to be comparable with the destruction cross sections for $\bar{H}$ collisions with H and He$^+$, for which there are critical distances. The three atomic cross sections are shown to be given quite reasonably by simple classical orbiting formulas at energies that are very low but still high enough for $L>0$ partial waves to be dominant. The cross sections for formation of the antiprotonic atoms (Pn or $\bar{p}$He) and their initial quantum numbers are found to be significantly different from the analogous cross sections for $\bar{p}$ projectiles. The cross sections for the molecular targets are significantly larger. [Preview Abstract] |
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O1.00108: Study of critical dynamics in a dynamic double-well system of driven cold atoms Myoung-sun Heo, Ki-hwan Lee, Changil Ryoo, Dahyun Yum, Yonghee Kim, Kihwan Kim, Wonho Jhe While critical phenomena in equilibrium systems has been well known both in theory and in experiments, those studies in non-equilibrium or far-from-equilibrium systems are still challenging subjects. These have been studied in a number of systems. Laser cooled confined atoms also can be a good candidate since we are able to easily change its temperature and numbers. By parametrically modulating magneto-optical trap we have observed several interesting phenomena such as dynamic double well, Hopf bifurcation and spontaneous symmetry-breaking(SSB). Particularly SSB is approximately identified as Ising-like phase transition. It is well known that fluctuation near critical point causes the relaxation time longer. We have measured critical slowing down approaching the critical point varying the total number. [Preview Abstract] |
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O1.00109: A classical spinor approach to the quantum/classical interface: background and new results Renan Cabrera, William E. Baylis We first review how classical results take quantum form\footnote {W. E. Baylis in \emph{Computational Noncommutative Algebra and Applications}, Proc. NATO Advanced Study Institute, NATO Science Series II, Vol. 136, ed. J. Byrnes (Kluwer Academic, Dordrecht 2004), pp. 127---154 and 363---387.} in the algebra of physical space (APS). APS is the Clifford (or geometric) algebra of 3-D Euclidean space, and its paravectors form a 4-D linear space with the Minkowski spacetime metric. It provides a spinor description of classical relativistic dynamics that bridges much of the traditional gap between classical and quantum formulations. We then provide details of new quantum and classical eigenspinor solutions for charges in electromagnetic fields, including the use of null coordinates to reduce the spinor equations of motion in the presence of electromagnetic plane waves. [Preview Abstract] |
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O1.00110: Computational Approaches for Solving the Second Quantized Schroedinger Equation Using Non-Orthogonal Basis Sets Andrew Giustini, Brian Granger The second quantized approach to quantum mechanics provides an efficient method of incorporating the exchange symmetry of the many body wavefunction. The usual formulation, however, requires that field operators be expanded in an orthogonal basis. Using the theory developed by Artacho and del Bosch (PRA 43, 5770 (1991)) we have developed a numerical method for solving the second quantized Schroedinger equation using nonorthogonal basis sets, such as B-splines and finite elements. We present an implementation of this approach in the Python programming language and discuss preliminary results of the efficiency and performance of the method. [Preview Abstract] |
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O1.00111: Interaction of a Single-Mode Laser With a Generalized Sturmian Description of Helium: Application of a Space-Time Basis Time Propagation Algorithm Justin McDowell, Eddie Red, Albert Wynn III, Charles Weatherford \noindent The method of Generalized Sturmians is applied to the calculation of the ground and excited states of the helium atom. In the process, the screening constant of each one-electron Sturmian orbital is non- iteratively uniquely determined. Also, the usual Hartree-Fock calculation is avoided. The entire helium spectrum is thus calculated with approximately the same accuracy for the excited and ground states. The interaction of a single-mode laser with helium is then simulated by solving the time-dependent Schr\"odinger equation using a space-time basis propagation algorithm. The method is implicit and is quite stable. The STIRAP (Stimulated Raman Adiabatic Passage) noniterative control scheme will be used to manipulate the helium excitation by the laser. [Preview Abstract] |
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O1.00112: A novel table-top device for the single-atom detection of Carbon-14 Fred W. Meyer, Ernst Galutschek Carbon-14 labeled compounds are widely used in the pharmaceutical industry, e.g., as tracers to determine the fate of these compounds \textit{in vivo}. The sensitivities of most present methods are inadequate to permit utilization of sufficiently small quantities of $^{14}$C to avoid the issues of radioactive waste and contamination, both of which are unacceptable for environmental, health and safety, and financial reasons. A new compact $^{14}$C detection apparatus has recently been developed that uses low-energy multicharged carbon beams with charge state of +3 or higher to eliminate molecular isobar interference at mass 14. After magnetic selection of the desired charge state, the ion beam, which will still be dominated by $^{14}$N multicharged ions of the same charge state, is directed to an insulator single-crystal surface at grazing incidence, where efficient negative ion formation takes place without appreciable energy loss of the scattered ions. Two stages of electrostatic analysis spatially separate the desired $^{14}$C$^{-}$ ions from scattered neutrals and other background prior to their detection on a two-dimensional position-sensitive detector (2-D PSD). Unique characteristics of the apparatus are its small size, low cost, high efficiency (i.e., throughput), and ease of sample preparation, in comparison with the conventional AMS approach. Initial test results using large-area LiF and KBr single crystal targets will be presented. [Preview Abstract] |
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O1.00113: Analysis of Electric Properties of DNA Nucleotides R. Zikic, X.-G. Zhang, P.S. Krstic, J.C. Wells, M. Fuentes-Cabrera Calculation of the quantum tunnelling conductance through the DNA nucleotides between gold nanoelectrodes and analysis of the corresponding molecular spectra reveals that the tunneling conductance at low electric bias can be separated into two simple and approximately independent factors. The first is an exponential factor due to the potential barrier between the molecule and the electrode. The second factor is different for each molecule, but follows a universal form that can be expressed in terms of the bending angle of the DNA base relative to the sugar-phosphate group. This factor is also oscillatory indicating interference and resonance effects inside the molecule. Distinguishable conductances of Adenine (A), Cytosine (C), Guanine (G), and Thymine (T) are correlated to their differences in geometric dimensions. [Preview Abstract] |
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O1.00114: Detection of brain magnetic fields with an atomic magnetometer Hui Xia, Dan Hoffman, Andrei Baranga, Michael Romalis We report detection of magnetic fields generated by evoked brain activity with an atomic magnetometer. The measurements are performed with a high-density potassium magnetometer operating in a spin-exchange relaxation free regime. Compared to SQUID magnetometers which so far have been the only detectors capable of measuring the magnetic fields from the brain, atomic magnetometers have the advantages of higher sensitivity and spatial resolution, simple multi-channel recording, and no need for cryogenics. Using a multi-channel photodetector array we recorded magnetic fields from the brain correlated with an audio tone administered with a non-magnetic earphone. The spatial map of the magnetic field gives information about the location of the brain region responding to the auditory stimulation. Our results demonstrate the atomic magnetometer as an alternative and low cost technique for brain imaging applications, without using cryogenic apparatus. [Preview Abstract] |
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O1.00115: Quantum Beats Magnetometer Scott Seltzer, Parker Meares, Michael Romalis In Earth’s magnetic field the Zeeman resonance in alkali atoms is split into several lines given by the Breit-Rabi equation. We demonstrate coherent optical pumping of quantum beats corresponding to interference between different Zeeman resonances. Using a circularly polarized pump laser transverse to the direction of the magnetic field and modulated at both the average Larmor frequency and the frequency corresponding to the second-order splitting between Zeeman lines, we pump potassium atoms into a superposition state corresponding to maximum transverse spin precession. This double-modulation technique has several advantages for an Earth-field atomic magnetometer. In addition to increasing the signal strength, it largely eliminates the heading error caused by changes in the relative strength of the Zeeman resonances depending on the orientation of the magnetometer relative to the magnetic field. [Preview Abstract] |
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O1.00116: Investigation of high energy efficiency of white LEDs Junho Jeong, Ho-Ki Jeong White LEDs with high energy efficiency on the single chip emitting blue light Since the white LEDs with UV light source made on the single chip emit a little of UV light, it produces physical mutagens and heat that reduces energy efficiency. In order to solve these problems, the white LEDs on single chip emitting blue light has been researched by using the law of the energy conservation related to electron scattering. Because blue wavelength, which passes through phosphors or p-n junctions, is converted to orange, yellow or red wavelengths without e-h direct recombination, energy loss is very low. And we have researched how to fabricate white LEDs high current (more than 1.0 A) and voltage as heat loss of conventional white LEDs. [Preview Abstract] |
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O1.00117: The use of a shaped mesh lens in electron optics. B.A. deHarak, N.L.S. Martin In electron impact ionization studies, an electron optical system is often used to (a) transport electrons from the interaction region to the entrance of an energy analyzer, and (b) adjust the energy of the electrons to that required by the analyzer. To obtain good resolution using electrostatic energy analyzers, such as the hemispherical sector type, low pass energies (a few eV) are required. However, such analyzers are often used in the detection of electrons with energies of tens of eV ({\it e.g.} those ejected from the He $2\ell2\ell'$ autoionizing levels with $E_{ej}\sim35$eV) and, if conventional electron optics are used, the required retardation increases the angular spread of trajectories at the analyzer entrance. This has the unwelcome effect of either degrading the energy resolution, or, if angular stops are used, substantially reducing the intensity. We have designed an electron optical system that uses shaped mesh lenses to achieve the required retardation without the attendant increase in angular spread. We will present simulations using SIMION 3D 7.0 Ion and Electron Optics Software. [Preview Abstract] |
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O1.00118: The ORNL MIRF upgrade project Fred W. Meyer, Mark E. Bannister, Jerry W. Hale, Jim W. Johnson, John W. Sinclair A major facility upgrade of the ORNL Multicharged Ion Research Facility (MIRF) has recently been completed. It consists of the installation of a 250-kV high-voltage platform with a new all-permanent magnet ECR ion source, a new beamline switchyard for transporting the higher energy beams to on-line experiments, and reconfiguration of the present CAPRICE ECR ion source for injecting extracted beams into a floating beamline to permit deceleration to energies as low as a few eV x q upon entry into grounded experimental chambers. With the two sources, the range of energies available at MIRF has been expanded to more than five orders of magnitude. The new ECR ion source installed on the HV platform was designed and built at CEA Grenoble and operates in the frequency range 12.75 -- 14.5 GHz. High-voltage-platform components and beamline components up to the various end-stations are controlled and monitored via Allen-Bradley ControlLogix programmable logic controllers (PLC's) that are integrated into a Linux-hosted, EPICS-based distributed control system. Additional design and performance details of the upgraded facility will be provided at the conference. [Preview Abstract] |
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O1.00119: Using Molecular Dissociation to Determine the Efficiency of a Delay Line Microchannel-plate Detector B. Gaire, A.M. Sayler, P.Q. Wang, Nora G. Johnson, M. Leonard, E. Parke, K.D. Carnes, I. Ben-Itzhak The detection efficiency of a delay line microchannel-plate (MCP) detector has been measured using the dissociation and ionization channels of a heteronuclear diatomic molecular ion. The method is based on the fact that molecular breakup always yields two hits on a detector, but due to finite detection efficiency some of these events are recorded as single particles while others are detected in coincidence. This method is applicable for both timing and position signals of the detector. To demonstrate the application of this method we will present the detection efficiency measured with HD$^{+}$ molecular-ion breakup. [Preview Abstract] |
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O1.00120: RF Trapping and Mass Spectrometry with Low Mass Ions Robert Thompson, Jeremie Choquette, Daniel Foster Low mass ions, or more specifically ions with a low mass to charge ratio, are of extensive interest for high resolution study as their simple electronic structure and strong interaction with electromagnetic fields make them ideal practical systems for study of a range of theoretical models. However, the low mass of these species can make for added complications, especially in traps designed to hold a range of ion masses, due to the fact that low mass-to-charge ratios tend to correlate with high stability parameter (the so-called q-factor) values. This presentation will focus on recent work examining two topics: (1) an alternate ion-trap mass spectrometry technique, the so-called q-scan mass spectrometry, that has advantages with low mass ions; and (2) a practical hole just below q = 0.908 in the nominal ion trapping stability region for a linear Paul trap. Both experimental and computational / theoretical results for each project will be presented. [Preview Abstract] |
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O1.00121: Magnetic Field Control and Stabilization Using a Current Transducer and the Applications for Optically Trapped Atoms C. Welford, M. Junker, D. Dries, J. Hitchcock, Y. P. Chen, R. G. Hulet Magnetic fields created by driving current through coils are used both to provide the trapping force in magnetic traps such as Ioffe-Prichard traps and as a means to control the atomic interactions in optical-dipole traps via Feshbach resonances. Field stability is important to achieve accurate measurements, since the resonant atomic frequency varies with field, as does the atomic scattering length. In addition, the ability to rapidly change the magnetic field has various applications in experiments using Feshbach resonanaces. A means for improving the current stability and rate of change has been implemented using a current transducer, by precisely measuring the current and providing feedback to a controlled FET. The bandwidth has been measured at 6 kHz and the device will be characterized to ensure there is no noise near trapping frequencies, which would otherwise cause heating and loss from the trap. [Preview Abstract] |
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O1.00122: Novel Method for Laser Beam Profile Determination Giorgi Veshapidze, Marc Trachy, Mudessar Shah, Brett DePaola A novel method for laser beam size determination has been developed, which greatly increases the accuracy of beam size estimation. The conventional technique of driving a knife-edge across the beam was used, but instead of differentiating and fitting the result with a Gaussian function, the raw data were fit to an analytical approximation to the complementary error function. The advantage of this technique over the conventional one is that differentiation, and the resulting scatter in the data, is not required. The results from the improved method are within the fitting uncertainty of the results obtained with the conventional method. However, the fitting error is substantially reduced. [Preview Abstract] |
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O1.00123: A stable 657nm laser for a Ca atom interferometer Brian Neyenhuis, Christopher Erickson, Rebecca Tang, Greg Doermann, Marshall van Zijll, Dallin Durfee We will present an extremely stable laser to be used in an atom interferometer. A 657nm grating-stabilized diode laser is locked to a high-finesse cavity using the Pound-Drever-Hall method. Utilizing a feedback circuit with a bandwidth of ~5 MHz we see a laser linewidth less than one kHz. In addition to a relatively high bandwidth, our circuit design allows for mode-hop-free scanning over a large range. We are also working on several improvements which should further reduce our linewidth; we are improving passive mechanical and thermal stability of the laser and the optical cavity and plan to change to a higher finesse cavity, we have designed and are testing a more stable current driver based on an updated Hall-Libbrecht design, and we calculating an optimized multiple-input feedback transfer function for our system. We will also present the measurement of the resonances of our optical cavity relative to the Ca intercombination line using a high-temperature vapor cell. [Preview Abstract] |
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O1.00124: Excitation of Atoms with Magnetic Microstructures Matthew Eardley, Ying-Ju Wang, Svenja Knappe, John Moreland, Leo Hollberg, John Kitching We report on the first observation of excitation of atoms in a gaseous state by motion of a magnetic, mechanical resonator. We excite Zeeman resonances in Rb$^{87}$ atoms, confined in a microfabricated buffer gas vapor cell, that have been optically pumped on the D1 line at 795nm. A cantilever with a magnetic tip is placed near the cell and driven on resonance to provide an oscillating magnetic field that, in combination with a DC magnetic bias field, de-pumps the atoms and thus promotes absorption that is detected on a photodiode. As we sweep the oscillation frequency of the cantilever across resonance we see a narrow peak in absorption that corresponds to excitation of the atoms by the cantilever motion. We characterize this system by measuring the sensitivity of the atoms to external magnetic fields. Further improvements will include microfabricated torsional cantilevers that will provide a more homogeneous excitation field, and the use of a magneto-optical trap to drop cold atoms near the cantilever and thereby maximize the interaction time. [Preview Abstract] |
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O1.00125: An Interferometric Technique for the Measurement of Acoustic Velocity R. Schiller, A. Vernaleken, M. G. Cohen, H. Metcalf We have developed a system for accurate measurement of acoustic velocity using an optical phase shifting technique employing acousto-optic modulators (AOMs)\footnote{E. Li et al., Opt. Lett. {\bf 30}, 189 (2005)}. We use a pair of AOMs driven at the same rf frequency but different phase $\Delta \phi$. They are aligned to cancel the optical frequency shifts they impose on a laser beam, but they still shift the optical phase by $\Delta \phi$. By maintaining $\Delta \phi$ constant and translating one modulator in the direction of acoustic propagation, a 2$\pi$ optical phase shift is produced for each acoustic wavelength $\lambda_a$ of translation. The acoustic velocity is the product of this measured $\lambda_a$ and the known rf frequency. We measure the optical phase shifts by inserting the pair of AOMs into one arm of a Mach-Zehnder interferometer, with one AOM mounted on a micrometer stage, and then detect the fringe shifts. We present results for longitudinal mode velocities in lead molybdate and tellurium dioxide. We will also discuss the importance of refractive index gradients that can produce anomalous results. [Preview Abstract] |
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O1.00126: Optical detection of NMR in liquids Igor Savukov, Seung-Kyun Lee, Michael Romalis We will describe the first observation of optical rotation of visible light induced by nuclear spin polarization in a liquid. We have detected NMR signals in hyperpolarized liquid $^{129}$Xe and thermally-polarized water by monitoring the rotation of the plane of polarization of a laser beam transmitted through the liquid. Optical detection of NMR offers several intrinsic advantages, such as excellent real-time spatial resolution and relative immunity to RF fields. One possible application of this technique is direct high-resolution imaging of hyperpolarized $^{129}$Xe for studies of long-range dipolar interactions. While the angle of rotation is not very large, on the order of a microradian in hyperpolarized $^{129}$Xe, sufficient signal to noise ratio can be achieved using shot-noise limited polarimetry with high laser power, which does not perturb nuclear spin precession. The nuclear spin optical rotation effect has a universal nature and can be observed in many substances. The size of the effect is a sensitive probe of atomic and molecular structure. [Preview Abstract] |
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O1.00127: An In-House Thermal Evaporation System for Anti-Reflection An In-House Thermal Evaporation System for Anti-Reflection Coating of Laser Diodes Sarah Nagel, Duong Nguyen, Thomas Killian, Randall Hulet Laser diodes have become a very popular and powerful tool in AMO research due to their frequency tunability, small size and low cost. These devices are often used in external cavity diode lasers (ECDL) and injection locked amplifers. In both cases, in order to maintain frequency stability, the reflectivity of the diode facet must be as low as possible. Anti-reflection coating these devices is a commerically available, but rather expensive, procedure. In this poster, we present an in-house thermal evaporation system designed for coating diode lasers with SiO in a vacuum chamber capable of reaching $10^{-5}$ torr. We monitor the coating process by measuing the threshold current of the laser diode \emph{in-situ}. We present reflectivity and coating stability results. [Preview Abstract] |
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O1.00128: CW all-Optical Quadruple Resonance Spetroscopy of Sodium Dimer Ergin Ahmed, Peng Qi, Marjatta Lyyra We report the first cw all-optical quadruple resonance excitation experiment with all excitations steps being coherently driven by a combination of four tunable lasers. We have constructed a theoretical model to simulate the experimentally observed signal based on density matrix formalism, which can be used also to identify optimal laser wavelengths as well as laser propagation geometry for observation of coherence effects such as the Autler-Townes splitting. This excitation technique is very general and can be used to probe transitions to highly excited electronic states and their transition dipole moments. Of special interest are excited states with ion pair character. Transitions to such electronic states have unusual electronic transition dipole moment functions. The Autler-Townes effect associated with this technique can be used as a probe of transition dipole moment functions to facilitate a study of charge separation as a function of internuclear distance with well-defined initial conditions. This technique also allows for general access to highly excited electronic states at large internuclear distance with high resolution and thus can be used to probe Rydberg states in high vibrational levels, which are difficult to reach otherwise starting from the thermal population in the ground state. In ultracold samples this technique can be used in reverse order from intermediate to small internuclear distance to form cold ground state molecules through photoassociation. [Preview Abstract] |
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O1.00129: Sub-Doppler Calibration of a DAVLL Signal using a Compact Design David French, Ansel Foxley, Anthony Gorges, Jacob Roberts We have developed a compact experimental design used to calibrate the error signal produced in a Dichroic Atomic Vapor Laser Lock (DAVLL)$^{1}$. While a DAVLL does not require frequency modulation and produces an error signal that spans a large frequency range, its lack of any sub-Doppler features in its error signal requires that it be periodically recalibrated in order to compensate for long term drifts. By adding beams in a saturated absorption spectrometer configuration through the DAVLL cell, this recalibration can be accomplished in an efficient way, despite the fact that the saturated absorption is measured in magnetic field of about 100G. Measurements of the long-term stability of this technique will be presented. This work was performed as part of an undergraduate research project. \\ $^{1}$K. L. Corwin, Z. T. Lu, C. F. Hand, R. J. Epstein, and C. E. Wieman, :Frequency-stabilized diode laser with the Zeeman shift in an atomic vapor," Appl. Opt. $\bf{37}$, 3295 (1998). [Preview Abstract] |
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O1.00130: Efficient Optically Pumped Cesium Vapor Laser B. Zhdanov, T. Ehrenreich, R.J. Knize We have demonstrated a cesium laser with 81{\%} slope efficiency relative to the input pump power. The maximum output power at 894 nm was 0.36 W with a pump power of 0.57 W (the overall optical efficiency was 63{\%}). Optically pumped alkali lasers have a number of desirable features as compared to solid state or fiber lasers: the quantum efficiency is high (95.3{\%} for Cs as compared to 76{\%} for a 1.06 $\mu $m Nd:YAG laser); the gain medium is a gas with excellent optical quality; thermal problems are reduced since the gas gain medium can be flowed to remove heat. We used the three-level pump scheme to create the population inversion on the D1 transition (6$P_{1/2}$ to 6$S_{1/2})$ in the Cs atomic vapor. A narrowband pump laser operating at 852 nm pumps the atoms to the 6$P_{3/2}$ state (D2 line) which is then rapidly quenched to the 6$P_{1/2}$ state by an ethane buffer gas. This creates a population inversion between the 6$P_{1/2}$ and 6$S_{1/2}$ states and lasing at 894 nm. The experimental set-up consisted of a Coherent MBR 110 Ti:Sapphire laser used as a pump source and a 2 cm long Cs vapor cell with AR coated windows positioned in the center of a 16 cm long stable laser cavity. The cell was filled with metallic cesium and 500 Torr of ethane at 20$^{o}$C and was placed inside a temperature controlled oven. The laser cavity was longitudinally pumped through the input cavity mirror. [Preview Abstract] |
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