Bulletin of the American Physical Society
2013 Joint Meeting of the APS Division of Atomic, Molecular & Optical Physics and the CAP Division of Atomic, Molecular & Optical Physics, Canada
Volume 58, Number 6
Monday–Friday, June 3–7, 2013; Quebec City, Canada
Session U5: Focus Session: Few and Many Body Dynamics in Collision Processes |
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Chair: John Tanis, Western Michigan University Room: 301 |
Friday, June 7, 2013 10:30AM - 11:00AM |
U5.00001: Initial-state selective fully differential study on ionization of lithium by ion impact Invited Speaker: Aaron LaForge The study of atomic break-up by charged particle impact gives insight into few body systems. Gaining kinematically complete information on simple systems is highly important as it provides the most fundamental and stringent test for state of the art theoretical models. Through the use of ``Reaction Microscopes'' (ReMi), fully differential data has become readily available for cold gas targets prepared via supersonic expansion. Here, we present a novel experimental improvement on the REMI by using laser-cooled atomic ensemble as the target prepared by a Magneto Optical Trap (MOT) for ion impact collisions. This new experimental setup, the MOTReMi, opens up new atomic systems previously unavailable using conventional techniques. Single ionization of the lithium states 2s, 2p, and 1s were investigated by 6 MeV H$^{+}$ and 1.5 MeV amu$^{-1}$O$^{8+}$ impact. In the measured doubly differential cross sections as a function of electron energy and transverse momentum transfer, significant initial state dependence was found. By comparison to several quantum mechanical models, there appear discrepancies for Li(2s) and Li(1s) while there is better agreement for the Li(2p) initial state. Futhermore, the three-dimensional angular distributions of the ejected electrons (i.e. the fully differential cross sections) show strong differences between lithium 2p ionization, lithium 2s ionization, and helium 1s ionization. For Li(2p) ionization, the differences are attributed to the m$_{l} =$ -1 sub-state being the predominant excited state. In this state, we observe orientational dichroism previously unobserved in ion-atom collisions. For Li(2s) ionization, there are pronounced interference effects which we ascribe to the nodal structure of the 2s wave function. [Preview Abstract] |
Friday, June 7, 2013 11:00AM - 11:12AM |
U5.00002: Isotopic effects in HD$^+$ collision-induced dissociation A.M. Summers, Ben Berry, Nora G. Kling, A. Max Sayler, Jack W. Maseberg, K.D. Carnes, I. Ben-Itzhak Collision-induced dissociation (CID) of few keV HD$^+$ on an Ar target atom results from either electronic excitation to a dissociative state or a ro-vibrational excitation into the continuum of the electronic ground state, with the latter mechanism typically occurring in close-encounter collisions. We probe for a preferred channel (isotopic effect) in the dissociation of HD$^+$ into either H$^+$+D or H+D$^+$ final products. Preliminary results indicate that H+D$^+$ is somewhat favored over H$^+$+D when vibrational excitation drives the breakup. Existing theory $\left[1\right]$ predicts a factor of four preference in the branching ratio favoring H+D$^+$ in head-on collisions. Our experiment, using a coincidence 3D momentum imaging technique, enables the measurement of this branching ratio as a function of the scattering angle, i.e. the collision distance. Recent modifications to this experimental setup will improve the quality of the CID data, in particular for the ro-vibrational excitation mechanism of interest in this work\\[4pt] [1] D. Rapp, J. Chem. Phys. \textbf{32}, 735 (1960) [Preview Abstract] |
Friday, June 7, 2013 11:12AM - 11:24AM |
U5.00003: Mechanisms of Dissociative Electron Attachment to CO$_2$ and NH$_3$ William McCurdy, Daniel Haxton, Spiridoula Matsika, Thomas Rescigno Recent experiments at LBNL on the angular dependence of dissociative electron attachment (DEA) to CO$_2$ and NH$_3$ have raised new questions about the mechanism of DEA in both cases. In the case of CO$_2$, for attachment via the shape resonance at 4 eV, the angular distribution of O$^-$ is peaked along the axis of the incident direction of the electron, even though the resonance through which the electron is attached is of $^2\Pi$ symmetry. We present an explanation based on both the attachment amplitude from \textit{ab initio} complex Kohn scattering calculations and the identification of two conical intersections, one involving a $\Sigma/\Pi$ intersection, that may both be involved in the surprisingly complicated mechanism for this low energy DEA process. In NH$_3$, complex Kohn scattering calculations identify the $^2$A$'$ Feshbach resonance near 5 eV that produces both H$^-$ and NH$_2^-$ fragments in the experiment, also involving at least one conical intersection. [Preview Abstract] |
Friday, June 7, 2013 11:24AM - 11:54AM |
U5.00004: Ionization and fragmentation of complex molecules studied with a density functional theory based approach Invited Speaker: Tom Kirchner Ion-impact induced ionization and fragmentation of complex molecules have important applications in many branches of science. If the molecule is H$_2$O an obvious topic to address is the radiobiological relevance of these processes, e.g. in the context of hadron therapy, to name just one example. From a more fundamental physics viewpoint ion-molecule collision systems constitute interesting many-body systems, whose analysis poses challenges to both experimentalists and theorists. This talk will describe a theoretical approach to ion-molecule collisions, which is based on density functional theory to describe the nonperturbative electron dynamics. The basis generator method applied in the past successfully to ion-atom collisions is adapted to deal with the multi-center problem one faces when one considers molecular targets [1]. Cross sections for single- and multiple-electron processes (capture and transfer to the continuum) are obtained directly from solving time-dependent Kohn-Sham-type orbital equations and using a Slater determinant based analysis. Fragmentation yields are predicted on the basis of a semi-phenomenological model which uses the calculated cross sections as input [2]. Results will be presented for various ions impacting on water molecules in the energy range of 10--5000 keV/amu and compared with experimental data and previous theoretical calculations where available. First applications of the model to collisions involving CH$_4$ molecules will also be discussed. \\[4pt] [1]~M. Murakami {\it et al.}, Phys. Rev. A \textbf{85}, 052704 (2012);\\[0pt] [2]~M. Murakami {\it et al.}, Phys. Rev. A \textbf{85}, 052713 (2012). [Preview Abstract] |
Friday, June 7, 2013 11:54AM - 12:06PM |
U5.00005: Ion/Anion Pair Production from Electron Impact J. Sartor, M. Keiling, M. Fogle, T.J. Gay, A.L. Landers One of the least studied dissociation pathways of a neutral molecule is the decay to an ion/anion pair, yet these reactions can provide new insight into fundamental molecular dynamics. We initiate these reactions with the pulsed field from a fast electron, where in principle all ion/anion pair-production modes are accessible and not limited by photo-absorption selection rules. We accomplish this by intersecting a bunched electron beam with a jet of gas over a wide range of energies, and use a fast-switched electric field to guide the ion products towards two position sensitive detectors. Using the positions and flight times of the ions, we completely determine the final state momenta. This not only allows for the discrimination of this channel from dominant contaminant reactions (particularly the electron producing ionization channels), but also yields the kinetic energy release and product angular distribution. Preliminary results for the reaction \centerline{ $e^-$ + O$_{2}$ $\Rightarrow$ e$^-$ + O$^+$ + O$^-$ } will be presented along with additional measurements currently underway, including the fundamental case of hydrogen. [Preview Abstract] |
Friday, June 7, 2013 12:06PM - 12:18PM |
U5.00006: Single differential projectile ionization cross sections d$\sigma $/dE$_{e}$ for 50 AMeV U$^{28+}$ in the ESR storage ring Siegbert Hagmann, Pierre-Michel Hillenbrand, Thomas Stoehlker, Yuri Litvinov The very high intensity beams of relativistic high Z ions with incident collision energies up to 2.7AGeV requested for experiments using the SIS100 synchrotron of FAIR require that 1.3 10$^{11}$ ions at 2.6Hz be injected from SIS12/18 into SIS100. The needed luminosity of the beam can only be achieved for such high Z ions when - considering the space charge limit ($\sim $A/q$^{2})$ - a low charge state q of the ion to be accelerated keeps the particle density at the highest feasible level. For a thorough understanding of beam loss it is imperative that the mechanisms active in projectile ionization be understood quantitatively to provide benchmarks for advanced\textit{ ab initio} theories beyond first order. We have embarked on an experimental investigation of single differential projectile ionization cross sections d$\sigma $/dE$_{e}$ (SDCS) for single and multiple ionization of U$^{28+\, }$in the ESR storage ring by measuring the electron loss to continuum (ELC) cusp at 0$^{0}$ with respect to the beam axis employing our imaging forward electron spectrometer. This was motivated by the high relative fraction of multiple ionization estimated to exceed 40{\%}. We report first results for absolute projectile ionization SDCS for U$^{28+}$. We find a remarkably high asymmetry for the ELC cusp. This is at strong variance with the line shape expected for validity of first order theories. [Preview Abstract] |
Friday, June 7, 2013 12:18PM - 12:30PM |
U5.00007: Three-body collision in ultracold Cs/Rb and Cs/Li mixtures Paul Julienne, Yujun Wang Three-body collisions, in particular three-body recombination, are important in determining the stability of ultracold atomic mixtures used for making ground state polar molecules. We apply a new three-body model, which uses a multichannel two-body representation of three-body interactions, to study three-body collisions in Cs/Rb and Cs/Li mixtures near isolated Feshbach resonances. We survey a range of two-body resonance widths spanning both narrow and broad resonances. Using accurate two-body models for Li + Cs [1] and Cs + Cs [2] interactions, we predict the positions of Efimov resonances in the Cs + Cs + Li ``Efimov-favored'' three-body system, for which the Efimov scaling factor is small enough that three consecutive Efimov resonances might be observed. For a Cs/Rb mixture, we show how the resonance width plays a role in determining the minimum of atomic losses in experiments involving narrow resonances. The difference between the loss minimum and maximum when magnetic field is tuned near a narrow resonance is not the same as the two-body width, consistent with recent experimental observations of such differences [3].\\[4pt] [1] Tung, et al., Phys. Rev. A 87,10702(2013)\\[0pt] [2] Berninger et al, arXiv:1212.5584\\[0pt] [3] Takekoshi, et al., Phys. Rev. A 85, 032506(2012) [Preview Abstract] |
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