Bulletin of the American Physical Society
40th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 54, Number 7
Tuesday–Saturday, May 19–23, 2009; Charlottesville, Virginia
Session B2: Focus Session: Slow Ion-Atom and Ion-Molecule Collisions |
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Chair: Brett DePaola, Kansas State University Room: Gilmer Hall 130 |
Wednesday, May 20, 2009 10:30AM - 11:00AM |
B2.00001: A contemporary theoretical view of ion-atom, -molecule interactions Invited Speaker: Energetic interactions among electrons, ions, atoms, and molecules within gaseous, plasma, and even material environments are fundamental to such diverse phenomena as low temperature plasma processing of semiconductors, collapsing giant molecular clouds forming stars, fluorescent lighting, cold atom condensates, radiation treatment of disease, and the chemistry of earth's atmosphere. Consequently, they have been studied using the contemporary theoretical and computational methods of the time for many years, yet progress continues as we are confronted with challenges presented by new, often more complex systems and by the drive to understand these foundational interactions at an ever more subtle and predictive level. I will illustrate some of this progress using examples from recent work treating very simple systems, for which our knowledge has been both subtly refined and significantly changed, and more complex systems, where complementary methods based on either elaborate treatments or broadly applicable simple models can be used to advance our knowledge. [Preview Abstract] |
Wednesday, May 20, 2009 11:00AM - 11:30AM |
B2.00002: Recent result in slow ion-atom and ion-molecule collisions Invited Speaker: Slow collisions of ions with atoms and molecules have been of great interest to basic atomic physics and play an important role in the early universe. Collisions resulting in charge exchange have long played an important role in the ionization equilibrium of magnetically confined plasmas. Moreover, the discovery of x rays from cometary and planetary atmospheres has highlighted the role of charge exchange processes in planetary science, solar, and astrophysics. As a result, laboratory studies have expanded to include high-resolution measurements of the x-ray emission associated with slow ion-atom and ion-molecule collisions. For example, laboratory measurements of the K-shell x-ray emission of Fe were conducted to understand the mechanism for x-ray production in the galactic ridge; measurements of the x-ray emission of L-shell sulfur ions colliding with molecules are now underway to understand the origins of Jupiter's auroral emission. Laboratory x-ray measurements have also uncovered some significant differences with predictions from atomic theory - the principal quantum number into which capture takes place is one to multiple levels higher than predicted, and the observed intensity pattern can differ significantly from predictions, especially at the lower collision energies. Measurements employing different experimental techniques, however, have yielded intensity patterns that differ quite strongly from each other and add further to the puzzles that remain to be solved before the physics of slow ion-atom and ion-molecule collisions is fully understood. [Preview Abstract] |
Wednesday, May 20, 2009 11:30AM - 11:42AM |
B2.00003: Estimates of Collisional Cooling and Quenching Rates for Atomic and Molecular Ion Collisions with Ultracold Atoms. Winthrop Smith, James Wells Translational cross sections and rate coefficients for cold ion-neutral elastic and charge-exchange collisions (either atomic or molecular) are $>>$ larger ($\sim$10$^6$ a.u.) than neutral-neutral collisions at the same CM energy. This is due to the long range polarization potential V(R) = -C$_{4}$/R$^{4}$, where C$_{4}$ is proportional to the polarizability of the neutral partner. Thus collisions between ultracold alkali atoms (trapped in a magneto-optic trap or MOT) and low-energy ions can be used for sympathetic cooling experiments. We are building a prototype hybrid-trap apparatus [1] that applies these principles to collisions of Ca$^{+}$ ions (which can be laser pre-cooled) with MOT-trapped ultracold Na atoms. Some calculations on this system and other related ion-neutral systems have been published [2] and some initial experiments on other ion-neutral species have begun [3]. Estimates of cooling and quenching rates in the low K-mK CM energy range for Ca+ on Na and other cases will be presented and possible experiments described. [1] Winthrop W. Smith, Oleg P. Makarov and Jian Lin, J. Modern Optics \textbf{52}, 2253 (2005). [2] R. C\^{o}t\'{e} and A. Dalgarno, Phys. Rev. A \textbf{62}, 012709 (2000); R. C\^{o}t\'{e}, Phys. Rev. Lett. \textbf{85}, 5316 (2000). [3] A. Grier, M. Cetina, F.Orucevic, and V. Vuletic, ArXiv atom-ph/0808.3620. [Preview Abstract] |
Wednesday, May 20, 2009 11:42AM - 11:54AM |
B2.00004: Jahn-Teller Interactions in the Dissociative Recombination of H$_{3}^{+}$ Stephen Pratt, Christian Jungen A simple analytical approach is presented to describe the dissociative recombination (DR) of an electron with H$_{3}^{+}$ and its isotopomers. The principal assumption is that resonant capture mediated by the Jahn-Teller interaction dominates the cross section. The only input required comes from spectroscopic data on the 3pE' Rydberg state of H$_{3}$ and the $\nu _{2}$ vibrational frequencies of H$_{3}^{+}$ and its isotopomers. The approach provides an independent prediction of the low-energy DR cross sections and rates, and is in good agreement with the latest experimental$^{1}$ and theoretical$^{2}$ determinations. Work at Argonne was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under contract No. DE-AC02-06CH11357. 1. K. Kreckel et al., Phys. Rev. Lett. \textbf{95}, 263201 (2005); see also, B. J. McCall et al., Nature \textbf{422}, 500 (2003), and B. J. McCall et al., Phys. Rev. A \textbf{70}, 052716 (2004). 2. S. Fonseca dos Santos, V. Kokoouline, and C. H. Greene, J. Chem. Phys. \textbf{127}, 124309 (2007). [Preview Abstract] |
Wednesday, May 20, 2009 11:54AM - 12:06PM |
B2.00005: Quantum-defect theory for ion-atom interactions Bo Gao We present a quantum-defect theory\footnote{B. Gao, Phys. Rev. A \textbf{78}, 012702 (2008).} (QDT) for ion-atom systems that provides a systematic understanding of ion-atom interaction over a wide range of energies that includes the cold and ultracold regime. Ion-atom bound spectrum, ion-atom scattering at ultracold temperatures including ultracold shape resonances and quantum reflection and tunnelling, can all be understood systematically within the QDT framework. We will give a flavor of the theory, with an emphasis on the universal spectra for ion-atom systems up to $l=10$. [Preview Abstract] |
Wednesday, May 20, 2009 12:06PM - 12:18PM |
B2.00006: Theory of Dissociative Recombination of linear triatomic ions with permanent dipole moment: Study of HCO$^{+}$ Nicolas Douguet, Viatcheslav Kokoouline, Chris H. Greene Dissociative recombination of HCO$^{+}$ ions in collisions with low energy electrons has been extensively studied in theory and experiment. Despite recent improvement in the theory, the theoretical calculations and experimental results are still different by about a factor of 2-3. The most recent theoretical approach used to describe the process of dissociative dissociation included all degrees of freedom of HCO$^{+}$ ion, as well as the Renner-Teller effect, which is responsible in HCO$^+$ for the large probability to capture the incident electron. It is also well known that the HCO$^{+}$ ion has a considerable permanent dipole moment ( D$\approx$4 Debye), which could not be taken into account in a standard quantum defect theory approach. In the present study, we explicitly included the effect of the permanent dipole on dynamics of the incident electron using the generalized quantum defect theory and present the new results obtained for the cross section. To our knowledge it is the first application of the generalized quantum defect theory to dissociative recombination of molecular ions. [Preview Abstract] |
Wednesday, May 20, 2009 12:18PM - 12:30PM |
B2.00007: Preliminary results on a new method for producing ultracold molecular ions Wade Rellergert, Eric Hudson We describe a new method for the production of ultracold molecular ions. This method utilizes sympathetic cooling due to the strong collisions between appropriately chosen molecular ions and laser-cooled neutral atoms to realize ultracold, internal ground-state molecular ions. In contrast to other experiments producing cold molecular ions, our proposed method efficiently cools both the internal and external molecular ion degrees of freedom. Preliminary results from experiments aimed at cooling trapped molecular ions using an Yb MOT are presented. [Preview Abstract] |
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