Bulletin of the American Physical Society
2005 36th Meeting of the Division of Atomic, Molecular and Optical Physics
Tuesday–Saturday, May 17–21, 2005; Lincoln, Nebraska
Session C3: Single Photon Atomic and Molecular Processes |
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Chair: Linda Young, Argonne National Laboratory Room: Burnham Yates Conference Center Olive Branch |
Wednesday, May 18, 2005 1:30PM - 1:42PM |
C3.00001: Spectra of Stick Molecules Reinhold Blumel Imagine an electron roaming freely on a ball-and-stick molecule made out of very thin wire. The result is a ``quantum network,'' or a ``quantum graph.'' It is shown that no matter how complex the stick molecule, its quantum spectrum can be computed explicitly and analytically using exact periodic-orbit expansion techniques. [Preview Abstract] |
Wednesday, May 18, 2005 1:42PM - 1:54PM |
C3.00002: Near-threshold photodetachment in parallel electric and magnetic fields Christian Bracher, John Delos We predict photodetachment cross sections and spatial photoelectron distributions in parallel electric and magnetic fields. Near-threshold photodetachment cross-sections of negative ions in strong external electric fields show oscillations related to interference of outgoing with returning electron waves. The interference pattern can also be imaged on a position-sensitive detector, and these observed interference patterns permit the determination of electron affinities with unprecedented accuracy. If a magnetic field is added parallel to the electric field, the spiraling motion of the electron produces trajectories with interesting caustic structures. Multiple paths connect the source to any point on the detector, so a complex interference pattern is found. Using semiclassical methods, we predict the structure of this interference pattern, and we show how it evolves as the electron energy and the field strengths change. [Preview Abstract] |
Wednesday, May 18, 2005 1:54PM - 2:06PM |
C3.00003: The vase cavity: a model for chaotic ionization Kevin Mitchell, Paul Hansen, John Delos In recent investigations of chaotic ionization, we predicted that an excited hydrogen gas in parallel electric and magnetic fields can ionize by emitting a train of electron pulses -- a chaos-induced pulse train. Here, we analyze another physical system, consisting of an open reflecting two-dimensional cavity whose dynamics model chaotic ionization. This system could be experimentally realized as a microwave or optical cavity (or even an electronic microjunction.) The reflecting boundary of the cavity is shaped like the profile of a vase, containing a bulbous interior region, a narrow neck, and a flange-shaped opening. A pulse enters the cavity as a circular wave. This wave bounces around chaotically inside the cavity along classical trajectories, and eventually exits as a chaos-induced pulse train, analogous to that predicted for the ionization problem. The structure of this pulse train can be analyzed through study of the topological properties of a homoclinic tangle. [Preview Abstract] |
Wednesday, May 18, 2005 2:06PM - 2:18PM |
C3.00004: Coulomb four-body problem near threshold in a classical framework Agapi Emmanouilidou, Jan Michael Rost We obtain the triple ionization probability from the ground state of Lithium and find it to be in remarkably good agreement with the experiment results [1]. In addition, we confirm the Wannier threshold law $\sigma \propto E^{\alpha}$ for four-body break-up of Lithium with an exponent of $\alpha=2.16$ in the energy range $0.9-2$ eV and it is shown that the experimentally obtained $\alpha$ [1] deviates from the theoretically obtained value due to its determination between $2-5$ eV above threshold. Most importantly in the framework of classical dynamics we develop a new classification scheme for multi-electron trajectories in terms of sequences of binary electron collisions [2]. We believe that this classification scheme is applicable to a general three-electron dynamics. Surprisingly, for the angular distribution of the ionized electrons, we do not find a preferential symmetric break-up with an inter-electronic angle of $120^{\circ}$ as predicted by Wannier's theory. Instead we find an angular pattern with two peaks at $90^{\circ}$ and $180^{\circ}$. However, we show how the angular pattern we obtain is consistent with our analysis in terms of binary collisions. [1] R.Wehlitz, T. Pattard, M.-T. Huang, I. A. Sellin, J. Burgdorfer, and Y. Azuma, Phys. Rev. A 61, 030704 (R) (2000). [2] A. Emmanouilidou and J. M. Rost, submitted, xxx.lanl.gov/abs/physics/0409034 [Preview Abstract] |
Wednesday, May 18, 2005 2:18PM - 2:30PM |
C3.00005: Parametrizations and dynamical analysis of angle-integrated cross sections for double photoionization Andrei Y. Istomin, Anthony F. Starace, N.L. Manakov, A.V. Meremianin As for one-electron ionization cross sections, the doubly and singly-differential cross sections (DDCSs and SDCSs) for double photoionization (DPI) may be conveniently described by four dipole and quadrupole asymmetry parameters, $\sigma_0$, $\beta$, $\gamma$, and $\delta$. Here we derive two {\it ab initio} parametrizations for these parameters: (i)~in terms of one- dimensional integrals of the polarization-invariant DPI amplitudes and (ii)~in terms of the exact two-electron matrix elements. The dynamical parameters are calculated including electron correlations to lowest order. Our results for SDCSs and DDCSs of He for excess energies of 100~eV and 450~eV are in reasonable agreement with experimental measurements and other theoretical calculations. Agreement with results of others for ratios of double to single ionization cross sections for K-shell DPI from multi-electron atoms is excellent in most cases. [Preview Abstract] |
Wednesday, May 18, 2005 2:30PM - 2:42PM |
C3.00006: Circular Dichroism of Dense Alkali Vapors Thad Walker, Earl Babcock Spin-Exchange Optical Pumping (SEOP) of He-3 utilizes high density (10$^{14}$-10$^{15}$ cm$^{-3})$ alkali vapors to enable manageable spin-transfer rates to He-3 of on the order of a few hours. For cells of a few cm in length, this implies that the vapors are typically 100 optical depths thick at line center. Successful optical pumping requires that the vapor become transparent when fully polarized. Recent experiments on SEOP with alkali mixtures showed that the key factor limiting the conversion of light angular momentum into polarized He-3 spins is the failure of the atoms to become fully polarized even in the presence of intense pumping light. This effect lowers the spin-exchange efficiency by typically a factor of 10. To study this effect further, we have measured the circular dichroism of Rb-He vapors in the vicinity of the D-lines. The measurements indicate that near the 795 D1 resonance the dichroism is substantially smaller than normally expected. A natural explanation is that the He buffer gas mixes the Rb fine-structure levels, allowing for fully polarized atoms to continue to absorb light. This work is supported by the Department of Energy. [Preview Abstract] |
Wednesday, May 18, 2005 2:42PM - 2:54PM |
C3.00007: Circularly-Polarized Photo-Fragmentation of Molecular Hydrogen T.J. Gay, J.E. Furst, D.H. Jaecks, J. Machacek, K.W. McLaughlin, O. Yenen The role of angular momentum in molecular photoionization/dissociation is best probed using circularly-polarized ionizing radiation. We have measured the Balmer-beta (486 nm) fluorescence from hydrogen atoms following photodissociation of hydrogen molecules by circularly-polarized light with energy ranging from 26 eV to 50 eV. Even though the photodissociation events are prompt in these dissociation channels (i.e., do not proceed via pre-dissociative states), there is significant oscillatory structure with incident photon energy in both the linear and circular polarization of the 486 nm fluorescence. No equivalent structure is observed in the H(n=4) total intensity measurements. Our results for the latter are in significant disagreement with earlier measurements [1]. We find evidence that the photodissociated H(n=4) atoms have an orientation opposite that of the incident light near the threshold for photodissociation, at $\sim $26eV. This work was done at the Advanced Light Source, Lawrence Berkeley Laboratory. [1] M.Glass-Maujean, J.Chem.Phys.\textbf{89}, 2839 (1988). [Preview Abstract] |
Wednesday, May 18, 2005 2:54PM - 3:06PM |
C3.00008: Auger Decay Studies of Core-Excited HBr by Angle-resolved Photoelectron Spectroscopy Ximao Feng, Anthony Wills, Emma Sokell, Marco Wiedenhoeft, Nora Berrah Angle-resolved, two-dimensional photoelectron spectra of HBr, covering the Br $3d\to \sigma $* resonances, $3d\to $\textit{nl}$\lambda $ Rydberg resonances and the $3d\to \varepsilon l$ continuum, have been measured,. The two $3d_{5/2,3/2}\to \sigma $* resonances are separated for the first time and are found to be around 70.84 and 71.87 eV, respectively. The atomic Auger lines are measured and their angular distribution parameters $\beta $ calculated. Their intrinsic anisotropy parameters $\alpha _{2}$ are found similar to those of the corresponding normal Auger decay lines in the isoelectronic counterpart Kr. Resonant enhancements on the valence photolines from all the $3d$ resonances have been observed and found to have different effects on the angular distribution parameters $\beta $ of the photolines. The potential energy curves of the $\sigma $* state and the related dissociative molecular states are quantitatively predicted. [Preview Abstract] |
Wednesday, May 18, 2005 3:06PM - 3:18PM |
C3.00009: Unified and Self-Consistent Treatment of Photoionization and Recombination: Fe XXI, Ne VIII, Ne IX Sultana Nahar Photoionization models for astrophysical plasmas require a self-consistent treatment of ionization balance between photoionization and electron-ion recombination. We will present new results using the unified method for electron-ion recombination which enables (i) treatment of both radiative and di-electronic recombination (RR and DR) in an ab initio manner, (ii) self-consistent calculations for photoionization and recombination using the same wavefunction expansion, (iii) level-specific recombination rate coefficients for a large number of bound levels. The method provides a single and accurate total (e-ion) recombination rate coefficient, to be computed at all temperatures of interest in astrophysical and laboratory plasmas. Total recombination rates from the unified method will be presented for Fe XXI. Importance of channel coupling through relativistic effects, not allowed in non-relativistic LS coupling, at very low temperature for this ion will be shown as seen in the experiment. Latest results for highly charged Li- and He-like ions of interest in X-ray astronomy will be presented for Ne VIII-IX and Mg X-XI. [Preview Abstract] |
Wednesday, May 18, 2005 3:18PM - 3:30PM |
C3.00010: Transition from Non-Markovian to Markovian Dynamics in a Dense Potassium Vapor Virginia Lorenz, Steven Cundiff We have observed non-Markovian dynamics in a dense potassium vapor, whose dephasing time is large compared to the collision duration $\tau_c$, allowing clear separation of time scales, using transient four-wave mixing with pulses short compared to all time scales. This is in contrast to molecular systems limited by ultrafast dephasing due to solvent motion [1]. For pulse delays $\tau<\tau_c$, dynamics are dominated by fluctuations of the transition frequency as the atoms form transient dimers (non-Markovian limit). For $\tau\gg\tau_c$, atomic motions make the system homogeneously broadened (Markovian limit). The theoretical TFWM signal is obtained for a 3-level system in the delta-function pulse limit via a perturbation expansion of the density matrix equations of motion, including non-Markovian dynamics through an assumed correlation function [2], $M_{ij}(t) = \Delta_{ij}\exp(-\Lambda_ {ij}t)$, where $i$ and $j$ are energy levels. Theory matches quite well with experimental results at both short (non- Markovian) and long (Markovian) time scales. [1] for a review see G. R. Fleming and M. Cho, ``Chromophore- solvent dynamics,'' Annu. Rev. Phys. Chem. \textbf{47}, 109-134 (1996). [2] S. Mukamel, \textit{Principles of Nonlinear Optical Spectroscopy}, Oxford Univ., NY (1995). [Preview Abstract] |
Wednesday, May 18, 2005 3:30PM - 3:42PM |
C3.00011: Hyperfine-State Coherence in the Presence of Spontaneous Photon Scattering R. Ozeri, C. Langer, J.D. Jost, B.L. DeMarco, A. Ben-Kish, R.B. Blakestad, J. Britton, J. Chiaverini, D. Hume, W.M. Itano, D. Leibfried, R. Reichle, T. Rosenband, P. Schmidt, D.J. Wineland Hyperfine-state superpositions are important to many atomic physics applications and experiments. In many of these experiments, off-resonance laser light is used either to manipulate or trap atoms. Spontaneous scattering of photons can therefore play an important role in hyperfine-state decoherence. We study experimentally the coherence of a hyperfine-state of a trapped $^9$Be$^+$ ion, in the presence of off-resonance light. It is shown that it is only Raman spontaneous scattering of photons which affects coherence. Raman scattering of photons is largely suppressed at a laser detuning which is much larger than the fine-splitting of the excited state. Coherence times that exceed the average scattering time of 19 photons are measured. This result implies that laser light can be used to manipulate hyperfine-state superpositions with very small decoherence. [Preview Abstract] |
Wednesday, May 18, 2005 3:42PM - 3:54PM |
C3.00012: Electron angular distributions and circular dichroism effects for double photoionization of polarized atoms N.L. Manakov, A.V. Meremianin, Andrei Y. Istomin, Anthony F. Starace We consider double photoionization (DPI) of a polarized He atom that is in the $n^{1,3}P$-state. For the axially-symmetric polarized $n^{1,3}P$-states and an arbitrary polarization of the photon beam, we have derived {\it ab initio} parametrizations for the transition amplitude and triply differential cross section (TDCS) in terms of scalar products of vectors that enter this problem. As follows from these parametrizations, a non-zero circular dichroism (CD) effect in DPI of a polarized atom exists even at equal energy sharing, in contrast to DPI from unpolarized atoms. Lowest-order perturbative account of electron correlations in the final state, combined with variational account of electron screening in the $2^{1,3}P$-states, is used to analyze the TDCS and CD quantitatively. [Preview Abstract] |
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