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 V4: Photon Interactions with Atoms, Ions, and Molecules-Theory |
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Chair: Alfred Msezane, Clark Atlanta University Room: Knoxville Convention Center 301E |
Friday, May 19, 2006 1:30PM - 1:42PM |
V4.00001: Term dependence of nondipole valence s-photoelectron angular distributions from half-filled shell atoms Valeriy Dolmatov, Emre Guler, Steven Manson To date, nondipole photoelectron angular distributions from open-shell atoms have remained poorly studied because of their complexity in both experimental and theoretical studies. To remedy this situation, we have initiated a theoretical research program for studying nondipole effects in these atoms. The aim of this presentation is to demonstrate a dramatic dependence of nondipole valence s-photoelectron angular distributions on final state terms of the open-shell atom-remainder. It turns out that electron correlation affects these nondipole angular distributions crucially but much differently depending on a final state term of the residual ion. As a result, there are marked differences between nondipole contributions to the valence s-photoelectron angular distributions ``belonging'' to different final state terms of the ion-remainder. This is demonstrated by our calculations for the 4s photoelectrons from the Mn(3d$^{5}$4s$^{2}$,$^{6}S$) atom for the 4s,$^{5}S$ and 4s,$^{7}S$ final state terms of the ion-remainder. Calculations were performed in a ``spin-polarized'' random-phase-approximation with exchange (SP RPAE) framework. [Preview Abstract] |
Friday, May 19, 2006 1:42PM - 1:54PM |
V4.00002: Nondipole effects in double photoionization of He at 450 eV excess energy Andrei Y. Istomin, Anthony F. Starace, N.L. Manakov, A.V. Meremianin, A.S. Kheifets, Igor Bray We calculate photoelectron angular distributions for double photoionization of He at an excess energy of 450 eV including nondipole effects~[1]. We employ our model-independent representations~[2] for the dipole and quadrupole transition amplitudes and use the convergent close-coupling (CCC) approach~ [3] to evaluate them\footnote{Use of the Compaq AlphaServer SC National Facility of the Australian Partnership for Advanced Computing is acknowledged.}. Our nondipole results for the triply-differential cross section are shown to have improved agreement (as compared to the dipole approximation results) with recent experiments using linearly polarized light~[4] for a number of kinematical configurations. [1] A.Y. Istomin et al., J. Phys. B {\bf 39}, L35 (2006). [2] A.Y. Istomin et al., Phys. Rev. Lett. {\bf 92}, 063002 (2004). [3] A.S. Kheifets and I. Bray, Phys. Rev. A {\bf 57}, 2590 (1998). [4] A. Knapp et al., J. Phys. B {\bf 38}, 615 (2005). [Preview Abstract] |
Friday, May 19, 2006 1:54PM - 2:06PM |
V4.00003: Photodetachment Wave Packet Motion in Crossed Fields Liang-You Peng, Qiaoling Wang, Anthony F. Starace We present a detailed quantum mechanical treatment of the photodetachment of H$^-$ by short laser pulses in crossed static electric and magnetic fields. We derive the effective detachment cross section appropriate for short laser pulses, which reduces in the limit of a long pulse to known results for the case of a monochromatic field. The time-evolution in coordinate space of the detached electron wave packet allows one to observe clearly the return of a portion of the wave packet to the origin at times which correspond to the orbital periods of the classical closed orbits~[1]. In addition, Fourier transform of the oscillatory part of the cross section reveals its classical connections. We will present movies of the wave packet motion for single and double pulse cases. [1] A.D. Peters and J.B. Delos, {\it Phys. Rev.} A {\textbf 47}, 3020 (1993). [Preview Abstract] |
Friday, May 19, 2006 2:06PM - 2:18PM |
V4.00004: Breakdown of the Impulse Approximation for Compton K-Shell Doubly Differential Cross Sections for Heavy Atoms L.A. LaJohn, R.H. Pratt The impulse approximation is known to be valid in the prediction of inelastic photon scattering (Compton) differential cross sections when the photon momentum transfer $|{\bf K}| = |{\bf K_i} - {\bf K_f}| $ is much larger than the average electron momentum $p_{av}$ (${\bf |K|} >> p_{av}$). It was found that this requirement can be relaxed to ${\bf p_ {av}/|K|} < 1$ for light atom doubly differential cross sections (DDCS) (P. Eisenberger and P. M. Platzman, PRA {\bf 2}, 415 1970). Compton K-shell DDCS for light atoms such as Copper obtained from impulse approximation (IA) (either relativistic or nonrelativistic versions), are in good agreement with exact (within the independent particle approximation) S-matrix (SM) values. For example, the relative difference ($\Delta $) between SM and the relativistic version of IA (RIA), when $P_{av}/{\bf |K|} =0.6$, at the Compton peak in the case of back angle scattering, is about $2\%$ for Cu (Z=29). However it increases to $14\%$ for Sm (Z=62) and $25\%$ for U (Z=92). This rapid increase in $\Delta $ with increasing Z can be explained in terms of the sum rule that is used to justify the expression for calculating DDCS in the IA and RIA theory. This sum rule becomes progressively less valid for RIA with increasing Z. [Preview Abstract] |
Friday, May 19, 2006 2:18PM - 2:30PM |
V4.00005: Generalizations and applications of Bethe's treatment of photoionization P.W. Langhoff, J.C. Arce, C.L. Winstead Extensions and elaborations are reported of the late Hans Bethe's non-stationary or initial-value treatment of photoionization based on Dirac variation-of-constants solution of the time-dependent Schr\"odinger equation [Ann. Physik, 5, 433 (1930)] . His method is applied to complex anisotropic targets, including molecules both randomly oriented and fixed in space, and to more general dynamical aspects of the time evolution of photo-excitation and ionization processes. Explicit expressions are derived for photoionization cross sections differential in ejected electron direction for polyatomic molecules in terms of a minimal set of body-frame angular distribution functions for incident dipole radiation of arbitrary polarization. A generalization of the familiar Bethe-Cooper-Zare expression for atomic anisotropy factors applicable to randomly-oriented molecules and other aggregates is obtained which provides useful connections with experiments performed on fixed-in-space molecules. Some representative applications are provided as illustrations of the formalism, including study of the kinematics of elementary excitation and ionization processes and of the natures of the associated transient Ehrenfest's forces operative in these cases. The conceptual and computational advantages of the approach that Bethe developed in such connections are indicated. [Preview Abstract] |
Friday, May 19, 2006 2:30PM - 2:42PM |
V4.00006: Photoionization of Hot Radicals Stephen Pratt, Haiyan Fan The combination of velocity map imaging and tunable vacuum ultraviolet light has been used to determine the internal energy dependence of a number of small hydrocarbon radical. In these experiments, radicals are produced by the photodissociation of a suitable halogen-containing precursor, and the resulting images allow the determination of the internal energy of the radical. Comparison of the halogen image, which reflects the true translational energy distribution, and the radical image, which is a convolution between the true distribution and the internal-energy-dependent relative photoionization cross section, allows the extraction of the internal energy dependence of the crtoss section. In larger radicals, intramolecular vibrational energy redistribution minimizes the effect of vibrational excitation of the radical. This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biological Sciences under contract No. W-31-109-Eng-38. [Preview Abstract] |
Friday, May 19, 2006 2:42PM - 2:54PM |
V4.00007: Unified Method for Electron-Ion Recombination, Photoionization, and Dielectronic Satellie Lines of Fe~XXIV, Ni~XXVI, N~V-VI, F~VII-VIII Sultana Nahar, Anil Pradhan Dielectronic satellite (DES) spectra of highly charged ions are useful diagnostics in high-temperature laboratory and astrophysical X-ray sources. The unified method for total electron-ion recombination has been extended to study the strengths and rates of DES lines. The method subsumes radiative and dielectronic recombination and enables self-consistent results for the inverse process of photoionization. It is demonstrated that the resonances in the unified recombination cross sections correspond directly to the observed DES spectra. The new approach includes interference effects of resonant and background contributions in an ab initio manner giving proper autoionization line profiles. The relativistic Breit-Pauli R-matrix calculations yield accurate satellite intensities as a function of temperature. Results for the DES spectra of Fe~XXIV and Ni~XXVI obtained using the new approach are compared with those from the individual resonance approximation. The new extension generalizes the unified method based on the close-coupling approximation using the R-matrix method to accurate and complete treatment of several atomic processes. Unified results will be presented for level-specific photoionization and electron-ion recombination of several hundreds levels of Li- and He-like ions N~V-VI and F~VII-VIII. [Preview Abstract] |
Friday, May 19, 2006 2:54PM - 3:06PM |
V4.00008: Dynamics of photoelectrons in a magnetic field Christian Bracher, Tobias Kramer, John B. Delos Near-threshold photodetachment from negative atomic ions provides a virtually pointlike source of electrons, and is ideally suited to study electron dynamics in externally applied electric and magnetic fields. These fields govern the motion of the emitted electron wave, and lead to characteristic modulations both in the total photocurrent and in the spatial electron distribution. These changes have been predicted and observed in an electric field environment (photodetachment microscopy). Here, we examine the effects of a purely magnetic field on the photodetachment cross sections. Theoretical predictions for the electron distribution reveal a surprising wealth of structure that is currently only partly understood. We present numerical and analytical results, and give a semiclassical interpretation of the observed features where possible. [Preview Abstract] |
Friday, May 19, 2006 3:06PM - 3:18PM |
V4.00009: Numerical studies of photoelectron angular distributions in near-resonant two-photon ionization from hyperfine coherent superposition states A.K. Mills, D.S. Elliott We present numerical studies of photoelectron angular distributions for near-resonant two-photon ionization from a prepared coherent superposition of atomic hyperfine components of the ground state. The photoelectron angular distributions exhibit dramatic variation as a function of the superposition state composition, polarizations of the two-color laser field, and detuning from the intermediate hyperfine states. We discuss the capability of current angle-resolved detection systems to measure the coherence of an ensemble of atoms in a prepared atomic beam, and we discuss possible studies of decoherence in such systems. [Preview Abstract] |
Friday, May 19, 2006 3:18PM - 3:30PM |
V4.00010: Trsnsition from a T-shape to a symmetric structure in the Coulomb four body problem close to threshold Agapi Emmanouilidou, Jan Michael Rost In a classical framework, we present a theoretical study for the photo-fragmentation process for the four-body Coulomb problem. Specifically, we explore the angular distribution of the inter-electronic angle of the escaping electrons in the photoionization process from the ground state of Lithium, close to threshold. According to Wannier's theory the inter-electronic angle is 120$^{\circ}$ at threshold. Surprisingly, we find that the inter-electronic angular distribution has a double hump structure with peaks at 90$^{\circ}$ and 180$^{\circ}$ at $E=0.9$ eV. We find that as the excess energy decreases this double hump structure shifts to an almost single hump structure with a peak at 120$^{\circ}$ at $E=0.1$ eV, with the latter structure more closely resembling the one predicted by Wannier. This double hump structure is quite unexpected since it takes place in the energy range where we find the Wannier threshold law to be valid with an exponent of $\alpha=2.16$ [1]. We can understand this T-shape structure in terms of an innovative classification scheme we have previously introduced [2]. Our scheme groups the triple ionizing trajectories according to their sequence of momentum transferring electron-electron collisions. [1] A.Emmanouilidou and J.M. Rost, ``Triple photoionization of Lithium close to threshold'' accepted as Letter in J.Phys.B. [2] A.Emmanouilidou and J.M. Rost, ``Multi-electron collision dynamics and the Coulomb four-body problem in a classical framework'' submitted. [Preview Abstract] |
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