Bulletin of the American Physical Society
38th Annual Meeting of the Division of Atomic, Molecular, and Optical Physics
Volume 52, Number 7
Tuesday–Saturday, June 5–9, 2007; Calgary, Alberta, Canada
Session P6: Photon Interactions with Atoms and Molecules |
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Chair: A. Aguilar, Lawrence Berkeley National Laboratory Room: TELUS Convention Centre Olde Scotch Room |
Friday, June 8, 2007 10:30AM - 10:42AM |
P6.00001: Double Photoionization of H$^-$ by a Single Photon Frank Yip, Daniel Horner, C. William McCurdy, Thomas Rescigno We present fully differential cross sections for the three-body breakup of H$^-$ by single photon absorption. Electron correlation drives the double photoionization process and thus should impact double photoionization results most strongly for H$^-$, which is a three-body atomic system bound only because of electron correlation. The absolute triple-differential and single-differential cross sections were obtained from \emph{ab initio} calculations making use of exterior complex scaling within a discrete variable representation partial wave basis. Results calculated at photon energies of 18eV and 30eV are compared with reported cross sections for helium calculated at 20eV above the double ionization threshold. [Preview Abstract] |
Friday, June 8, 2007 10:42AM - 10:54AM |
P6.00002: Perturbative and Non-Perturbative Calculations of Photoionization of H$_2^+$ M. Foster, J. Colgan, O. Hagan, J.L. Peacher, D.H. Madison, M.S. Pindzola We present both non-perturbative and perturbative calculations for photoionization of H$^+_2$. For the perturbative approach, we have investigated two different final state wavefunctions for the ionized electron. The first wavefunction is a product of two Coulomb functions (2C) where each Coulomb function represents the two-body interaction between the ionized electron and one of the residual protons in the nucleus. The second final state wavefunction, we investigated was a distorted wave for the ionized electron calculated using a spherically symmetric potential for the two residual protons. These methods are compared to the results computed using the non-perturbative time-dependent method. The time-dependent method solves the time-dependent Schrodinger equation for H$^+_2$ using the variational principle in spherical coordinates centered on the center of mass of the H$^+_2$ system. [Preview Abstract] |
Friday, June 8, 2007 10:54AM - 11:06AM |
P6.00003: Production of Excited Atomic Hydrogen and Deuterium from $H_2 $ and $D_2 $ Photodissociation T.J. Gay, J.D. Bozek, J.E. Furst, H. Gould, A.L.D. Kilcoyne, J.R. Machacek, F. Martin, K.W. McLaughlin, J.L. Sanz-Vicario We have measured the production of both $Ly\alpha $ and $H\alpha $ fluorescence from atomic H and D for the photodissociation of $H_2 $ and $D_2 $ by linearly-polarized photons with energies between 24 and 60 eV. In this energy range, excited photofragments result primarily from the production of doubly-excited molecular species which promptly autoionize or dissociate into two neutrals. Our data are compared with \textit{ab initio} calculations of the dissociation process, in which both doubly-excited state production and prompt ionization through non-resonant channels are considered. Agreement between our experimental data and that of earlier work [1], and with our theoretical calculations, is qualitative at best. [1] E.Melero Garc\'{\i}a, J.\'{A}lvarez Ruiz, S.Menmuir, E.Rachlew, P.Erman, A.Kivim\"{a}ki, M.Glass-Maujean, R.Richter, and M.Coreno, J.Phys.B \textbf{39}, 205 (2006). Support provided by the NSF (Grant PHY-0354946), DOE (LBNL/ALS) and ANSTO (Access to Major Research Facilities Programme). [Preview Abstract] |
Friday, June 8, 2007 11:06AM - 11:18AM |
P6.00004: Single photon induced symmetry breaking of H$_{2}$ dissociation Thorsten Weber et al. Symmetries are essential building blocks of our physical, chemical and biological models. For macroscopic objects symmetries are always only approximate. By reducing the complexity in the microcosm these symmetries often become strict. In H$_{2}$ and H$_{2}^{+}$, the smallest and most abundant molecules in the universe, this complexity is reduced to the minimum. They have perfectly symmetric ground states. What does it take to break this symmetry? In our study we show how and why the inversion symmetry of the hydrogen molecule can be broken by absorption of a linearly polarized photon, which itself has inversion symmetry. The emission of an electron with subsequent dissociation of the remaining H$_{2}^{+}$ shows that under some circumstances no symmetry to the ionic and neutral fragment. This is the smallest and most fundamental molecular system for which such symmetry breaking is possible. The mechanisms identified behind this symmetry breaking are general for all molecules. Fully differential angular cross sections, which have been experimentally obtained with the COLTRIMS technique (using linear polarized light ranging from 32 to 50 eV at the Advanced Light Source) are compared with a state-of-the art quantum mechanical approach implementing B-spline basis sets. [Preview Abstract] |
Friday, June 8, 2007 11:18AM - 11:30AM |
P6.00005: Global Franck-Condon breakdown: nonresonant molecular photoionization processes Aloke Das, David Hardy, Alejandro Aguilar, A.L.D. Kilcoyne, John D. Bozek, Erwin D. Poliakoff We report photoelectron spectroscopy results of nonresonant Franck-Condon breakdown in the photoionization of CO and ICN. Most importantly, the deviations occur over a surprisingly wide range of energies. For the case of CO$^{+}(X^{2}\Sigma ^{+})$, the v$^{~+~}$=~1/v$^{~+~}$=~0 vibrational branching ratio is found to vary significantly ($>$50{\%}) over a 200~eV range. While it is well understood that resonances can lead to coupling between photoelectron and molecular vibration, there is little information on nonresonant sources of coupling. It appears that Cooper minima may be responsible for the observations. Moreover, for ICN, the vibrationally resolved deviations from Franck-Condon behavior are vibrationally mode-specific. Studies on alternative molecular targets are planned to see whether they exhibit photoelectron dynamics that are geometry-dependent. [Preview Abstract] |
Friday, June 8, 2007 11:30AM - 11:42AM |
P6.00006: X-Ray Absorption in Carbon Ions Near the K-Edge M.F. Hasoglu, Sh. A. Abdel-Naby, D. Nikolic, T.W. Gorczyca, B.M. McLaughlin K-shell photoabsorption calculations are important for determining the elemental abundances of the interstellar medium (ISM) from observed X-ray absorption spectra. Previously, we performed reliable K-shell photoabsorption calculations for oxygen [1-3] and neon [4,5] ions. We have executed detailed R-matrix calculations for carbon ions, including Auger broadening, by using an optical potential, and relaxation effects, by using pseudoorbitals with the necessary pseudoresonance elimination. This work was funded by NASA's Astronomy Physics Research and Analysis (APRA) and Solar and Heliospheric Physics (SHP) Supporting Research and Technology (SR\&T) programs. References: [1] T. W. Gorczyca and B. M. McLaughlin. J Phys. B. 33 L859 (2000) [2] A. M. Juett, et al., Astrophys. J. 612, 308 (2004) [3] J. Garcia et al., Astrophys. J. Supp. S. 158, 68 (2005) [4] T. W. Gorczyca., Phys. Rev. A. 61, 024702 (2000) [5] A. M. Juett, et al., Astrophys. J. 648, 1066 (2006) [Preview Abstract] |
Friday, June 8, 2007 11:42AM - 11:54AM |
P6.00007: Enhancement in low energy valence photoionization of Ar confined in C$_{60}$ Himadri Chakraborty, Mohamed Madjet, Jan-Michael Rost, Steve Manson Encapsulation of atoms in C$_{60}$ offers a unique possibility to examine the behavior of an atom in a nanometric confinement. We perform calculations on the photoionization of an Ar atom endohedrally confined in C$_{60}$. The ground state of the Ar@C$_{60}$ compound is described by the Local Density Approximation (LDA) after freezing sixty C$^{2+}$ ions to form a classical jellium background. A time dependent LDA method is then employed to calculate the response of the compound to the dipole photon. The valence 3p ground state electron of Ar shows very little mixing with the C$_{60}$ electrons to retain its atomic character even in the confinement. However, the low energy part of the 3p ionization cross section is found to enhance considerably, by more than an order of magnitude, when compared with its counterpart for a free atom. Further scrutiny reveals that this increase in the Ar 3p cross section is accompanied by a compensatory depletion of the surface plasmon in the ionization cross section of the confining shell, pointing to a strong dynamical coupling between the atom and the shell. [Preview Abstract] |
Friday, June 8, 2007 11:54AM - 12:06PM |
P6.00008: Half-filled shell atoms as intense sources of spin-polarized photoelectrons Valeriy Dolmatov, Steven Manson Intense beams of spin-polarized electrons are much needed because they are used in unique fundamental experimental studies in the area of photon-atom, electron-atom, and electron-molecular collisions. In the present paper, we predict a spin-polarized beam of photoelectrons of significant intensity results from photoionization of a closed $ns$$^{2}$-subshell of a spin-aligned atom having a multielectron half-filled subshell in its ground state. The polarization results from the specific properties of a half-filled shell atom due to the unbalanced exchange interaction between spin-up and spin-down electrons in the atom, both at the independent-particle and multielectron correlation levels. This mechanism causing the preferable spin orientation of outgoing photoelectrons (a) differs from the commonly known mechanisms yielding spin-polarized photoelectrons from atoms, (b) has advantages over the latter since, in the present case, it exhibits much larger intensity, and (c) the degree of spin-polarization depends neither on polarization of the incoming radiation nor on the angular distribution of emitted photoelectrons. Calculated results for the photoionization of the valence $4s{^2}$ subshell of a spin-oriented Mn($4s^{2}$ $^{6}S$) atom are presented. [Preview Abstract] |
Friday, June 8, 2007 12:06PM - 12:18PM |
P6.00009: Electron-Ion Recombination, Photoionization and Dielectronic Satellie Lines of Ca~XVIII and Ca~XIX Using Unified Method Sultana Nahar The unified method is used to study photoionization, electron-ion recombination and dielectronic satellite (DES) lines of highly charged Ca~XVIII and Ca~XIX. The method which (i) subsumes both the radiative and dielectronic recombinations, (ii) provides self-consistent sets of photoionization and recombination cross sections, has been extended recently to study (iii) the DES spectra with natural profiles and blending of the lines. The method is implemented through relativistic Breit-Pauli R-matrix method using coupled channel wavefunctions. Present calculations include all fine structure levels with $n \leq 10$ and $0 \leq l \leq 9$ which correspond to 98 levels of total angular momenta 1/2 $\leq J \leq$ 17/2 of Ca~XVIII and 193 levels with $0 \leq J \leq 10$ of Ca~XIX. Results will be presented with important features for level-specific total and partial photoionization cross sections, total and level-specific recombination rate coefficients, DES spectrum and rate coefficients. They include for He-like Ca~XVIII, (i) cross sections and rates for the diagnostic w, x, y, z lines and (ii) resonance strengths and rates for the 22 KLL DES lines, and for Li-like Ca~XVII, the cross sections and rates for the ultraviolet lines observed in astrophysical spectra. Comparison is made with available data. [Preview Abstract] |
Friday, June 8, 2007 12:18PM - 12:30PM |
P6.00010: Benchmarking resonance Phenomena in Atomic Photoionization and Recombination Anil Pradhan, Sultana Nahar, Maximiliano Montenegro Resonances are crucial in the study and applications of atomic photoionization and electron-ion recombination. They form naturally at quantum energies with increment of the effective quantum numbers by unity and bring out the features characteristic to atomic system and its state. A number of highly acurate experimental measurements of atomic photoionization cross sections are being carried out with accelerator based advanced light sources and in synchrotrons, and of electron-ion recombination of heavy ions in storage rings, in addition to studies of Rydberg series of resonances and threshold behavior. We will present theoretical results from ab initio calculations using the unified and self-consistent relativistic R-matrix method for several atomic systems with detailed comparisons with these experiments of resonance structures in both photoionization and electron-ion recombination. We will describe further extensions of the method to complex and heavy atoms and ions with multiple series of overlapping and correlated resonances. As an example, we will present the preliminary results on Fe~IV photoionization cross sections where the overlapping Rydberg series of 16 core states bars identification of individual series and compare with the experiment measurement at ALS. [Preview Abstract] |
Friday, June 8, 2007 12:30PM - 12:42PM |
P6.00011: Prediction of Compton doubly and tripply differential cross sections and Compton profiles at high energy from modified nonrelativistic theories: Effect of treating the momentum of the ejected electron relativistically L.A. LaJohn, R.H. Pratt With increasing atomic number and incident photon energy ($\omega_1$), the nonrelativistic (nr) matrix element based on the interaction Hamiltonian [$H_{int}=(e^2A^2/2) - e(p\cdot A)$] becomes insufficient for the accurate prediction of Compton cross sections, even in the Compton peak region, except when v/c (v=velocity of the ejected electron) is small, in which case it remains valid. Under such circumstances one could use the more exact relativistic S-matrix (SM) theory. However we find that for doubly and triply differential cross sections in the vicinity of the Compton peak, an $A^2$ matrix element based on Schr\"odinger wavefunctions works even for $v/c \rightarrow 1$, if the momentum of the ejected electron is treated relativistically. However an entirely nr treatment (including for momentum) of the Compton profiles (CP), as a function of $p_z$ ($p_z$ = z component of the incident electron energy), unlike for the cross sections, is in surprisingly good agreement with relativistic SM (with relativistic $p_z$) results even when $v/c \rightarrow 1$, due to partial cancellation of relativistic factors in CP and $p_z$. [Preview Abstract] |
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