Bulletin of the American Physical Society
39th Annual Meeting of the APS Division of Atomic, Molecular, and Optical Physics
Volume 53, Number 7
Tuesday–Saturday, May 27–31, 2008; State College, Pennsylvania
Session I6: Photoionization and Photodetachment Processes |
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Chair: Nora Berrah, Western Michigan University Room: Nittany Lion Inn Boardroom II |
Thursday, May 29, 2008 8:00AM - 8:12AM |
I6.00001: Photoionization of atoms confined inside bucky-ball onions Valeriy Dolmatov, Patrick Brewer, Steven Manson We report on a theoretical study of photoionization of the atom $A$ placed at the center of nested fullerenes $C_{60}$, $C_{240}$, and $C_{540}$; the formation termed ``bucky-ball onion''. The fullerene cages are represented by spherical well potentials of adjusted inner radii, widths, and potential depths. The \textit{inner-shell} photoionization largely rules out the impact of both the change in the structure of the deepest inner-shell due to the confinement and polarizability of a bucky-ball on the photoionization process. In turn, the \textit{near-threshold} ionization eliminates the impact of the granular structure of the bucky-ball cage on the photoionization process. This makes our model workable, to a good approximation. Progressively complicating trends in the abundance of appearing confinement type resonances in the photoionization spectra with increasing number of nested bucky-balls is unraveled and the corresponding interpretation is given. The $1s$- and $2p$-near- threshold photoionization of $Ar$ from $Ar@C_{60}$, $Ar@C_{60}@C_{240}$, and $Ar@C_{60}@C_{240}@C_{540}$ is chosen as the case study. [Preview Abstract] |
Thursday, May 29, 2008 8:12AM - 8:24AM |
I6.00002: Photoionization and Electron-Ion Recombination: Fe~XVII, S~XIV AND S~XV Using Unified Method Sultana Nahar New results on photoionization and electron-ion recombination from unified method for Fe~XVII, S~XIV, and S~XV will be reported. The unified method, based on close-coupling approximation and R-matrix method, (i) subsumes both the radiative and dielectronic recombinations and (ii) provides self-consistent sets of photoionization and recombination cross sections, $\sigma_{PI}$ and $\sigma_{RC}$. Important features will be illustrated for level-specific total and partial photoionization cross sections, total and level-specific recombination rate coefficients, such as of diagnostic w, x, y, z X-ray lines of S~XV and ultraviolet lines of S~XIV observed in astrophysical spectra. Results are obtained for the first time for fine structure levels with $n \leq 10$ and $0 \leq l \leq 9$ which are 98 levels of S~XIV of total angular momenta 1/2 $\leq J \leq$ 17/2 and 188 levels of S~XV of $0 \leq J \leq 10$. Total recombination rates agree well with the available rates. $\sigma_{PI}$ of Fe~XVII for hundreds of fine structure levels being calculated using relativistic Breit-Pauli R-matrix method will also be reported. The wavefunction includes ground and 59 core excitations of n=2 and 3 complexes. Impact of n=3 core excitations will be illustrated. [Preview Abstract] |
Thursday, May 29, 2008 8:24AM - 8:36AM |
I6.00003: Photo-Fragmentation of Lithium Atoms in FEL Radiation Fields Lewis Foster, James Colgan, Mitch Pindzola, Alexander Dorn Multi-electron ejection from lithium induced by absorption of a single photon is a fundamental few-body reaction that tests the correlated interaction dynamics between atomic constituents. Experiments have been proposed using intense FEL radiation at FLASH in combination with reaction microscopes to measure four-body dynamics. These proposed experiments will first measure the double ionization dynamics from both the even parity Li ground state and the laser excited odd parity Li(2p $^2P^o$) state. We will present theoretical calculations using the time-dependent close-coupling method (TDCC) to assist in the experimental search for interesting correlation effects for both double and triple ionization of lithium. [Preview Abstract] |
Thursday, May 29, 2008 8:36AM - 8:48AM |
I6.00004: Inner-Shell Photodetachment of Iron and Ruthenium Negative Ions Ileana Dumitriu, R.C. Bilodeau, T. Gorczyca, C.W. Walter, N.D. Gibson, A. Aguilar, Z. Pesic, D. Rolles, N. Berrah Transition metals such as Ru and Fe are of wide application in oceanography, cosmochemistry, and geology due to their catalytic properties and participation of d-orbital electrons in their bonding properties. The first inner-shell photodetachment studies in Fe$^{-}$ and Ru$^{-}$ conducted using the Ion Photon Beamline (IPB) on ALS beamline10.0.1 will be presented. The negative ions extracted from the SNICS ion source are mass selected and merged collinearly with the photon beam. Inner-shell photodetachment and subsequent Auger decay produce positive ions which are detected as a function of photon energy over a range of 48 to 72 eV. Excitations from p-electrons to open d-shells were carried out in both Fe$^{-}$ and Ru$^{-}$. Two shapes resonances were observed in Fe$^{-}$ and no clear resonance was observed in Ru$^{-}$. The absolute cross-section for the production of Fe$^{+}$ and Ru$^{+}$ will also be presented. [Preview Abstract] |
Thursday, May 29, 2008 8:48AM - 9:00AM |
I6.00005: Photodetachment from the S$^{-}$ ion at the $^{2}$P$_{1/2 }\to $ $^{3}$P$_{2}$ threshold John N. Yukich, James E. Wells Numerous experiments have investigated photodetachment spectroscopy in a magnetic field at the $^{2}$P$_{3/2 }\to \quad ^{3}$P$_{2}$ threshold of ions such as S$^{-}$ and O$^{-}$. The energy of this threshold is known as the atom's \textit{electron affinity}. In this work we have investigated detachment at the lowest-lying threshold for the S$^{-}$ ion, the $^{2}$P$_{1/2 }\to \quad ^{3}$P$_{2}$ threshold. Our experimental apparatus includes a Penning ion trap in which the ions are created, trapped and stored, and a single-mode, ring dye laser. Our observations yield a quantitative measurement for the threshold energy and an indirect measurement for the spin-orbit splitting of the S$^{-}$ ion. [Preview Abstract] |
Thursday, May 29, 2008 9:00AM - 9:12AM |
I6.00006: Kinetic Energy Release Effects on the Angular Distributions from the Photoionization of H$_2^+$ James Colgan, Alain Huetz, Tim Reddish, Michael Pindzola Recent experimental and theoretical work examining the double photoionization of H$_2$ has uncovered surprising changes in the resulting differential cross sections as the kinetic energy released to the outgoing protons is varied. In this work, a complementary study is made of the angular distributions arising from the photoionization of H$_2^+$ at various internuclear separations $R$. Different internuclear separations correspond to different amounts of kinetic energy released to the exploding protons. We find that the angular distributions for the $\sigma \rightarrow \pi$ transition in H$_2^+$ are relatively insensitive to changes in $R$, but that the angular distributions for the $\sigma \rightarrow \sigma$ transition display dramatic variations with $R$. The reasons for this effect and the connections with early work on H$_2^+$ photoionization will be discussed. [Preview Abstract] |
Thursday, May 29, 2008 9:12AM - 9:24AM |
I6.00007: Large internuclear distance effects on photoionization of diatomic molecules Li$_2^+$ and H$_2^+$ Frank Yip, Thomas Rescigno, C. William McCurdy The role of internuclear separation in diatomic molecules dramatically impacts the magnitude and angular distributions of single photoionization cross sections from these targets. We consider photoionization of Li$_2^+$, which has a large equilibrium bond distance at $R=5.86$a$_0$, and the one-electron analog H$_2^+$ fixed at this internuclear distance. The highly non-spherical nature of such molecules causes higher angular momentum components to contribute more significantly compared to smaller diatomic bond distances even at low energies within 1eV of the photoionization threshold, resulting in angular distributions that appear to be $f$-wave dominated. At slightly higher energies, the photoelectron de Broglie wavelength becomes comparable with the bond distance, introducing interference effects in the resulting cross sections at energy ranges far lower than necessary for interference pheonomena to appear with typical molecular bond distances. [Preview Abstract] |
Thursday, May 29, 2008 9:24AM - 9:36AM |
I6.00008: Photoionization of Variable-Size Rare Gas Clusters H. Zhang, D. Rolles, Z.D. Pesic, J.D. Bozek, N. Berrah The electronic structure of variable-size rare gas clusters was studied using angle resolved photoelectron spectroscopy. Photoelectron spectra subsequent to the photoionization of valence and core levels of different-size (60-5000 atoms) clusters have been measured. The photoelectron angular distribution parameters as a function of photon energy and cluster size have been determined. Particularly, the angular distribution parameters of the two-spin orbit components for Xe 5p, 4d, Kr 4p, 3d, Ar 2p have been obtained. The results demonstrate that the photoelectron angular distributions of the clusters are more isotropic than those of the corresponding free atoms, for the photon energy right above the corresponding thresholds. For the valence electrons, differences between the two spin-orbit components in Ar, Kr clusters were found. For Ar clusters, the photoelectron angular distributions of the 3p and 3s valence levels were also obtained. [Preview Abstract] |
Thursday, May 29, 2008 9:36AM - 9:48AM |
I6.00009: Ionization of Methane, Ethane, Butane and Octane in Strong and Ultrastrong, Relativistic Fields Sasikumar Palaniyappan, Robert Mitchell, Robert Sauer, Isaac Ghebregziabher, Samantha White, Matthew DeCamp, Barry Walker Strong and ultrastrong field photoionization of methane is reported at intensities from 10$^{14}$ W/cm$^{2}$ to 10$^{19}$ W/cm$^{2}$. The molecular fragment ions, C$^{+}$, and C$^{+2}$ are created in an intensity window from 10$^{14}$ W/cm$^{2}$ to 10$^{15}$W/cm$^{2 }$and show a high degree of correlation generally consistent with a Coulomb explosion mechanism. The ionization of the remaining valence electrons in carbon (C$^{+3}$, C$^{+4})$ has at least two contributing mechanisms, one correlated to the molecular fragments and one resulting from tunneling ionization of the carbon ion. The photoelectron energy spectrum near the saturation intensity of C$^{+4}$ extends out to 1 keV. In ultrastrong fields (10$^{19}$W/cm$^{2})$, removal of the K-shell to form C$^{+5}$ is uncorrelated with molecular channels. The ionization proceeds via tunneling and non-sequential rescattering ionization. The photoelectron energy spectrum for C$^{+5}$ is shown to extend beyond the rest mass of the electron with kinetic energies up to 0.6 MeV. We also measured C$^{+n}$ (n$<$5) ion yields from ethane, butane and octane at intensities from 10$^{14}$W/cm$^{2 }$to 10$^{17}$ W/cm$^{2 }$from a linearly polarized field. These results are very similar to that from methane, which strongly indicates that the response of methane to an ultrastrong laser field can be generalized to even larger molecules. [Preview Abstract] |
Thursday, May 29, 2008 9:48AM - 10:00AM |
I6.00010: Interplay between Electronic and Nuclear Motion in the Photodouble Ionization of H$_{2}$ T.J. Reddish, J. Colgan, P. Bolognesi, L. Avaldi, M. Gisselbrecht, M. Lavoll\'ee, M.S. Pindzola, A. Huetz Photodouble ionization of molecular hydrogen results in a ``Coulomb explosion,'' as the two protons rapidly separate in opposite directions. The internuclear distance, $R$, between the two nuclei at the instant of photodouble ionization can be accessed through the kinetic energies of the emitted protons. A systematic analysis of the variation with $R$ of the fully differential cross section (FDCS) for this process is presented for a geometry where the 4-body interaction is completely probed. Dramatic variations in the FDCS with different $R$ are observed for geometries where the molecule is at approximately 20$^{\circ}$ to the polarization axis. Excellent agreement is found between experiment and Time-Dependent Close-Coupling theory after convolution of the latter over the relevant solid angles. We show that the observed variations are purely due to the $\varepsilon_{\Sigma}$ component of the polarization vector $\varepsilon$ along the molecular axis and a physical interpretation is proposed by analogy with single ionization of H$_{2}^{+}$, where similar variations in the angular distributions of the outgoing electron are found as a function of $R$. [Preview Abstract] |
Thursday, May 29, 2008 10:00AM - 10:12AM |
I6.00011: Near-ultraviolet photoionization of Xe@C$_{60}$: Sizable oscillator strength transfer from shell to atom Himadri Chakraborty, Mohamed Madjet, Jan-Michael Rost, Steve Manson The encapsulation of an atom in a fullerene cage offers a unique laboratory to probe the behavior of an atom in confinement. We perform calculations on the photoionization of a Xe atom endohedrally confined in C$_{60}$. The time-dependent local density approximation is employed to treat the correlated motion of 240 valence electrons of the fullerene \textit{plus} all Xe electrons in the background of a uniformly charged classical jellium hull that centrally holds the Xe nucleus. In the energy region from Xe 5p ionization threshold to about 40 eV, the atomic cross section is found to enhance considerably by drawing strength from the collective motion of C$_{60}$ electrons. This is confirmed by identifying an equivalent depletion of the surface plasmon in the ionization cross section of the confining shell. Results corroborate our prediction [1] that the correlation moves oscillator strength from the shell to the atom, not in the opposite path. [1] Madjet et al., Phys. Rev. Letts. \textbf{99}, 243003 (2007). [Preview Abstract] |
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