Bulletin of the American Physical Society
41st Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 55, Number 5
Tuesday–Saturday, May 25–29, 2010; Houston, Texas
Session B5: Photoionization |
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Chair: William McCurdy, University of California, Davis Room: Arboretum I-III |
Wednesday, May 26, 2010 10:30AM - 10:42AM |
B5.00001: Double Photoionization of Atomic Beryllium Frank L. Yip, C. William McCurdy, Thomas N. Rescigno One-photon double ionization (DPI) of beryllium represents the next step in the evolution of DPI investigations that began with helium in order to sensitively probe electron correlation. Beryllium is the simplest atomic species of the alkaline earth elements which, in general, possess two electrons outside of a fully occupied inner shell that spherically screens the nucleus. This provides a natural basis for comparison to $1s^2$ helium DPI. However, the valence state of beryllium has $n=2$, thus making the valence excited target $2s2p$ more accessible relative to the $2s^2$ ground state as compared to ground-state and metastable helium. Also, the symmetry of photoionizing from either the $^1S$ or $^1P$ initial state will have consequences for the angular distributions for double ionization. Triply differential cross sections (TDCS) are presented for DPI from both ground state $2s^2$ and excited state $2s2p$ beryllium calculated using exterior complex scaling (ECS) for the valence electrons. [Preview Abstract] |
Wednesday, May 26, 2010 10:42AM - 10:54AM |
B5.00002: Nuclear Recoil Cross Sections from Time-dependent Studies of Two-Photon Double Ionization of Helium Dan H. Horner, Thomas N. Rescigno, C. William McCurdy In preparation for benchmark comparisons with new experimental data from experiments at FLASH, we use accurate triple differential cross sections (TDCS), extracted from non-perturbative solutions of the time-dependent Schr\"odinger equation, to compute the nuclear recoil cross section at energies a few electron volts below the threshold for sequential double ionization. The nuclear recoil cross sections produced by two-photon double ionization of helium reveal aspects of the underlying TDCS used in their computation -- even though the recoil of the He$^{++}$ nucleus reflects an integration over all electron ejection directions and energy sharings. In this range of photon energies, the nuclear recoil patterns prefigure some important aspects of the sequential mechanism that dominates at higher energies, above the sequential ionization threshold. [Preview Abstract] |
Wednesday, May 26, 2010 10:54AM - 11:06AM |
B5.00003: Triple differential cross sections for the double photoionization of Li James Colgan, Anatoli Kheifets, Dmitry Fursa, C.W. Hines, Igor Bray, Michael Pindzola A joint theoretical study is presented of the triple differential cross sections arising from the double photoionization of lithium. The time-dependent close-coupling (TDCC) [1] and convergent close-coupling (CCC) [2] approaches are used to predict the cross sections for photon energies of 90 eV, where the non-ionized electron remains in the ground state. It is shown that for the equal energy sharing case the double photoionization process can be described by two symmetrized singlet and triplet amplitudes. The two theoretical approaches generally show good agreement, and detailed comparisons will be given for both equal and unequal energy sharing cases. We also report on our progress in calculating fully differential cross sections for the triple photoionization of lithium. The Los Alamos National Laboratory is operated by Los Alamos National Security, LLC for the National Nuclear Security Administration of the U.S. Department of Energy under Contract No. DE-AC5206NA25396. \\[4pt] [1] J. Colgan, D. C. Griffin, C. P. Ballance, and M. S. Pindzola, Phys. Rev. A {\bf 80}, 063414 (2009). \\[0pt] [2] A. S. Kheifets, D. V. Fursa, and I. Bray, Phys. Rev. A {\bf 80}, 063413 (2009). [Preview Abstract] |
Wednesday, May 26, 2010 11:06AM - 11:18AM |
B5.00004: Double Ionization of H$_2$ in Intense Short-Pulse Laser Fields Xiaoxu Guan, Klaus Bartschat Bartschat, Barry I. Schneider We report our development of a nonperturbative time-dependent method to treat one- and two-photon double ionization of the hydrogen molecule by intense ultrashort laser pulses. The two-center two-electron system is discretized in prolate spheroidal coordinates combined with a FE-DVR basis. The solution to the time-dependent laser-driven problem is obtained in the Born-Oppenheimer approximation by propagating the initial state using an effective Arnoldi algorithm. We discuss the dependence of the fully differential cross section for double ionization on the directions of both the molecular and the laser polarization axes. Our results are compared with other recent theoretical predictions. [Preview Abstract] |
Wednesday, May 26, 2010 11:18AM - 11:30AM |
B5.00005: Double Photoionization of Naphthalene and Azulene Ralf Wehlitz, Pavle Jurani\'c We have determined the double-to-single photoionization ratio of naphthalene (C$_{10}$H$_{8}$) and its isomer azulene from threshold to high photon energies using synchrotron radiation. While the overall photon-energy dependence of the ratio is very similar for both molecules, the absolute values of their ratios are clearly different indicating an isomer effect. Previously, an isomer effect for double ionization by electron impact has been observed for C$_{3}$H$_{4}$.\footnote{S. W. J. Scully {\it et al.}, Phys.\ Rev.\ A {\bf 72}, 030701 (2005)} Our results demonstrate that double photoionization is sensitive to the structure of a molecule and not just to its constituents. [Preview Abstract] |
Wednesday, May 26, 2010 11:30AM - 11:42AM |
B5.00006: Time-dependent close-coupling calculations of the double photoionization of He@C$_{60}$ John Ludlow, Teck-Ghee Lee, M.S. Pindzola The double photoionization of a helium atom enclosed in a C$_{60}$ molecule, He@C$_{60}$, has been investigated using the time-dependent close-coupling method [1], with the C$_{60}$ molecule represented by a spherical shell potential and the helium atom treated non-perturbatively. Comparisons are made with the double photoionization of a bare helium atom. The ratio of the total double photoionization cross section of He@C$_{60}$ to that of a bare helium atom is found to exhibit oscillations [2] due to the interference between three photoelectron waves: the photoelectron wave emerging directly from the ionized 1s subshell and the photoelectron waves reflected from the inner and outer boundaries of the C$_{60}$ shell. Single energy differential cross sections and triple differential cross sections are calculated. The investigation is extended to examine the double photoionization of a Helium atom enclosed in a positively or negatively charged C$_{60}$ molecule. \\[4pt] [1] M. S. Pindzola {\it et al.}, J. Phys. B {\bf 40}, R39 (2007).\\[0pt] [2] M. Ya. Amusia, E. Z. Liverts and V. B. Mandelzweig, Phys. Rev. A {\bf 74}, 042712 (2006). [Preview Abstract] |
Wednesday, May 26, 2010 11:42AM - 11:54AM |
B5.00007: Asymmetric Core-Photoelectron Angular Distributions of Fixed-in-Space CO$_2$ F.P. Sturm, M. Schoeffler, S. Lee, T. Osipov, N. Neumann, H.-K. Kim, S. Kirschner, B. Rudek, J.B. Williams, J.D. Daughhetee, C.L. Cocke, K. Ueda, A.L. Landers, Th. Weber, M.H. Prior, A. Belkacem, R. Doerner We report a kinematically complete experiment of carbon 1s photoionization of CO$_{2}$ including Auger decay and fragmentation. By measuring in coincidence of CO$_{2}$ C(1s) photoelectrons and ion fragments using synchrotron light at several energies above the C(1s) threshold, we determine photoelectron angular distributions as well as Auger electron angular distributions with full solid angle in the molecular fixed frame. Our measurement confirms earlier results showing a weak but definite asymmetry of the photoelectron emission with respect to the central carbon atom. Additionally, it reveals new asymmetric features in the photoelectron angular distribution which change as a function of the kinetic energy release. Our explanation for this puzzling asymmetry is that the angular dependence of the outgoing photoelectron is modified by the instantaneous dipole moment of the vibrating {CO$_{2}$}$^{+}$ ion, registering information about its asymmetric vibrational motion at the time of photoejection. [Preview Abstract] |
Wednesday, May 26, 2010 11:54AM - 12:06PM |
B5.00008: Perturbation Theory Analysis of Attosecond Photoionization Evgeny A. Pronin, Anthony F. Starace, Mikhail V. Frolov, Nikolai L. Manakov Ionization of an atom by a few-cycle attosecond XUV pulse is analyzed using perturbation theory (PT), keeping terms in the transition amplitude up to second order in the pulse electric field [{\it Phys. Rev. A \bf 80}, 063403 (2009)]. Within the PT approach, we present an {\it ab initio} parametrization of the ionized electron angular distribution (AD) (using rotational invariance and symmetry arguments) that gives analytically the dependence of the AD on the carrier envelope phase (CEP), the pulse polarization, and the ionized electron direction, $\bf{\hat p}$. For an elliptically-polarized pulse, interference of the first and second order amplitudes causes a CEP-dependent asymmetry (with respect to $\bf{\hat p}\to -\bf{\hat p}$) and both elliptic and circular dichroism effects. For ionization of the H atom by linearly-polarized pulses, our PT results are in excellent agreement with results of numerical solutions of the time-dependent Schr\"{o}dinger equation of Peng \textit{et al}. [{\it New J. Phys.}~{\bf 10}, 025030 (2008)]. [Preview Abstract] |
Wednesday, May 26, 2010 12:06PM - 12:18PM |
B5.00009: Angular Dependance of Auger Decay of Double Core Vacancies in N$_{2}$ James Cryan, James Glownia, Philip Bucksbaum, Ryan Coffee We present a first experimental step toward angle-resolved multiple-core vacancy Auger electron spectroscopy. We observe Auger decay for both single-site and double-site $K^{-2}$ vacancy pairs in N$_2$ and find that the single-site double vacancy undergoes an Auger process whose angular pattern qualitatively resembles previous measurements of $1\sigma_{g}^{-1}$ Auger decay. In addition, we measure the angle dependence of the Auger decay of single core vacancies to quasi-bound molecular dication states. Ultimately, the combination of impulsive molecular alignment and x-ray free-electron lasers enables angle resolution for few femtosecond chemical dynamics. [Preview Abstract] |
Wednesday, May 26, 2010 12:18PM - 12:30PM |
B5.00010: Dynamics of Micro-Plasmas Generated in Noble Gases by Strong-Field Laser Pulses Dmitri A. Romanov, Ryan Compton, Alex Filin, Robert J. Levis The ultrafast dynamics of micro-plasmas generated by an $\sim $80 fs laser pulse in noble gases has been investigated using four-wave mixing (FWM). The evolution patterns of the FWM signal are indicative of the gas species and ambient conditions. The signal is observed to reach higher intensity levels faster for Xe, with progressively lower scattering intensity and longer time dynamics for the noble gas series Xe, Kr, Ar, Ne, and He. The theoretical model for interpreting this temporal dynamics is based on initial tunnel ionization followed by electron impact ionization cooling. The model reproduces well the measured degree of ionization in atmospheric-pressure laser-induced plasmas and predicts quantitatively the intensity of four-wave mixing as a function of time for the series of five noble gases. The model also predicts the dynamics as a function of pump laser intensity and gas pressure. The findings open the way for effective control of micro-plasma dynamics. [Preview Abstract] |
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