Bulletin of the American Physical Society
42nd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 56, Number 5
Monday–Friday, June 13–17, 2011; Atlanta, Georgia
Session P3: Atomic and Molecular Photoprocesses |
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Chair: Jose Crespo Lopez-Urrutia, Max-Planck Institute for Nuclear Physics Room: A703 |
Thursday, June 16, 2011 2:00PM - 2:12PM |
P3.00001: Photo Double Ionization of Helium Atoms beyond the Dipole Approximation Th. Weber, M.S. Schoeffler, A. Belkacem, R. Doerner, T. Jahnke, A. Landers, C.L. Cocke This work tries to unravel the emission patterns and ionization mechanisms of highly correlated electrons in the simplest atomic many-body system (Helium) at high photon energies (900eV). We try to answer the following questions: 1.) Is there a third ionization mechanism beyond shake-off and knock-off, as predicted 30 years ago by Amusia et al., the quasi free (or equal energy sharing) mechanism? 2.) At which energies does the dipole approximation break down? So far all experiments have been done in an energy range where non-dipole effects are small. Accordingly, most of the advanced theories have restricted themselves to the dipole approximation. Hence, one objective of this experiment was to provide the first experimental data sensitive to non-dipole contributions to the two electron continuum. The goal was to gain a deeper understanding of the initial ground state of the simplest many-particle system helium. Moreover we were looking for a back-to-back-emission of the electrons, which would implicate a break down of the dipole-approximation and which would indicate processes where single photons couple to both electrons simultaneously via quadrupol transitions. [Preview Abstract] |
Thursday, June 16, 2011 2:12PM - 2:24PM |
P3.00002: Study of double core holes in CO molecules created by intense short free electron laser pulses Li Fang, Brendan Murphy, Timur Osipov, Nora Berrah, Edwin Kukk, Motomichi Tashiro, Masahiro Ehara, Kiyoshi Ueda, Kevin. C. Prince, Robert Richter, Raimund Feifel, Peter Salen, Peter van der Meulen, Henning Schmidt, Richard D. Thomas, Mats Larsson The intense short x-ray laser pulses from the Linac Coherent Light Source sequentially double core-ionize CO molecules within the time scale of the Auger decay of the single-core-hole state, producing double core holes. We will present the evidence of double core holes located at different atomic sites in CO molecules via both Auger and photoelectron spectroscopy. The spectral features corresponding to the double core holes are consistent with our theoretical calculations, showing evidence of various site arrangement of multiple core holes. Our results prove experimentally the feasibility of double-core-hole spectroscopy as a new chemical analysis tool. This work was supported in part by the DOE-SC-BES. [1] L.~Fang \textit{et al.}, PRL \textbf{105}, 083005 (2010). [2] J.~Cryan \textit{et al.}, PRL \textbf{105}, 083004 (2010). [Preview Abstract] |
Thursday, June 16, 2011 2:24PM - 2:36PM |
P3.00003: Indirect double photoionization of water T.N. Resccigno, H. Sann, A.E. Orel, R. D\"orner The vertical double ionization thresholds of small molecules generally lie above the dissociation limits corresponding to formation of two singly charged fragments. This gives the possibility of populating singly charged molecular ions by photoionization in the Franck-Condon region at energies below the lowest dication state, but above the dissociation limit into two singly charged fragment ions. This process can produce a superexcited neutral fragment that autoionizes at large internuclear separation. We study this process in water, where absorption of a photon produces an inner-shell excited state of H$_2$O$^+$ that fragments to H$^+$+OH*. The angular distribution of secondary electrons produced by OH* when it autoionizes produces a characteristic asymmetric pattern that reveals the distance, and therefore the time, at which the decay takes place. [Preview Abstract] |
Thursday, June 16, 2011 2:36PM - 2:48PM |
P3.00004: Threshold photodetachment of HOCO$^-$: a theoretical study S. Miyabe, D.J. Haxton, K.V. Lawler, A.E. Lawler, C.W. McCurdy, T.N. Rescigno The HOCO radical is an important intermediate in combustion. Information about its electronic structure has been obtained from studies of the photodetachment of HOCO$^-$. We present the results of a theoretical study of HOCO$^-$ photodetachment, with a view toward understanding the origin of two peaks observed in the photoelectron kinetic energy spectrum very close to threshold. Fixed-nuclei variational electron-HOCO scattering calculations are used to compute photodetachment cross sections and laboratory-frame photoelectron angular distributions. We show that the observed peaks cannot, as previously assumed, be narrow shape resonances and argue that they can be attributed to vibrational Feshbach resonances of dipole-bound trans-HOCO$^-$. [Preview Abstract] |
Thursday, June 16, 2011 2:48PM - 3:00PM |
P3.00005: Resonant photodissociation in substituted benzenes Tim Scarborough, Collin McAcy, David Foote, Cornelis Uiterwaal Cyclic aromatic molecules are abundant in organic chemistry, with a wide variety of applications, including pharmacology, pollution studies and genetic research. Among the simplest of these molecules is benzene (C$_{6}$H$_{6})$, with many relevant molecules being benzene-like with a single atomic substitution. In such a substitution, the substituent determines a characteristic perturbation of the electronic structure of the molecule. We discuss the substitution of halogens into the ring (C$_{6}$H$_{5}$X), and its effects on the dynamics of ionization and dissociation of the molecule without the focal volume effect [1]. In particular, using 800-nm, 50-fs laser pulses, we present results in the dissociation of fluorobenzene, chlorobenzene, bromobenzene and iodobenzene into the phenyl ring (C$_{6}$H$_{5})$ and the atomic halogen, and the subsequent ionization of these fragments. The impact of the ``heavy atom effect'' on a $^{1}(\pi $,$\pi $*) $\to \quad ^{3}(n$,$\sigma $*) singlet-triplet intersystem crossing will be emphasized. Currently under investigation is whether such a dissociation can be treated as an effective source of the neutral substituent.\\[4pt] [1] J. Strohaber and C. Uiterwaal, Phys. Rev. Lett. \textbf{100} 023002 (2008). [Preview Abstract] |
Thursday, June 16, 2011 3:00PM - 3:12PM |
P3.00006: Imaging of graphene ribbons and carbon nanotubes by strong field ionization Agnieszka Jaron-Becker, Henok Ghebrechristos, Min Seok Choi One of the goals of strong field physics is imaging the structure and dynamics of quantum systems. We present numerical results and a theoretical analysis of ionization dependence on the orientation of carbon nanotubes and graphene ribbons with respect to the polarization of the electric field of a laser. Properties of considered finite systems like e.g. the radius and length of carbon nanotubes as well as impurities for graphene ribbons can be related to qualitative and quantitative changes in the orientation dependent ionization signal. [Preview Abstract] |
Thursday, June 16, 2011 3:12PM - 3:24PM |
P3.00007: Dynamics of Photoelectrons in Magnetic Fields Christian Bracher, Alexandros Fragkopoulos Near-threshold photodetachment of negative ions provides a practical means to operate an almost point-like, coherent, monochromatic source of electrons. Due to their charge, the emitted electron matter-waves are easily manipulated by external electromagnetic fields. In our contribution, we present the influence of a homogeneous magnetic field on the probability density and current profiles of the electrons, studied using exact and semiclassical methods. Notwithstanding the simplicity of their classical cyclotron motion, the Lorentz force refracts the electron wave in surprisingly complex ways, giving rise to caustic singularities and an intricate interference superstructure, and causes a remarkable modulation of the photodetachment cross section. We consider two cases: An electron source in a purely magnetic field environment, and electron waves effectively confined to a two-dimensional layer, spreading under the influence of crossed electric and magnetic fields, akin to the Hall configuration. [Preview Abstract] |
Thursday, June 16, 2011 3:24PM - 3:36PM |
P3.00008: An analysis of the breakdown of the relativistic factorized-form expression for Compton scattering doubly differential cross sections L.A. LaJohn, R.H. Pratt The factorizable form of the relativistic impulse approximation (RIA) expression for Compton scattering doubly differential cross sections (DDCS), an expression that is used to obtain the Compton profile from DDCS, and then the bound electron momentum distribution, loses its validity for K-shell ionization of moderate to high nuclear charge Z atoms. This factorizable expression again has the nonrelativistic form DDCS=KJ, where K is a kinematic factor and J represents the Compton profile; it can overestimate the Compton peak magnitude by as much as $50\%$ for K-shell ionization as Z becomes large. In this study we have evaluated the error due to using this factorizable RIA expression for DDCS, as a function of Z, scattering angle $\theta $, and incident photon energy $\omega_i$. We provide an explanation for why this approximation breaks down as J becomes broader with increasing Z. We show, and explain why, the magnitude of this error is proportional to $Z^2$ for all $\omega_i$ and $\theta $ for low to moderate Z, and still for all Z when $\theta $ is between $0^{\circ}$ and at least $35^{\circ}$ if $\omega_i < 1$ MeV. These results provide useful information for when one can use this factorizable RIA expression for DDCS and also how the expression can be corrected when it fails. [Preview Abstract] |
Thursday, June 16, 2011 3:36PM - 3:48PM |
P3.00009: Ionization of kicked Rydberg atoms via a turnstile mechanism Korana Burke, Kevin Mitchell, Shuzhen Ye, Brendan Wyker, F. Barry Dunning We present a theoretical and experimental study of the chaotic of quasi-one-dimensional potassium Rydberg wavepackets via phase space turnstile mechanism. Turnstiles form a general mechanism for numerous chaotic systems, and this study is first to explicitly illuminate their relevance to atomic. We create time-dependent Rydberg wavepackets, subject to alternating applied electric fields (kicks), and measure the fraction. We show that the ionization of the electron not only on the initial electron energy, but also on the phase position of the electron with respect to the turnstile--that of the electron packet inside the turnstile ionizes quickly, one period of the applied field, while that part outside the ionizes after multiple kicking periods. The dependence of ionization on the kicking period can also be understood in terms the turnstile geometry. [Preview Abstract] |
Thursday, June 16, 2011 3:48PM - 4:00PM |
P3.00010: ABSTRACT WITHDRAWN |
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