S. H. Southworth

Argonne National Laboratory, Лимонт, Illinois, United States

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Publications (192)433.24 Total impact

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    Full-text · Dataset · Oct 2015
  • Antonio Picón · Jordi Mompart · Stephen H Southworth
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    ABSTRACT: Recent techniques in x-ray free electron lasers allow the generation of highly coherent, intense x-ray pulses with time lengths on the order of femtoseconds. Here we explore the possibilities of using such x-ray pulses to control matter based on coherence. In particular we propose a theoretical scheme to perform stimulated Raman adiabatic passage in the x-ray regime by using inner-hole excited states. Numerical results in two well-known systems, the neon atom and the carbon monoxide molecule, show a robust control of population transfer. In the molecule, vibrational selectivity is achieved with femtosecond x-ray pulses. This work supports the possibility of using two-color x-ray pulses for coherent control. © 2015 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    No preview · Article · Aug 2015 · New Journal of Physics
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    ABSTRACT: Photoionization cross sections and partial ion yields of Xe and XeF2 from Xe 3d5/2, Xe 3d3/2, and F 1s subshells in the 660–740 eV range are compared to explore effects of the F ligands. The Xe 3d-ϵf continuum shape resonances dominate the photoionization cross sections of both the atom and molecule, but prominent resonances appear in the XeF2 cross section due to nominal excitation of Xe 3d and F 1s electrons to the lowest unoccupied molecular orbital (LUMO), a delocalized anti-bonding MO. Comparisons of the ion products from the atom and molecule following Xe 3d photoionization show that the charge-state distribution of Xe ions is shifted to lower charge states in the molecule along with production of energetic F+ and F2+ ions. This suggests that, in decay of a Xe 3d core hole, charge is redistributed to the F ligands and the system dissociates due to Coulomb repulsion. The ion products from excitation of the F 1s-LUMO resonance are different and show strong increases in the yields of Xe+ and F+ ions. The subshell ionization thresholds, the LUMO resonance energies, and their oscillator strengths are calculated by relativistic coupled-cluster methods and agree well with measurements.
    Full-text · Article · Jun 2015 · The Journal of Chemical Physics
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    Full-text · Technical Report · Jun 2015
  • B. Zimmermann · V. McKoy · S. H. Southworth · E. P. Kanter · B. Krässig · R. Wehlitz
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    ABSTRACT: We describe a theoretical approach to molecular photoionization that includes first-order corrections to the dipole approximation. The theoretical formalism is presented and applied to photoionization of H2 over the 20- to 180-eV photon energy range. The angle-integrated cross section σ, the electric dipole anisotropy parameter βe, the molecular alignment anisotropy parameter βm, and the first-order nondipole asymmetry parameters γ and δ were calculated within the single-channel, static-exchange approximation. The calculated parameters are compared with previous measurements of σ and βm and the present measurements of βe and γ+3δ. The dipole and nondipole angular distribution parameters were determined simultaneously using an efficient, multiangle measurement technique. Good overall agreement is observed between the magnitudes and spectral variations of the calculated and measured parameters. The nondipole asymmetries of He 1s and Ne 2p photoelectrons were also measured in the course of this work.
    No preview · Article · May 2015 · Physical Review A
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    ABSTRACT: Theoretical predictions show that depending on the populations of the Fe 3dxy, 3dxz, and 3dyz orbitals two possible quintet states can exist for the high-spin state of the photoswitchable model system [Fe(terpy)2]2+. The differences in the structure and molecular properties of these 5B2 and 5E quintets are very small and pose a substantial challenge for experiments to resolve them. Yet for a better understanding of the physics of this system, which can lead to the design of novel molecules with enhanced photoswitching performance, it is vital to determine which high-spin state is reached in the transitions that follow the light excitation. The quintet state can be prepared with a short laser pulse and can be studied with cutting-edge time-resolved X-ray techniques. Here we report on the application of an extended set of X-ray spectroscopy and scattering techniques applied to investigate the quintet state of [Fe(terpy)2]2+ 80 ps after light excitation. High-quality X-ray absorption, nonresonant emission
    Full-text · Article · Mar 2015 · The Journal of Physical Chemistry C
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    ABSTRACT: X-ray spectroscopies, when combined in laser-pump, X-ray-probe measurement schemes, can be powerful tools for tracking the electronic and geometric structural changes that occur during the course of a photoinitiated chemical reaction. X-ray absorption spectroscopy (XAS) is considered an established technique for such measurements, and X-ray emission spectroscopy (XES) of the strongest core-to-core emission lines (Kα and Kβ) is now being utilized. Flux demanding valence-to-core XES promises to be an important addition to the time-resolved spectroscopic toolkit. In this paper we present measurements and density functional theory calculations on laser-excited, solution-phase ferrocyanide that demonstrate the feasibility of valence-to-core XES for time-resolved experiments. We discuss technical improvements that will make valence-to-core XES a practical pump–probe technique.
    Full-text · Article · Feb 2015 · The Journal of Physical Chemistry C
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    ABSTRACT: We have studied the photoinduced low spin (LS) to high spin (HS) conversion of aqueous Fe(bpy)3 with pulse-limited time resolution. In a combined setup permitting simultaneous X-ray diffuse scattering (XDS) and spectroscopic measurements at a MHz repetition rate we have unraveled the interplay between intramolecular dynamics and the intermolecular caging solvent response with 100 ps time resolution. On this time scale the ultrafast spin transition including intramolecular geometric structure changes as well as the concomitant bulk solvent heating process due to energy dissipation from the excited HS molecule are long completed. The heating is nevertheless observed to further increase due to the excess energy between HS and LS states released on a subnanosecond time scale. The analysis of the spectroscopic data allows precise determination of the excited population which efficiently reduces the number of free parameters in the XDS analysis, and both combined permit extraction of information about the structural dynamics of the first solvation shell.
    No preview · Article · Aug 2014 · Faraday Discussions
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    Antonio Picón · Phay J Ho · Gilles Doumy · Stephen H Southworth
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    ABSTRACT: We have unveiled coherent multiphoton interferences originating from different quantum paths taken by the Auger electron induced by a high-intensity x-ray/XUV pulse under the presence of a strong optical field. These interferences give rise to a clear signature in the angle-resolved Auger electron spectrum: an asymmetry with respect to the energy of the Auger decay channel. In order to illustrate this effect we have considered the resonant Auger decay of the transition $2p^{5} \!\leftrightarrow\! 1s^{-1}2p^{6}$ in Ne$^{+}$. The simulations show that these interferences are very sensitive to the parameters of the x-ray and optical fields.
    Full-text · Article · Aug 2013 · New Journal of Physics
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    ABSTRACT: X-ray free-electron lasers (FELs) produce femtosecond x-ray pulses with unprecedented intensities that are uniquely suited for studying many phenomena in atomic, molecular, and optical (AMO) physics. A compilation of the current developments at the Linac Coherent Light Source (LCLS) and future plans for the LCLS-II and Next Generation Light Source (NGLS) are outlined. The AMO instrumentation at LCLS and its performance parameters are summarized. A few selected experiments representing the rapidly developing field of ultra-fast and peak intensity x-ray AMO sciences are discussed. These examples include fundamental aspects of intense x-ray interaction with atoms, nonlinear atomic physics in the x-ray regime, double core-hole spectroscopy, quantum control experiments with FELs and ultra-fast x-ray induced dynamics in clusters. These experiments illustrate the fundamental aspects of the interaction of intense short pulses of x-rays with atoms, molecules and clusters that are probed by electron and ion spectroscopies as well as ultra-fast x-ray scattering.
    Full-text · Article · Aug 2013 · Journal of Physics B Atomic Molecular and Optical Physics
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    ABSTRACT: We theoretically show that core-excited state populations can be efficiently manipulated with strong optical fields during their decay, which takes place in a few femtoseconds. We focus on the $1s^{-1}3p$ resonant excitation in neon, where the $1s^{-1}3p$ and $1s^{-1}3s$ core-excited states are coupled by an optical field. By analyzing the Auger electron spectrum we observe the inner-shell population transfer induced by the optical coupling. We also show that the angular anisotropy of the Auger electron is imprinted in the multipeak structure induced by the optical-dressed continuum, namely sidebands.
    Full-text · Article · Feb 2013 · Physical Review A
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    ABSTRACT: We have used dipole-quadrupole interference spectroscopy to observe an optically forbidden quadrupole Rydberg series of helium autoionizing levels. The technique measures the forward-backward asymmetry of photoelectron angular distributions produced in the vacuum ultraviolet photoionization of helium. The resonant behavior of this asymmetry in the region of a quadrupole autoionizing level enables the determination of the position, width, and Fano line-profile parameter q of the level. We have obtained these quantities for the He 2(1,0)n+ 1 D Rydberg series for n=2–7. We find that for n≥3 all three quantities have the expected n scaling, with a quantum defect of 0.31. For n≥3 the average q parameter lies close to zero, whereas for n=2 it is negative.
    No preview · Article · Nov 2012 · Physical Review A
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    ABSTRACT: We report on extending hard X-ray emission spectroscopy (XES) along with resonant inelastic X-ray scattering (RIXS) to study ultrafast phenomena in a pump-probe scheme at MHz repetition rates. The investigated systems include low-spin (LS) FeII complex compounds, where optical pulses induce a spin-state transition to their (sub)nanosecond-lived high-spin (HS) state. Time-resolved XES clearly reflects the spin-state variations with very high signal-to-noise ratio, in agreement with HS–LS difference spectra measured at thermal spin crossover, and reference HS–LS systems in static experiments, next to multiplet calculations. The 1s2p RIXS, measured at the Fe 1s pre-edge region, shows variations after laser excitation, which are consistent with the formation of the HS state. Our results demonstrate that X-ray spectroscopy experiments with overall rather weak signals, such as RIXS, can now be reliably exploited to study chemical and physical transformations on ultrafast time scales.
    Full-text · Article · Sep 2012 · Journal of Electron Spectroscopy and Related Phenomena
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    ABSTRACT: We have studied the photoinduced low spin (LS) to high spin (HS) conversion of [Fe(bipy)(3)](2+) in aqueous solution. In a laser pump/X-ray probe synchrotron setup permitting simultaneous, time-resolved X-ray diffuse scattering (XDS) and X-ray spectroscopic measurements at a 3.26 MHz repetition rate, we observed the interplay between intramolecular dynamics and the intermolecular caging solvent response with better than 100 ps time resolution. On this time scale, the initial ultrafast spin transition and the associated intramolecular geometric structure changes are long completed, as is the solvent heating due to the initial energy dissipation from the excited HS molecule. Combining information from X-ray emission spectroscopy and scattering, the excitation fraction as well as the temperature and density changes of the solvent can be closely followed on the subnanosecond time scale of the HS lifetime, allowing the detection of an ultrafast change in bulk solvent density. An analysis approach directly utilizing the spectroscopic data in the XDS analysis effectively reduces the number of free parameters, and both combined permit extraction of information about the ultrafast structural dynamics of the caging solvent, in particular, a decrease in the number of water molecules in the first solvation shell is inferred, as predicted by recent theoretical work.
    Full-text · Article · Sep 2012 · The Journal of Physical Chemistry A
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    ABSTRACT: Charge production, charge redistribution, and ion fragmentation are explored in the decay of a Xe K-shell vacancy in XeF2. Coincidence measurements of all ionic fragments in XeF2 provide evidence that an interatomic-Coulombic-decay-like (ICD-like) process plays a role in the cascade decay. The signature of the ICD-like process is an enhancement of the total number of electrons ejected as compared to the case of atomic Xe. The results indicate that the F atoms participate in the decay cascade within the first few femtoseconds after core-hole formation and that fragmentation begins during the decay process.
    Full-text · Article · Sep 2012 · Physical Review A
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    ABSTRACT: The photoionization of an inner-shell electron in a heavy atom triggers a vacancy cascade with the emission of x-ray fluorescence and Auger electrons leading to its final charge states. If the atom is part of a molecule or cluster, the decay process may involve removal of the valence electrons on the neighboring atoms thereby forming several charge centers and resulting in the Coulomb explosion of the system. This phenomenon in molecules where the valence electrons on the neighboring atoms play a significant role in the decay process is called Interatomic Coulombic Decay (ICD) [1]. The focus of this work is to explore the ICD effect in XeF2 [2] following K-shell ionization of the Xe atom near 34.5 keV. We compare the total charge produced following Xe Kα or Kβ fluorescence decay from atomic Xe and from Xe in XeF2 molecules. We present both experimental and calculational evidence that the fluorine atoms get involved in the decay process and the molecules start undergoing structural changes during the vacancy cascade.[4pt] [1] L. S. Cederbaum et al., Phys. Rev. Lett. 79, 4778 (1997).[0pt] [2] C. Buth et al., J. Chem. Phys. 119, 10575 (2003).
    Full-text · Conference Paper · Jun 2012
  • Antonio Picon · Gilles Doumy · Stephen Southworth · Linda Young · Christian Buth
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    ABSTRACT: The emerging x-ray free electron lasers (FELs) such as the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory can reach very high intensities and ultrashort pulse durations. We analyze how to control Auger decay using a secondary intense near-infrared (NIR) laser with electromagnetically induced transparency for x rays. A three-level λ-type model is used, where a core electron is coupled to a Rydberg state by the x rays while the NIR pulse couples the Rydberg states among each other. We use the model to predict the Auger electron spectrum of a neon atom and thus enhance our understanding and control of electron correlations. This work opens up new prospects to study and control the nonlinear interaction of ultraintense and ultrashort x rays with atoms.
    No preview · Article · Jun 2012
  • Antonio Picon · Gilles Doumy · Stephen Southworth · Linda Young · Christian Buth
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    ABSTRACT: The emerging x-ray free electron lasers (FELs) such as the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory can reach very high x-ray intensities and ultrashort pulse durations. We develop a theory for the strong coupling of x rays with an atom, which couples core electrons with Rydberg states. In addition, we consider a near-infrared (NIR) laser that couples the Rydberg states among each other. We can theoretically describe several atomic systems with this setup using three-level (λ-type and cascade-type are considered) models, which allow us to use electromagnetically induced transparency for x rays induced by the NIR laser. The theoretical models also allow us to calculate the NIR-laser-controlled Auger electron spectrum. We apply these models to predict the Auger electron spectrum of Ne (λ-type) and Ne^+ (cascade-type). This work opens up new prospects to study and analyze the interaction of ultraintense and ultrashort x rays with atoms.
    No preview · Article · Jun 2012
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    ABSTRACT: Resonance fluorescence is scattering of photons off atoms and molecules driven by a near-resonant external electric field; it is a cornerstone of spectroscopy and quantum optics. For intense x rays from existing and upcoming x-ray free electron lasers (FELs) such as the Linac Coherent Light Source (LCLS) in Menlo Park, California, USA, the cyclic excitation and decay of a core electron (Rabi flopping) can compete with spontaneous core-hole decay. We develop a two-level description of x-ray resonance fluorescence and exemplify it for neon cations strongly driven by LCLS light tuned to the 1s,p-1->1s-1,p transition at 848 eV. We compute the time-dependent spectrum of resonance fluorescence in order to study the coherent and fundamentally nonlinear process of Rabi flopping at x-ray frequencies. We predict resonance fluorescence spectra for two different scenarios: first, chaotic pulses generated at present-day LCLS and, second, Gaussian pulses which will become available soon with self-seeding techniques. In the latter case, as an example of the exciting opportunities deriving from the use of seeding methods, we predict a clear signature of Rabi flopping in the spectrum of resonance fluorescence.
    No preview · Article · Jun 2012
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    ABSTRACT: The spectrum of resonance fluorescence is calculated for a two-level system excited by an intense, ultrashort x-ray pulse made available for instance by free-electron lasers such as the Linac Coherent Light Source. We allow for inner-shell hole decay widths and destruction of the system by further photoionization. This two-level description is employed to model neon cations strongly driven by x rays tuned to the 1s 2p-1 --> 1s-1 2p transition at 848 eV; the x rays induce Rabi oscillations which are so fast that they compete with Ne 1s-hole decay. We predict resonance fluorescence spectra for two different scenarios: first, chaotic pulses based on the self-amplified spontaneous emission principle, like those presently generated at x-ray free-electron-laser facilities and, second, Gaussian pulses which will become available in the foreseeable future with self-seeding techniques. As an example of the exciting opportunities derived from the use of seeding methods, we predict, in spite of above obstacles, the possibility to distinguish at x-ray frequencies a clear signature of Rabi flopping in the spectrum of resonance fluorescence.
    Full-text · Article · May 2012 · Physical Review A

Publication Stats

3k Citations
433.24 Total Impact Points

Institutions

  • 1985-2015
    • Argonne National Laboratory
      • • Division of Physics
      • • Division of Materials Science
      Лимонт, Illinois, United States
  • 1990-2006
    • National Institute of Standards and Technology
      • Sensor Science Division
      Maryland, United States
  • 2003
    • The University of Tennessee Medical Center at Knoxville
      Knoxville, Tennessee, United States
    • University of Pittsburgh
      • Physics and Astronomy
      Pittsburgh, Pennsylvania, United States
  • 1998
    • University of Nebraska at Lincoln
      • Department of Physics and Astronomy
      Lincoln, Nebraska, United States
  • 1995
    • Lawrence Livermore National Laboratory
      Livermore, California, United States
  • 1994
    • University of Oulu
      • Physics
      Uleoborg, Northern Ostrobothnia, Finland
  • 1986-1989
    • Los Alamos National Laboratory
      • • Chemistry Division
      • • Materials Science and Technology Division
      Los Alamos, California, United States
  • 1988
    • United States Bureau of Reclamation
      Washington, Washington, D.C., United States
  • 1987
    • University of Freiburg
      Freiburg, Baden-Württemberg, Germany
  • 1983-1984
    • CSU Mentor
      • Department of Chemistry
      Long Beach, California, United States
  • 1979-1984
    • University of California, Berkeley
      • Department of Chemistry
      Berkeley, California, United States
  • 1982-1983
    • Lawrence Berkeley National Laboratory
      Berkeley, California, United States