S. H. Southworth

Argonne National Laboratory, Лимонт, Illinois, United States

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Publications (156)384.67 Total impact

  • 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.
    Physical Review A 05/2015; 91(5). DOI:10.1103/PhysRevA.91.053410 · 2.81 Impact Factor
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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.
    Faraday Discussions 08/2014; 171. DOI:10.1039/C4FD00097H · 4.61 Impact Factor
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    ABSTRACT: We have performed time-resolved resonant X-ray emission spectroscopy after UV photoexcitation of a colloidal solution of ZnO nanoparticles. The results point to electron trapping sites located at oxygen vacancies in the lattice.
    International Conference on Ultrafast Phenomena; 01/2014
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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.
    Physical Review A 11/2012; 86(5). DOI:10.1103/PhysRevA.86.053408 · 2.81 Impact Factor
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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.
    The Journal of Physical Chemistry A 09/2012; 116(40):9878-87. DOI:10.1021/jp306917x · 2.69 Impact Factor
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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.
    Physical Review A 09/2012; 86(3-3):033401. DOI:10.1103/PhysRevA.86.033401 · 2.81 Impact Factor
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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).
    43rd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics, Orange County; 06/2012
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    ABSTRACT: The Advanced Photon Source (APS) operates with a timing structure advantageous for ultrafast dynamics experiments and, as a result, X-ray time-resolved studies on the 100-picosecond timescale have flourished. The standard operating mode, 24-bunch mode, runs 65% of the time and a hybrid singlet mode runs 15% of the time, yielding a total of 80% of beamtime easily accessible for studies of ultrafast dynamics using laser-pump/X-ray probe techniques. This article highlights recent enhanced capabilities for time-resolved studies at the APS.
    Synchrotron Radiation News 03/2012; 25(2). DOI:10.1080/08940886.2012.663316
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    ABSTRACT: We show that high fluence, high-intensity x-ray pulses from the world's first hard x-ray free-electron laser produce nonlinear phenomena that differ dramatically from the linear x-ray-matter interaction processes that are encountered at synchrotron x-ray sources. We use intense x-ray pulses of sub-10-fs duration to first reveal and subsequently drive the 1s↔2p resonance in singly ionized neon. This photon-driven cycling of an inner-shell electron modifies the Auger decay process, as evidenced by line shape modification. Our work demonstrates the propensity of high-fluence, femtosecond x-ray pulses to alter the target within a single pulse, i.e., to unveil hidden resonances, by cracking open inner shells energetically inaccessible via single-photon absorption, and to consequently trigger damaging electron cascades at unexpectedly low photon energies.
    Physical Review Letters 12/2011; 107(23):233001. DOI:10.1103/PhysRevLett.107.233001 · 7.51 Impact Factor
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    ABSTRACT: An era of exploring the interactions of high-intensity, hard X-rays with matter has begun with the start-up of a hard-X-ray free-electron laser, the Linac Coherent Light Source (LCLS). Understanding how electrons in matter respond to ultra-intense X-ray radiation is essential for all applications. Here we reveal the nature of the electronic response in a free atom to unprecedented high-intensity, short-wavelength, high-fluence radiation (respectively 10(18) W cm(-2), 1.5-0.6 nm, approximately 10(5) X-ray photons per A(2)). At this fluence, the neon target inevitably changes during the course of a single femtosecond-duration X-ray pulse-by sequentially ejecting electrons-to produce fully-stripped neon through absorption of six photons. Rapid photoejection of inner-shell electrons produces 'hollow' atoms and an intensity-induced X-ray transparency. Such transparency, due to the presence of inner-shell vacancies, can be induced in all atomic, molecular and condensed matter systems at high intensity. Quantitative comparison with theory allows us to extract LCLS fluence and pulse duration. Our successful modelling of X-ray/atom interactions using a straightforward rate equation approach augurs favourably for extension to complex systems.
    Nature 07/2010; 466(7302):56-61. DOI:10.1038/nature09177 · 41.46 Impact Factor
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    ABSTRACT: The first experiments at the LCLS succeeded in not only revealing the nature of interactions between intense x-rays and atomic or molecular systems, but also properties of the LCLS x-ray pulses. Our spectroscopic measurements of the interaction with neon atoms have revealed information on x-ray photon energy, the pulse duration, and the focal spot size. When analyzed in conjunction with diagnostics of the electron beam, the data yields information on the photon energy bandwidth and shot-to-shot photon energy jitter. The intrinsic bandwidth of the LCLS pulse is strongly dependent on the LINAC tuning, thus high resolution photoelectron spectroscopy is a valuable diagnostic that can be implemented in a pass through configuration.
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    ABSTRACT: The goal of this experiment was to characterize the response of a prototypical atom, neon, to the unprecedented flux of microfocussed x-rays produced at the Linac Coherent Light Source (LCLS) at the SLAC Linear Accelerator Laboratory. In agreement with results from theoretical modeling, we find atoms inside the focal volume to undergo multiple successive ionization events, leading to fully stripped Ne^10+ at 2-keV x-ray energies, and charge states up to Ne^8+ below the K-ionization threshold. We also observe photoproduction of hollow neon ions through successive K-shell ionization on timescales shorter than Auger decay. We demonstrate intensity-induced x-ray transparency as a consequence of ever slower vacancy decay clocks limiting an ion's consecutive K absorption within a single 200-fs x-ray pulse.
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    ABSTRACT: Ultrafast X-ray science is an exciting frontier that promises the visualization of electronic, atomic and molecular dynamics on atomic time and length scales. A largely unexplored area of ultrafast X-ray science is the use of light to control how X-rays interact with matter. To extend control concepts established for long-wavelength probes to the X-ray regime, the optical control field must drive a coherent electronic response on a timescale comparable to femtosecond core-hole lifetimes. An intense field is required to achieve this rapid response. Here, an intense optical control pulse is observed to efficiently modulate photoelectric absorption for X-rays and to create an ultrafast transparency window. We demonstrate an application of X-ray transparency relevant to ultrafast X-ray sources: an all-photonic temporal cross-correlation measurement of a femtosecond X-ray pulse. The ability to control X-ray-matter interactions with light will create new opportunities for present and next-generation X-ray light sources.
    Nature Physics 01/2010; 6(1). DOI:10.1038/nphys1430 · 20.15 Impact Factor
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    ABSTRACT: We have measured the absorption of x-rays in neon gas in the presence of a strong laser pulse. The femtosecond x-rays were tuned to energies near the neon 1s-3p resonance, and the laser intensity of 1013 W/cm2 was below the intensity required to alone ionize neon. We observed strong modification of the x-ray absorption when the neon was subjected to laser light that was temporally overlapped with the x-rays.
    Journal of Physics Conference Series 11/2009; 194(3):2011-. DOI:10.1088/1742-6596/194/3/032011
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    ABSTRACT: We report a gas phase technique to establish the temporal overlap of ultrafast infrared laser and hard x-ray pulses. We use tunnel ionization of a closed shell atom in the strong field at the focus of an infrared laser beam to open a distinct x-ray absorption resonance channel with a clear fluorescence signature. The technique has an intrinsic response of a few femtoseconds and is nondestructive to the two beams. It provides a step-functionlike cross-correlation result. The details of the transient provide a diagnostic of the temporal overlap of the two pulses.
    Applied Physics Letters 04/2009; 94(17):171113-171113-3. DOI:10.1063/1.3125256 · 3.30 Impact Factor
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    ABSTRACT: We characterize the time evolution of ion spatial distributions in a laser-produced plasma. Krypton ions are produced in strong, linearly and circularly polarized optical laser fields (10<sup>14</sup>–10<sup>15</sup> W / cm <sup>2</sup>) . The Kr <sup>+</sup> ions are preferentially detected by resonant x-ray absorption. Using microfocused, tunable x rays from Argonne’s Advanced Photon Source, we measure ion densities as a function of time with 10 μ m spatial resolution for times ≤50 ns . For plasma densities of the order of 10<sup>14</sup> cm <sup>-3</sup> , we observe a systematic expansion of the ions outward from the laser focus. We find the expansion timescale to be independent of the plasma density though strongly dependent on the plasma shape and electron temperature. The former is defined by the laser focus, while the latter is controlled by the laser polarization state. We have developed a fluid description assuming a collisionless quasineutral plasma, which is modeled using a particle-in-cell approach. This simulation provides a quantitative description of the observed behavior and demonstrates the role of the very different electron temperatures produced by circularly and linearly polarized light. These results demonstrate the utility of this method as an in situ probe of the time and spatial evolution of laser-produced plasmas.
    Journal of Applied Physics 11/2008; 104(7-104):073307 - 073307-7. DOI:10.1063/1.2991339 · 2.18 Impact Factor
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    ABSTRACT: We demonstrate a hard x-ray probe of laser-aligned small molecules. To align small molecules with optical lasers, high intensities at nonresonant wavelengths are necessary. We use 95 ps pulses focused to 40 μm from an 800 nm Ti:sapphire laser at a peak intensity of 1012 W/cm2 to create an ensemble of aligned bromotrifluoromethane (CF3Br) molecules. Linearly polarized, 120 ps x-ray pulses, focused to 10 μm, tuned to the Br 1s→σ* preedge resonance at 13.476 keV, probe the ensemble of laser-aligned molecules. The demonstrated methodology has a variety of applications and can enable ultrafast imaging of laser-controlled molecular motions with Ångstrom-level resolution.
    Applied Physics Letters 03/2008; 92(9):094106-094106-3. DOI:10.1063/1.2890846 · 3.30 Impact Factor
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    R Santra · C Buth · E R Peterson · R W Dunford · E P Kanter · B Krässig · S H Southworth · L Young
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    ABSTRACT: Strong optical laser fields modify the way x rays interact with matter. This allows us to use x rays to gain deeper insight into strong-field processes. Alternatively, optical lasers may be utilized to control the propagation of x rays through a medium. Gas-phase systems are particularly suitable for illustrating the basic principles underlying combined x-ray and laser interactions. Topics addressed include the impact of spin-orbit interaction on the alignment of atomic ions produced in a strong laser field, electromagnetically induced transparency in the x-ray regime, and laser-induced alignment of molecules.
    Journal of Physics Conference Series 12/2007; 88(1):012052. DOI:10.1088/1742-6596/88/1/012052
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    ABSTRACT: Tunable, polarized, microfocused x-ray pulses were used to record x-ray absorption spectra across the K edges of Kr{sup +} and Kr{sup 2+} produced by laser ionization of Kr. Prominent 1s {yields} 4p and 5p excitations are observed below the 1s ionization thresholds in accord with calculated transition energies and probabilities. Due to alignment of 4p hole states in the laser-ionization process, the Kr{sup +} 1s {yields} 4p cross section varies with respect to the angle between the laser and x-ray polarization vectors. This effect is used to determine the Kr{sup +} 4p{sub 3/2} and 4p{sub 1/2} quantum state populations, and these are compared with results of an adiabatic strong-field ionization theory that includes spin-orbit coupling.
    Physical Review A 10/2007; 76(Oct. 2007). DOI:10.1103/PhysRevA.76.043421 · 2.81 Impact Factor

Publication Stats

2k Citations
384.67 Total Impact Points


  • 1996–2015
    • Argonne National Laboratory
      • • Division of X-ray Science
      • • Division of Physics
      Лимонт, Illinois, United States
  • 1990–2006
    • National Institute of Standards and Technology
      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
  • 1986–1989
    • Los Alamos National Laboratory
      • Chemistry Division
      Los Alamos, California, United States
  • 1987
    • University of Freiburg
      Freiburg, Baden-Württemberg, Germany
  • 1985
    • European Synchrotron Radiation Facility
      Grenoble, Rhône-Alpes, France
  • 1983–1984
    • CSU Mentor
      • Department of Chemistry
      Long Beach, California, United States
    • Lawrence Berkeley National Laboratory
      Berkeley, California, United States
  • 1979–1984
    • University of California, Berkeley
      • Department of Chemistry
      Berkeley, California, United States