S. H. Southworth

Argonne National Laboratory, Lemont, Illinois, United States

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Publications (179)404.24 Total impact

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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; · 3.82 Impact Factor
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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/extreme ultraviolet 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 in Ne+. The simulations show that these interferences are very sensitive to the parameters of the x-ray and optical fields.
    New Journal of Physics 08/2013; 15(8):083057. · 4.06 Impact Factor
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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.
    Journal of Physics B Atomic Molecular and Optical Physics 08/2013; 46(16):164003. · 2.03 Impact Factor
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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.
    Physical Review A 02/2013; 87(1). · 3.04 Impact Factor
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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). · 3.04 Impact Factor
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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.
    Journal of Electron Spectroscopy and Related Phenomena 09/2012; 188:166-171. · 1.71 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. · 2.77 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):033401. · 3.04 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 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.
    06/2012;
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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.
    06/2012;
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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.
    06/2012;
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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.
    Physical Review A 05/2012; 86(3). · 3.04 Impact Factor
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    ABSTRACT: We report on laser-pump/x-ray-probe investigations of photoexcitation and photodissociation dynamics of solvated molecules using high-repetition-rate techniques at the Advanced Photon Source.
    International Conference on Ultrafast Structural Dynamics; 03/2012
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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 01/2012; 25(2).
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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. · 7.73 Impact Factor
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    ABSTRACT: We describe our implementation of a high repetition rate (54 kHz-6.5 MHz), high power (>10 W), laser system at the 7ID beamline at the Advanced Photon Source for laser pump/x-ray probe studies of optically driven molecular processes. Laser pulses at 1.06 μm wavelength and variable duration (10 or 130 ps) are synchronized to the storage ring rf signal to a precision of ~250 fs rms. Frequency doubling and tripling of the laser radiation using nonlinear optical techniques have been applied to generate 532 and 355 nm light. We demonstrate that by combining a microfocused x-ray probe with focused optical laser radiation the requisite fluence (with <10 μJ/pulse) for efficient optical excitation can be readily achieved with a compact and commercial laser system at megahertz repetition rates. We present results showing the time-evolution of near-edge x-ray spectra of a well-studied, laser-excited metalloporphyrin, Ni(II)-tetramesitylporphyrin. The use of high repetition rate, short pulse lasers as pump sources will dramatically enhance the duty cycle and efficiency in data acquisition and hence capabilities for laser-pump/x-ray probe studies of ultrafast structural dynamics at synchrotron sources.
    The Review of scientific instruments 07/2011; 82(7):073110. · 1.52 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. · 38.60 Impact Factor
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    ABSTRACT: The paradigm “structure determines function” has long been a motivation for X-ray structural studies. However, the understanding of function can go well beyond static pictures with atomic resolution, as one explores the time domain with short-pulse X-rays. The U.S. Department of Energy's Advanced Photon Source (APS) at Argonne National Laboratory is home to a suite of science techniques and facilities that exploit the short-pulse nature of X-rays to probe structural dynamics in chemical, biological, condensed matter, and atomic/molecular systems.
    Synchrotron Radiation News 04/2010; 23(2):18-25.
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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.
    03/2010;

Publication Stats

2k Citations
404.24 Total Impact Points

Institutions

  • 1997–2014
    • Argonne National Laboratory
      • Division of Physics
      Lemont, Illinois, United States
  • 2006
    • University of Nevada, Las Vegas
      • Department of Chemistry
      Las Vegas, NV, United States
    • University of Pittsburgh
      • Physics and Astronomy
      Pittsburgh, PA, United States
  • 1991–2006
    • National Institute of Standards and Technology
      Maryland, United States
  • 2003
    • Tulane University
      New Orleans, Louisiana, United States
  • 1999
    • University of Freiburg
      Freiburg, Baden-Württemberg, Germany
  • 1989
    • Brookhaven National Laboratory
      • Chemistry Department
      New York City, NY, United States
  • 1986–1989
    • Los Alamos National Laboratory
      Los Alamos, California, United States
  • 1983–1984
    • CSU Mentor
      Long Beach, California, United States
    • Lawrence Berkeley National Laboratory
      Berkeley, California, United States
  • 1979–1982
    • University of California, Berkeley
      • Department of Chemistry
      Berkeley, CA, United States