H. Schwoerer

Stellenbosch University, Stellenbosch, Western Cape, South Africa

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Publications (75)196.83 Total impact

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    ABSTRACT: The ultrafast dynamics of the photochromic reaction of dithizonatophenyl-mercury(II) was recently reported. For purpose of investigating the effect of electronically different substituents (o-F, m-F, p-F, p-Cl, o-CH3, m-CH3, p-CH3, m,p-diCH3, p-OCH3, o-SCH3 and p-SCH3) on this reaction, a series of phenyl substituted dithizones were synthesized and complexed with phenylmercury(II). A variation of more than 3 ps in ground state repopulation times was observed, with the ortho-methyl derivative absorbing both at shortest wavelength and having the fastest repopulation time, while the para-S-methyl derivative lies at the opposite extremity. An increase in both decay times and max values is generally reflected by an increase in electron density in the chromophore. Ultrafast rates also proved to be dependent on solvent polarity, while a profound solvatochromic effect was observed in the transition state absorbance. Density functional theory realistically simulated isomer stabilities, electronic spectra and molecular orbitals. Increased electron density enhances stability in the photo-excited blue isomer relative to the orange resting state, as seen from a comparison between orange and blue isomer total bonding energies. A linear trend between computed HOMO energies and experimental max of related aliphatic substituted derivatives was found.
    The Journal of Physical Chemistry A 01/2014; · 2.77 Impact Factor
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    ABSTRACT: We present an investigation of the microscopic interplay between excitons and charge carriers by means of combined photoluminescence (PL) and charge carrier transport measurements on organic thin film transistors (OTFTs). For this purpose, the prototypical organic semiconductor diindenoperylene was utilized as the active material. The OTFT accumulation layer provides a spatially defined interaction zone for charges and photogenerated excitons leading to a PL intensity reduction of up to 4.5%. This effect correlates with the accumulated hole carrier density and provides a lower estimate of about 1.1×10−10 cm3/s for the recombination rate of nonradiative exciton-hole processes. It is rationalized that these processes are preferentially mediated by trapped holes.
    Physical review. B, Condensed matter 06/2013; 87(24). · 3.66 Impact Factor
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    ABSTRACT: A table-top femtosecond, non-relativistic, electron diffraction setup is combined with a low-jitter, photo-triggered streak camera to follow the optically induced structural dynamics in complex solids. A temporal resolution of 550 fs is experimentally demonstrated, while the route to streaking with sub-250 fs temporal resolution is outlined. The streaking technique allows for parallel capturing of temporal information as opposed to the serial data acquisition in a conventional scanning femtosecond electron diffraction. Moreover, its temporal resolution is not corrupted by increasing the number of electrons per pulse. Thus, compared to the conventional scanning approach, a substantial increase in signal-to-noise ratio (SNR) can be achieved. These benefits are demonstrated by studying a photo-induced charge density wave phase transition in 4Hb-TaSe2 using both methods. Within the same data acquisition time a three-fold increase in SNR is achieved when compared to the scanning method, with ways for a further improvement outlined.
    Applied Physics Letters 03/2013; 102(12). · 3.52 Impact Factor
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    ABSTRACT: We report on ultrafast photoindued charge density wave (CDW) dynamics in the transition-metal dichalcogenide 4Hb-TaSe2, studied with ultrafast electron diffraction. Fluence dependence of the lattice superstructure suppression show a phase transition from the commensurate to the incommensurate phase of 4Hb-TaSe2. Unusually long recovery times of perturbed charge density waves indicate th importance of a coupling between the two dimensional CDWs.
    03/2013;
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    ABSTRACT: The photophysics of charge transfer between the electron donating, surface adsorbed D149 dye and an electron accepting porous ZnO film was investigated by measuring excited state lifetimes using ultrafast transient absorption spectroscopy. We systematically varioed the production scheme of the sample including the electrolyte.
    03/2013;
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    ABSTRACT: We investigate the ultrafast dynamics of the photoinduced electron transfer between surface-adsorbed indoline D149 dye and porous ZnO as used in the working electrodes of dye-sensitized solar cells. Transient absorption spectroscopy was conducted on the dye in solution, on solid state samples and for the latter in contact to a I(-) /I(3) (-) redox electrolyte typical for dye-sensitized solar cells to elucidate the effect of each component in the observed dynamics. D149 in a solution of 1:1 acetonitrile and tert-butyl alcohol shows excited-state lifetimes of 300±50 ps. This signature is severely quenched when D149 is adsorbed to ZnO, with the fastest component of the decay trace measured at 150±20 fs due to the charge-transfer mechanism. Absorption bands of the oxidized dye molecule were investigated to determine regeneration times which are in excess of 1 ns. The addition of the redox electrolyte to the system results in faster regeneration times, of the order of 1 ns.
    ChemPhysChem 10/2012; · 3.35 Impact Factor
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    ABSTRACT: The dynamics of the photoinduced commensurate-to-incommensurate charge density wave (CDW) phase transition in 4H_{b}-TaSe_{2} are investigated by femtosecond electron diffraction. In the perturbative regime, the CDW re-forms on a 150-ps time scale, which is two orders of magnitude slower than in other transition-metal dichalcogenides. We attribute this to a weak coupling between the CDW carrying T layers and thus demonstrate the importance of three-dimensionality for the existence of CDWs. With increasing optical excitation, the phase transition is achieved, showing a second-order character, in contrast to the first-order behavior in thermal equilibrium.
    Physical Review Letters 10/2012; 109(16):167402. · 7.73 Impact Factor
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    ABSTRACT: The photophysics of charge transfer between electron donating, surface adsorbed D149 dye and electron accepting ZnO as well as the regeneration of the dye by various redox mediators was investigated using ultrafast transient absorption spectroscopy.
    Laser Science; 10/2012
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    ABSTRACT: The dynamics of the photoinduced commensurate to incommensurate charge density wave (CDW) phase transition in 4Hb-TaSe2 are investigated by femtosecond electron diffraction. In the perturbative regime the CDW reforms on a 150 ps timescale, which is two orders of magnitude slower than in other transition-metal dichalcogenides. We attribute this to a weak coupling between the CDW carrying T-layers and thus demonstrate the importance of three-dimensionality for the existence of CDWs. With increasing optical excitation the phase transition is achieved showing a second order character in contrast to the first order behavior in thermal equilibrium.
    Physical Review Letters 09/2012; 109(16). · 7.73 Impact Factor
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    ABSTRACT: Temporally resolved observation of micro-scopic structural dynamics of solids with ultrafast electron diffraction (UED) requires extremely short pulsed, highly charged, monoenergetic electron beams with sufficient transverse coherence length of several unit cells of the investigated samples. However, Coulomb repulsion defeats these parameters in free propagation of an electron pulse initially bright on the photo cathode. We demonstrate a new electron pulse compressor design based on a simple and compact RF structure incorporating a pair of gallium arsenide photoconductive semiconductor switches that are triggered by femtosecond laser pulses, thereby providing a longitudinal voltage gradient of up to 20 V/ps. Our proof of principle experiment achieved compression of bunches containing 26,000 electrons to a duration of below 750 fs and a beam diameter of 300 lm in the temporal and spatial focus of the device while maintaining the good beam col-limation required for time resolved electron diffraction experiments. The simplicity of the compressor provides a strong incentive for its further development toward prac-tical implementation in sub-relativistic UED experiments requiring the highest possible source brightness.
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    ABSTRACT: We present the first well-resolved experimental observation of the non-linear formation of a laser-driven plasma wave, its breaking leading to self-injection and acceleration of electrons in the wave's electric field in the regime of “Bubble-acceleration”.
    Frontiers in Optics; 10/2011
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    ABSTRACT: The initial photochromic reaction of dithizonatophenylmercury(II) in solution was investigated by femtosecond transient absorption spectroscopy. Ultrafast excitation within less than 100 fs caused a radiationless photoreaction with a time constant of 1.5 ps, which is interpreted as C=N isomerization through a conical intersection. The orthogonally twisted intermediate state was observed through its excited-state absorption. Bifurcation along pathways towards the ground states of the orange cis and blue trans configurations occurs below the funnel of the conical intersection. The photochromism of the title compound in a very polar solvent such as methanol is observed for the first time.
    ChemPhysChem 08/2011; 12(14):2653-8. · 3.35 Impact Factor
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    ABSTRACT: The Advanced Laser-Plasma High-Energy Accelerators towards X-rays (ALPHA-X) programme is developing laserplasma accelerators for the production of ultra-short electron bunches with subsequent generation of high brilliance, short-wavelength radiation pulses. Ti:sapphire laser systems with peak power in the range 20-200 TW are coupled into mm- and cm-scale plasma channels in order to generate electron beams of energy 50-800 MeV. Ultra-short radiation pulses generated in these compact sources will be of tremendous benefit for time-resolved studies in a wide range of applications across many branches of science. Primary mechanisms of radiation production are (i) betatron radiation due to transverse oscillations of the highly relativistic electrons in the plasma wakefield, (ii) gamma ray bremsstrahlung radiation produced from the electron beams impacting on metal targets and (iii) undulator radiation arising from transport of the electron beam through a planar undulator. In the latter, free-electron laser action will be observed if the electron beam quality is sufficiently high leading to stimulated emission and a significant increase in the photon yield. All these varied source types are characterised by their high brilliance arising from the inherently short duration (~1-10 fs) of the driving electron bunch.
    Proc SPIE 05/2011;
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    ABSTRACT: We present the first detailed demonstrations of octave-spanning SC generation in all-normal dispersion photonic crystal fibers (ANDi PCF) in the visible and near-infrared spectral regions. The resulting spectral profiles are extremely flat without significant fine structure and with excellent stability and coherence properties. The key benefit of SC generation in ANDi PCF is the conservation of a single ultrashort pulse in the time domain with smooth and recompressible phase distribution. For the first time we confirm the exceptional temporal properties of the generated SC pulses experimentally and demonstrate their applicability in ultrafast transient absorption spectroscopy. The experimental results are in excellent agreement with numerical simulations, which are used to illustrate the SC generation dynamics by self-phase modulation and optical wave breaking. To our knowledge, we present the broadest spectra generated in the normal dispersion regime of an optical fiber.
    Optics Express 02/2011; 19(4):3775-87. · 3.55 Impact Factor
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    ABSTRACT: We have developed a compact streak camera suitable for measuring the duration of highly charged subrelativistic femtosecond electron bunches with an energy bandwidth in the order of 0.1%, as frequently used in ultrafast electron diffraction (UED) experiments for the investigation of ultrafast structural dynamics. The device operates in accumulation mode with 50 fs shot-to-shot timing jitter, and at a 30 keV electron energy, the full width at half maximum temporal resolution is 150 fs. Measured durations of pulses from our UED gun agree well with the predictions from the detailed charged particle trajectory simulations.
    The Review of scientific instruments 10/2010; 81(10):105103. · 1.52 Impact Factor
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    ABSTRACT: We present experimental results from an all-optical diagnostic method to directly measure the evolution of the hot-electron distribution driving the acceleration of ions from thin foils using high-intensity lasers. Central parameters of laser ion acceleration such as the hot-electron density, the temperature distribution and the conversion efficiency from laser pulse energy into hot electrons become comprehensively accessible with this technique.
    New Journal of Physics 10/2010; 12(10):103027. · 4.06 Impact Factor
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    ABSTRACT: Experimental measurements of magnetic fields generated in the cavity of a self-injecting laser-wakefield accelerator are presented. Faraday rotation is used to determine the existence of multimegagauss fields, constrained to a transverse dimension comparable to the plasma wavelength ∼λp and several λp longitudinally. The fields are generated rapidly and move with the driving laser. In our experiment, the appearance of the magnetic fields is correlated with the production of relativistic electrons, indicating that they are inherently tied to the growth and wave breaking of the nonlinear plasma wave. This evolution is confirmed by numerical simulations, showing that these measurements provide insight into the wakefield evolution with high spatial and temporal resolution.
    Physical Review Letters 09/2010; 105(11):115002. · 7.73 Impact Factor
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    ABSTRACT: We investigate the structural response of SrTiO3 when Cooper pairs are broken in an epitaxially grown YBa2Cu3O7 top layer due to both heating and optical excitation. The crystal structure is investigated by static, temperature-dependent and time-resolved x-ray diffraction. In the static case, a large strain field in SrTiO3 is formed in the proximity of the onset of the superconducting phase in the top layer, suggesting a relationship between both effects. For the time-dependent studies, we likewise find a large fraction of the probed volume of the SrTiO3 substrate strained if the top layer is superconducting. Upon optical breaking of Cooper pairs, the observed width of the rocking curve is reduced and its position is slightly shifted towards smaller angles. The dynamical theory of x-ray diffraction is used to model the measured rocking curves. We find that the thickness of the strained layer is reduced by about 200 nm on a sub-ps to ps timescale, but the strain value at the interface between SrTiO3 and YBa2Cu3O7 remains unaffected.
    New Journal of Physics 08/2010; 12(8):083043. · 4.06 Impact Factor
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    ABSTRACT: In this paper a new method of determining the energy spread of a relativistic electron beam from a laser-driven plasma wakefield accelerator by measuring radiation from an undulator is presented. This could be used to determine the beam characteristics of multi-GeV accelerators where conventional spectrometers are very large and cumbersome. Simultaneous measurement of the energy spectra of electrons from the wakefield accelerator in the 55-70 MeV range and the radiation spectra in the wavelength range of 700-900 nm of synchrotron radiation emitted from a 50 period undulator confirm a narrow energy spread for electrons accelerated over the dephasing distance where beam loading leads to energy compression. Measured energy spreads of less than 1% indicates the potential of using a wakefield accelerator as a driver of future compact and brilliant ultrashort pulse synchrotron sources and free-electron lasers that require high peak brightness beams. (C) 2009 American Institute of Physics. [doi: 10.1063/1.321
    Physics of Plasmas 10/2009; 16. · 2.38 Impact Factor
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    ABSTRACT: We have designed a femtosecond electron gun suitable for ultrafast electron diffraction experiments, operating in the 30–100 kV regime. The concept is based on recompression of chirped expanding electron pulses emitted from a direct current photogun using a novel dispersion-corrected reflectron concept. We show, using detailed numerical simulations, that our design is capable of producing electron pulses containing 200 000 electrons with a full width at half maximum pulse duration of 130 fs, a root mean squared (rms) pulse radius of 140 μ m , and transverse coherence length of 1.5 nm at 100 kV. Our analysis includes the bunch properties at the sample, as well as interactions of the main pulse of high charge density with diffracted electrons. Since our design employs only static electron optics, we believe that it will be easier to implement than concepts based on radio frequency compression.
    Journal of Applied Physics 07/2009; · 2.21 Impact Factor

Publication Stats

1k Citations
196.83 Total Impact Points

Institutions

  • 2008–2014
    • Stellenbosch University
      • Department of Physics
      Stellenbosch, Western Cape, South Africa
  • 2010
    • Massachusetts Institute of Technology
      Cambridge, Massachusetts, United States
  • 2000–2009
    • Friedrich-Schiller-University Jena
      • Department of Optics and Quantum Electronics
      Jena, Thuringia, Germany
  • 2006
    • Heinrich-Heine-Universität Düsseldorf
      • Institut für Laser- und Plasmaphysik
      Düsseldorf, North Rhine-Westphalia, Germany
  • 2003
    • Max Planck Institute of Quantum Optics
      Arching, Bavaria, Germany
    • University of Leipzig
      Leipzig, Saxony, Germany