T. Elsaesser

Technische Universität Berlin, Berlín, Berlin, Germany

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Publications (489)1283.13 Total impact

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    ABSTRACT: Near-field scanning optical microscopy (NSOM) is applied for analyzing GaN-based diode lasers. The measurements are carried out at the front facets of standard devices without any additional preparation. Four different schemes for luminescence and photocurrent detection are applied. The results allow for a detailed analysis of the epitaxial layer sequence, the waveguide mode, the impact of defect absorption, and efficiencies of carrier transfer into the quantum well. Moreover, the effective potential profile as formed by both layer structure and doping profile is imaged. Features being spatially separated by only 30 nm are safely resolved. Our results pave the way towards non-destructive nanoscopic analysis of wide-bandgap optoelectronic devices.
    Semiconductor Science and Technology 09/2014; 29(11):112001. · 2.21 Impact Factor
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    ABSTRACT: X-Ray diffraction provides insight into the distribution of electronic charge in crystals. Equilibrium electron distributions have been determined with high spatial resolution by recording and analysing a large number of diffraction peaks under stationary conditions. In contrast, transient electron densities during and after structure-changing processes are mainly unknown. Recently, we have introduced femtosecond X-ray powder diffraction from polycrystalline samples to determine transient electron density maps with a spatial resolution of 0.03 nm and a temporal resolution of 100 fs. In a pump–probe approach with a laser-driven tabletop hard X-ray source, optically induced structure changes are resolved in time by diffracting the hard X-ray probe pulses at different time delays from the excited powder sample and recording up to several tens of reflections simultaneously. Time-dependent changes of the atomic arrangement in the crystal lattice as well as modified electron densities are derived from the diffraction data. As a prototypical field-driven process, we address here quasi-instantaneous changes of electron density in LiBH4, LiH and NaBH4 in response to a non-resonant strong optical field. The light-induced charge relocation in LiBH4 and NaBH4 exhibits an electron transfer from the anion (BH−4) to the respective cation. The distorted geometry of the BH4 tetrahedron in LiBH4 leads to different contributions of the H atoms to electron transfer. LiH displays a charge transfer from Li to H, i.e., an increase of the ionicity of LiH in the presence of the strong electric field. This unexpected behavior originates from strong electron correlations in LiH as is evident from a comparison with quasi-particle bandstructures calculated within the Coulomb-hole-plus-screened-exchange (COHSEX) formalism.
    Faraday Discussions 07/2014; · 4.19 Impact Factor
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    ABSTRACT: Ultrafast vibrational dynamics of [Formula: see text] ions, the key units in boron hydride materials for hydrogen storage, are studied in diluted polar liquid solution and in NaBH4 crystallites by femtosecond infrared spectroscopy. Two-color pump-probe experiments reveal v = 1 lifetimes of 3 ps for the asymmetric [Formula: see text] stretching mode ν3 and of 3.6 ps for the asymmetric bending mode ν4 in the solvent isopropylamine. We provide direct evidence for the [Formula: see text] stretching relaxation pathway via the asymmetric bending mode ν4 by probing the latter after femtosecond excitation of ν3. Pump-probe traces measured in the crystalline phase show signatures of radiative coupling between the densely packed [Formula: see text] oscillators, most clearly manifested in an accelerated subpicosecond depopulation of the v = 1 state of the ν4 mode. The radiative decay is followed by incoherent vibrational relaxation similar to the liquid phase. The excess energy released in the relaxation processes of the [Formula: see text] intramolecular modes is transferred into the environment with thermal pump-probe signals being much more pronounced in the dense solid than in the diluted solution.
    The Journal of chemical physics. 07/2014; 141(3):034506.
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    ABSTRACT: We report the first observation of terahertz higher harmonics in graphene by mapping the nonlinear response with broadband electrooptic sampling. The nonlinear response in the non-perturbative regime is determined by intra- and interband electron motions.
    CLEO: QELS_Fundamental Science; 06/2014
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    ABSTRACT: We study the nonlinear terahertz response of LiNbO3 using 2D spectroscopy. Dissecting the nonlinear response into different orders in the electric field shows a strong shift current and higher harmonics of the THz fundamental.
    CLEO: QELS_Fundamental Science; 06/2014
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    ABSTRACT: Infrared emission from 980-nm single-mode high power diode lasers is recorded and analyzed in the wavelength range from 0.8 to 8.0 μm. A pronounced short-wavelength infrared (SWIR) emission band with a maximum at 1.3 μm originates from defect states located in the waveguide of the devices. The SWIR intensity is a measure of the non-equilibrium carrier concentration in the waveguide, allowing for a non-destructive waveguide mapping in spatially resolved detection schemes. The potential of this approach is demonstrated by measuring spatially resolved profiles of SWIR emission and correlating them with mid-wavelength infrared (MWIR) thermal emission along the cavity of devices undergoing repeated catastrophic optical damage. The enhancement of SWIR emission in the damaged parts of the cavity is due to a locally enhanced carrier density in the waveguide and allows for an analysis of the spatial damage patterns. The figure shows a side view of a diode laser during catastrophic degradation as recorded by a thermocamera within 5 successive current pulses. The geometry of the device is given in grayscale. The position of the laser chip is indicated by the dotted line. The thermal signatures of the internal degradation of the diode laser are overlaid in color. The bi-directional spread of the damage along the laser cavity is clearly visible.
    Laser & Photonics Review 05/2014; · 7.98 Impact Factor
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    ABSTRACT: The terahertz (THz) response of the ferroelectric prototype material lithium niobate (LiNbO3) is studied in the nonperturbative regime of light-matter interaction. Applying two-dimensional THz spectroscopy with few-cycle pulses of an amplitude E≈100 kV/cm and a center frequency of 2 THz, we dissect the overall nonlinear response into different orders in the electric field. The underlying nonlinear current is of interband character and consists of a strong low-frequency shift current (SC) and higher harmonics of the THz fundamental. The SC component originates from the lack of inversion symmetry and the strong interband decoherence for long electron trajectories in k space as shown by theoretical calculations.
    Physical Review Letters 04/2014; 112(14):146602. · 7.73 Impact Factor
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    ABSTRACT: Internal degradation of 980 nm emitting single-spatial-mode diode lasers during ultrahigh power operation is investigated for continuous wave and pulsed operation (2 µJ, 20 W). In situ analysis of the evolution of the optical nearfields with pico-second time resolution enables the observation of the transition from single- to multi-spatial-mode operation at elevated emission powers. Moreover, internal degradation events and defect propagation are monitored by thermal imaging. The results complete earlier findings obtained at broad-area lasers and allow for the establishment of generalized models covering both classes of edge-emitting devices.
    Photonics West, San Francisco; 02/2014
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    Rene Costard, Ismael A. Heisler, Thomas Elsaesser
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    ABSTRACT: The properties of biomembranes depend in a decisive way on interactions of phospholipids with hydrating water molecules. To map structural dynamics of a phospholipid–water interface on the length and time scale of molecular motions, we introduce the phospholipid symmetric and asymmetric phosphate stretch vibrations as probes of interfacial hydrogen bonds and electrostatic interactions. The first two-dimensional infrared spectra of such modes and a line shape analysis by density matrix theory reveal two distinct structural dynamics components; the first 300 fs contribution is related to spatial fluctuations of charged phospholipid head groups with additional water contributions at high hydration levels; the second accounts for water–phosphate hydrogen bonds persisting longer than 10 ps. Our results reveal a relatively rigid hydration shell around phosphate groups, a behavior relevant for numerous biomolecular systems.
    Journal of Physical Chemistry Letters 01/2014; 5(3):506–511. · 6.69 Impact Factor
  • T Elsaesser, M Woerner
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    ABSTRACT: Ultrashort soft and hard x-ray pulses are sensitive probes of structural dynamics on the picometer length and femtosecond time scales of electronic and atomic motions. Recent progress in generating such pulses has initiated new directions of condensed matter research, exploiting a variety of x-ray absorption, scattering, and diffraction methods to probe photoinduced structural dynamics. Atomic motion, changes of local structure and long-range order, as well as correlated electron motion and charge transfer have been resolved in space and time, providing a most direct access to the physical mechanisms and interactions driving reversible and irreversible changes of structure. This perspective combines an overview of recent advances in femtosecond x-ray diffraction with a discussion on ongoing and future developments.
    The Journal of Chemical Physics 01/2014; 140(2):020901. · 3.12 Impact Factor
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    ABSTRACT: The nonlinear interaction between intense terahertz (THz) pulses and epitaxial multilayer graphene is studied by field-resolved THz pump–probe spectroscopy. THz excitation results in a transient induced absorption with decay times of a few picoseconds, much faster than carrier recombination in single graphene layers. The decay times increase with decreasing temperature and increasing amplitude of the excitation. This behaviour originates from the predominant coupling of electrons to the electromagnetic field via the very strong interband dipole moment while scattering processes with phonons and impurities play a minor role. The nonlinear response at field amplitudes above 1 kV cm−1 is in the carrier-wave Rabi flopping regime with a pronounced coupling of the graphene layers via the radiation field. Theoretical calculations account for the experimental results.
    New Journal of Physics 01/2014; 16(1). · 3.67 Impact Factor
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    ABSTRACT: The nonlinear dynamics of electrons in multilayer epitaxial graphene is investigated by time-resolved terahertz (THz) spectroscopy in a regime where the interaction of electrons with the external field dominates over scattering processes. The predominantly coherent electron response to the THz field involves both intra- and interband currents, leading to coherently driven interband transitions of carriers and to the generation of higher harmonics of the THz carrier frequency. The overall behavior of the graphene layers is always absorptive, even after generation of an initial electron-hole distribution by femtosecond midinfrared excitation. The results are in agreement with theoretical calculations of the nonperturbative THz response.
    Physical Review B 12/2013; 89(4). · 3.66 Impact Factor
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    ABSTRACT: Excitonic and spin excitations of single semiconductor quantum dots currently attract attention as possible candidates for solid state based implementations of quantum logic devices. Due to their rather short decoherence times in the picosecond to nanosecond range, such implementations rely on using ultrafast optical pulses to probe and control coherent polarizations. We combine ultrafast spectroscopy and near-field microscopy to probe the nonlinear optical response of a single quantum dot on a femtosecond time scale. Transient reflectivity spectra show pronounced oscillations around the quantum dot exciton line. These oscillations reflect phase-disturbing Coulomb interactions between the exitonic quantum dot polarization and continuum excitations. The results show that although semiconductor quantum dots resemble in many respects atomic systems, Coulomb many-body interactions can contribute significantly to their optical nonlinearities on ultrashort time scales.
    Proc SPIE 12/2013;
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    ABSTRACT: We study the quasi-instantaneous change of electron density in the unit cells of LiH and NaBH 4 in response to a nonresonant strong optical field. We determine for the first time the related transient electron density maps, applying femtosecond x-ray powder diffraction as a structure probe. The light-induced charge relocation in NaBH 4 exhibits an electron transfer from the anion (BH À 4) to the Na þ cation. In contrast, LiH displays the opposite behavior, i.e., an increase of the ionicity of LiH in the presence of the strong electric field. This behavior originates from strong electron correlations in LiH, as is evident from a comparison with quasiparticle band structures calculated within the Coulomb-hole-plus-screened-exchange formalism. Physical processes driven by external electric fields play a key role for the electronic and optical properties of condensed matter. Nonperturbative interactions with fields of an amplitude comparable to the inner-atomic fields in solids allow for studying new regimes of charge transport such as coherent ballistic electron motions [1] and/or for inducing a highly nonlinear optical response [2]. The latter results in phenomena such as field-driven electron emis-sion [3], interband tunneling [4], high harmonic generation [5], and/or light-driven charge relocations [6]. Recently, intense optical pulses of a few femtosecond duration have been applied to drive charge transport in insulators [7,8]. The electron density ðr; tÞ in crystalline matter transi-ently deformed by strong electric fields contains both temporal (or frequency) and spatial (or k-space) aspects. So far, most experimental studies had their focus on the temporal (or frequency) aspects, i.e., on transient macro-scopic polarizations and currents [1–8]. The results have given only indirect insight into Coulomb mediated elec-tronic correlations which—in contrast—have been the sub-ject of extensive theoretical work [9–20]. Moreover, the real space dynamics of charges on atomic length scales, which are a probe of electronic correlations, have remained mostly unresolved. Here, ultrafast time-resolved x-ray and electron diffraction experiments can provide direct insight as the structure factors governing transient diffraction pat-terns are determined by the Fourier transform of the time-dependent spatial electron distribution [21–24]. Following this approach, we have mapped field-driven electron relo-cations in the ionic material LiBH 4 by femtosecond x-ray powder diffraction [25]. In Ref. [6], experiments on a single reflection have been combined with a theory for the transient charge density map ðr; tÞ. In this Letter, we apply femtosecond x-ray powder dif-fraction to reveal the field-driven dynamics of correlated electrons in the prototype ionic materials LiH and NaBH 4 . The simultaneous measurement of intensity changes of different diffraction rings provides the transient structure factors from which we derive spatially resolved electron density maps. LiH is the simplest heteronuclear solid which has been studied in nuclear physics and used in various hydrogen storage systems. According to theory, it should display strong adiabatic [9–20] and nonadiabatic electron correlations [26,27]. We demonstrate for the first time a quasi-instantaneous increase of the ionicity of LiH in the presence of the strong electric field, i.e., a transient electron transfer from the Li to the neighboring H atoms. A comparison with model calculations shows that a mean-field theory, i.e., the Hartree-Fock approximation, fails to account for this behavior. However, calculations including Coulomb correlations on the most basic level, i.e., quasipar-ticle band structures calculated within the Coulomb-hole-plus-screened-exchange (COHSEX) formalism [14,17], predict correctly the increase of the ionicity of LiH. In contrast, NaBH 4 displays a charge transfer from the nega-tive BH À 4 to the Na þ ions, reducing the ionicity of the material as expected for a predominant admixture of states in the lowest conduction band. The experiments make use of a femtosecond pump-probe scheme where a nonresonant femtosecond pulse provides the external electric field interacting with the material, and the resulting change in the distribution of electronic charge is monitored by diffracting hard x-ray pulses from the sample which consists of a powder of small crystallites [28]. Both pump and probe pulses are derived from an amplified Ti:sapphire laser system delivering sub-50-fs pulses centered at 800 nm with an energy per pulse of 5 mJ and a repetition rate of 1 kHz. The excitation pulses at 800 nm have a peak amplitude of the electric field of 1 GV=m. The main fraction of the laser output drives a
    Physical Review Letters 11/2013; 111(22):217401. · 7.73 Impact Factor
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    ABSTRACT: We study the quasi-instantaneous change of electron density in the unit cells of LiH and NaBH_{4} in response to a nonresonant strong optical field. We determine for the first time the related transient electron density maps, applying femtosecond x-ray powder diffraction as a structure probe. The light-induced charge relocation in NaBH_{4} exhibits an electron transfer from the anion (BH_{4}^{-}) to the Na^{+} cation. In contrast, LiH displays the opposite behavior, i.e., an increase of the ionicity of LiH in the presence of the strong electric field. This behavior originates from strong electron correlations in LiH, as is evident from a comparison with quasiparticle band structures calculated within the Coulomb-hole-plus-screened-exchange formalism.
    Physical Review Letters 11/2013; 111(21):217401. · 7.73 Impact Factor
  • Christian Greve, Thomas Elsaesser
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    ABSTRACT: NH and OH stretching excitations of hydrated double-stranded DNA oligomers containing guanine-cytosine (GC) base pairs in a Watson-Crick geometry are studied by two-dimensional (2D) infrared spectroscopy. The 2D spectra measured at a low hydration level (~4 water molecules/base pair) are dominated by NH stretch contributions from the NH2 groups of G and C and the NH group of G. Partially hydrated NH2 groups display red-shifted NH stretch frequencies and a mixing of the wavefunctions of the two local NH oscillators via the mechanical vibrational coupling. The NH stretch lifetimes are of the order of 200-300 fs. Weak couplings exist between NH stretch oscillators within a base pair while interactions between neighboring GC pairs in the double helix are negligible. The absence of spectral diffusion on a 1 ps time scale suggests a relatively rigid structure of the hydrogen bonds between DNA and residual water molecules. 2D spectra recorded with fully hydrated DNA oligomers exhibit NH and OH stretch contributions with a weak influence of water fluctuations on the NH stretch lineshapes. The femtosecond spectral diffusion of OH stretch excitations is slower than in bulk H2O and originates from structural fluctuations of the water shell and the formation of a vibrationally hot ground state by vibrational relaxation. We compare our findings with measurements on hydrated adenine-thymine DNA oligomers and anhydrous GC base pairs in solution.
    The Journal of Physical Chemistry B 10/2013; · 3.38 Impact Factor
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    ABSTRACT: Extremely early phases of the catastrophic optical damage (COD) process in 808-nm emitting GaAs/Al0.35Ga0.65As high-power diode lasers are prepared by the application of short single current pulses. Typical energy entries during these pulses are on the order of 100 nJ within several 100 ns. The resulting defect pattern is investigated by high-resolution microscopy. The root of the COD is found to be located at the waveguide of the laser structure. Analysis of material composition modifications as a result of early COD phase points to melting being involved in the process. During recrystallization, an Al-rich pattern is formed that encloses a volume of a few cube micron of severely damaged material.
    IEEE Journal of Selected Topics in Quantum Electronics 07/2013; 19(4):1500508-1500508. · 3.47 Impact Factor
  • Rene Costard, Thomas Elsaesser
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    ABSTRACT: The relaxation of OH bend excitations of H2O confined in reverse micelles is studied by femtosecond broadband pump-probe spectroscopy for the two ionic systems dioctyl sodium sulfosuccinate (AOT) and dioleoylphosphatidylcholine (DOPC) and for the nonionic tetraethylene glycol dodecyl ether (Brij-30). In the ionic AOT/DOPC reverse micelles, the OH bending lifetime T1 decreases from T1 > 615 fs for a 3:1 ratio of water and AOT/DOPC molecules (w0 = 3) to T1 = 345 fs for a 16:1 ratio (w0 = 16). In contrast, H2O in Brij-30 reverse micelles shows a much shorter T1 = 400 fs at w0 = 2 which decreases to T1 = 250 fs at w0 = 8. OH bend relaxation proceeds mainly via librational overtones of the bend-excited water molecules with a rate correlating with the energy mismatch between the v = 1 OH bend state and the librational overtone. In the ionic systems, the lower librational frequencies at small w0 result in a larger mismatch and longer T1 times. In the nonionic case, the w0-independent librational frequencies with a small energy mismatch lead to shorter T1 times. For w0 ≥ 8, the energy flow into the first hydration shell of the bend-excited molecules makes an additional contribution to the relaxation rate in all systems.
    The Journal of Physical Chemistry B 05/2013; · 3.38 Impact Factor
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    ABSTRACT: form only given. Many functional processes in condensed matter involve atomic motions and charge relocations on ultrashort time scales. Ultrafast spectroscopy has given insight into the dynamics of such events but provides - if at all-very limited structural information. In contrast, x-ray diffraction methods with a femtosecond time resolution allow for spatially resolving transient structures and charge distributions in a most direct way. This knowledge is highly relevant for understanding the structure-function relationship of crystalline materials. Femtosecond x-ray diffraction is based on a pump-probe approach in which a femtosecond pump pulse initiates the structural change and a delayed hard x-ray pulse is diffracted from the excited sample to generate a diffraction pattern reflecting the momentary crystal structure [1]. In particular, transient x-ray powder diffraction patterns consist of many reflections measured simultaneously and, thus, allow for deriving time-dependent electron density maps [2].In this talk, recent results on ultrafast electron and lattice motions in ionic crystals are presented. Such measurements reveal the interplay of lattice and charge motions in the photoexcited prototype material KDP (KH2PO4) [3,4]. Upon photoexcitation, the low-frequency TO soft mode is elongated impulsively and modulates the electronic charge distribution on the length scale of interatomic distances, much larger than the vibrational amplitude. The results demonstrate a net transfer of electronic charge from the P-atoms to the O-H...O units in the crystal lattice. The different length scales of vibrational elongations and charge relocations originate from the long-range Coulomb forces the ionic lattice exerts on the highly polarizable valence electrons of the (PO4) units, and are rationalized with the help of Cochran's model of ferroelectricity in ionic materials [5]. As a second example, the field-driven transfer of valence electrons between ions in a su- erposition of quantum states will be addressed for the materials LiBH4 [6] and LiH. Such crystals consist of light elements only with a small number of inner electrons. As a result, the valence electrons make a major contribution to the x-ray diffraction signal. In LiBH4, the strong external field provided by a sub-40 fs laser pulse at photon energies far below the bandgap drives an interionic transfer of electronic charge from the (BH4)to the Li+ ion, i.e., over a distance of some 250 pm. The charge density map shows that this transfer occurs exclusively during the pump pulse and is fully reversible. The strong electric field of the pump pulse generates a coherent superposition of valence and conduction band quantum states in which electronic charge is shifted compared to the initial valence band states. This fully reversible transfer mechanism makes a major contribution to the overall optical polarizability of the material.
    The European Conference on Lasers and Electro-Optics; 05/2013
  • International Quantum Electronics Conference; 05/2013

Publication Stats

7k Citations
1,283.13 Total Impact Points


  • 2002–2014
    • Technische Universität Berlin
      • • Department of solid state Physics
      • • Department of Theoretical Physics
      Berlín, Berlin, Germany
    • Fraunhofer Heinrich-Hertz-Institute HHI
      Berlín, Berlin, Germany
  • 1994–2014
    • Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy
      Berlín, Berlin, Germany
  • 2013
    • University of Rhode Island
      • Department of Chemistry
      Kingston, RI, United States
  • 2011
    • Humboldt-Universität zu Berlin
      • Department of Physics
      Berlin, Land Berlin, Germany
    • Bundesanstalt für Materialforschung und -prüfung
      Berlín, Berlin, Germany
  • 2010
    • Weierstrass Institute for Applied Analysis and Stochastics
      Berlín, Berlin, Germany
  • 2008
    • Universität Potsdam
      • Institute of Physics and Astronomy
      Potsdam, Brandenburg, Germany
  • 2007
    • Ludwig-Maximilians-University of Munich
      München, Bavaria, Germany
  • 1983–2007
    • Technische Universität München
      • • Walter Schottky Institut (WSI)
      • • Faculty of Physics
      München, Bavaria, Germany
  • 2005
    • The University of Tokyo
      • Institute of Industrial Science
      Tokyo, Tokyo-to, Japan
  • 2004
    • Paul Drude Institute for Solid State Electronics
      Berlín, Berlin, Germany
  • 1998
    • AT&T Labs
      Austin, Texas, United States
  • 1990
    • Universität Stuttgart
      Stuttgart, Baden-Württemberg, Germany
  • 1983–1989
    • Deutsches Herzzentrum München
      München, Bavaria, Germany
  • 1988
    • Georg-August-Universität Göttingen
      • Institute of Inorganic Chemistry
      Göttingen, Lower Saxony, Germany