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Berlin, Germany

The Helmholtz-Zentrum Berlin for Materials and Energy is a research centre and part of the Helmholtz Association of German Research Centres. The institute carries out research into the structure and dynamics of novel materials and also investigates solar cell technology.Several large scale facilities are available, the most important of which are the 10 MW BER-II research reactor at the Lise Meitner campus in Wannsee and the 3rd generation BESSY synchrotron in Adlershof. The institute also specialises in research at high magnetic fields and low temperatures and is a world leader in providing sample environment for neutron scattering and physical property measurements.Both the reactor and synchrotron operate as user facilities. Due to the high competition for experiments, beam time is awarded after peer review of two page proposals which state the scientific case for each measurement. User groups are expected to run experiments on a 24-hour basis to maximise the use of the facility. Onsite guesthouses exist at both campuses. Wikipedia.

Boin M.,Helmholtz Center Berlin
Journal of Applied Crystallography | Year: 2012

A collection of routines for calculating neutron scattering and absorption cross sections on the basis of crystal structure descriptions is presented and implemented in the new and reusable nxs program library. An example program providing a graphical user interface to the nxs functions is created to demonstrate their usage. The flexibility of the library and the possibilities for multiple areas of application are shown by further examples involving Monte Carlo neutron simulations concerned with imaging experiment validation and neutron instrument development. © 2012 International Union of Crystallography Printed in Singapore-all rights reserved.

Stolterfoht N.,Helmholtz Center Berlin
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2013

Simulations of ion guiding through an insulating cylindrical nanocapillary are performed in continuation of a recent theoretical work. The ions are assumed to move on classical trajectories affected by the electric field primarily produced by the charge patch deposited near the capillary entrance. The deposited charges are transported along the capillary wall using a nonlinear conductivity law. Calculations for different capillary tilt angles from 0 to 8 are performed and compared with previous experimental results. The main focus of the analysis is to reveal unknown guiding mechanisms by a detailed investigation of the calculated results. Surprisingly, after reaching a maximum, the field component perpendicular to the capillary axis is found to decrease with increasing charge inserted into the capillary. At equilibrium, this field is nearly constant in all directions of the capillary along the entrance charge patch. The extension of this charge patch increases with increasing tilt angle although a simple picture of undeflected ions predicts the opposite behavior. These unexpected results simplify the theoretical treatment so that analytical expressions could be derived describing essential properties of the ion guiding. In particular, unknown parameters previously introduced in semiempirical models are interpreted. © 2013 American Physical Society.

Wernet P.,Helmholtz Center Berlin
Physical Chemistry Chemical Physics | Year: 2011

The interest in following the evolution of the valence electronic structure of atoms and molecules during chemical reactions on a femtosecond time scale is discussed. By explicitly mapping the occupied part of the electronic structure with femtosecond pump-probe schemes one essentially follows the electrons making the bonds while the bonds change. This holds the key to unprecedented insight into chemical bonding in short-lived intermediates and reveals the coupled motion of electrons and nuclei. Examples from the recent literature on small molecules and anionic clusters in the gas phase and on atoms and molecules on surfaces using lab-based femtosecond laser methods are used to demonstrate the case. They highlight how the evolution of the valence electronic structure can be probed with time-resolved photoelectron spectroscopy with ultraviolet (UV) probe photon energies of up to 6 eV. It is shown how new insight can be gained by extending the probing wavelength into the vacuum-ultraviolet (VUV) region to photon energies of 20 eV and more by accessing the whole occupied valence electronic structure with time-resolved VUV photoelectron spectroscopy. Finally, the importance of soft X-ray free-electron lasers with probe photon energies of several hundred eV and femtosecond pulses and in particular the key role of femtosecond time-resolved soft X-ray emission spectroscopy or resonant inelastic X-ray scattering for mapping the electronic structure during chemical reactions is discussed. © 2011 the Owner Societies.

Schulze T.F.,Helmholtz Center Berlin | Schmidt T.W.,University of New South Wales
Energy and Environmental Science | Year: 2015

All photovoltaic solar cells transmit photons with energies below the absorption threshold (bandgap) of the absorber material, which are therefore usually lost for the purpose of solar energy conversion. Upconversion (UC) devices can harvest this unused sub-threshold light behind the solar cell, and create one higher energy photon out of (at least) two transmitted photons. This higher energy photon is radiated back towards the solar cell, thus expanding the utilization of the solar spectrum. Key requirements for UC units are a broad absorption and high UC quantum yield under low-intensity incoherent illumination, as relevant to solar energy conversion devices, as well as long term photostability. Upconversion by triplet-triplet annihilation (TTA) in organic chromophores has proven to fulfil the first two basic requirements, and first proof-of-concept applications in photovoltaic conversion as well as photo(electro)chemical energy storage have been demonstrated. Here we review the basic concept of TTA-UC and its application in the field of solar energy harvesting, and assess the challenges and prospects for its large-scale application, including the long term photostability of TTA upconversion materials. © 2015 The Royal Society of Chemistry.

Seiffert S.,Helmholtz Center Berlin | Seiffert S.,Free University of Berlin | Sprakel J.,Wageningen University
Chemical Society Reviews | Year: 2012

Supramolecular polymer networks are three-dimensional structures of crosslinked macromolecules connected by transient, non-covalent bonds; they are a fascinating class of soft materials, exhibiting properties such as stimuli-responsiveness, self-healing, and shape-memory. This critical review summarizes the current state of the art in the physical-chemical characterization of supramolecular networks and relates this knowledge to that about classical, covalently jointed and crosslinked networks. We present a separate focus on the formation, the structure, the dynamics, and the mechanics of both permanent chemical and transient supramolecular networks. Particular emphasis is placed on features such as the formation and the effect of network inhomogeneities, the manifestation of the crosslink relaxation dynamics in the macroscopic sample behavior, and the applicability of concepts developed for classical polymer melts, solutions, and networks such as the reptation model and the principle of time-temperature superposition (263 references). © 2012 The Royal Society of Chemistry.

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