Multi-conjugate solar adaptive optics at the Vacuum Tower Telescope on Tenerife

We present a breadboard multi-conjugate adaptive optics (MCAO) system for high angular resolution solar observations which we operate at the Vacuum Tower Telescope. We have developed methods to estimate quantitatively the performance of solar adaptive optics from science data. Several sets of short exposure images of the solar photosphere were analyzed to assess the performance of the MCAO. We demonstrate that a 30 arcsec field of view is substantially improved when the MCAO system is turned on. This compares favourably with an improvement of a 10 arcsec field with conventional solar adaptive optics. We also show how irradiance fluctuations in the MCAO compensated focus can be suppressed. To cite this article: O. von der Lühe et al., C. R. Physique 6 (2005).     

Interactions Between Flame Topology and Turbulent Transport in High-Pressure Premixed Combustion

Flow, Turbulence and Combustion - Direct numerical simulations of a turbulent premixed stoichiometric methane-oxygen flame were conducted. The chosen combustion pressure was 20 bar, to...     

Anomalous thermodynamics of Coulomb-interacting massless Dirac fermions in two spatial dimensions

It is argued that the specific heat of N massless Dirac fermions in two spatial dimensions interacting with 1/r Coulomb interactions is suppressed logarithmically relative to its noninteracting counterpart. The (dimensionless) coefficient of the logarithm is calculated in a closed form in the leading order in large N expansion, but to all orders in the effective fine structure constant, alpha(F), a procedure which takes into account finite temperature screening. This effect is expected to occur in a single-layer graphene embedded in a dielectric medium. Its dependence on the dielectric constant is calculated analytically.     

Quantum Mechanical Calculations of Redox Potentials of the Metal Clusters in Nitrogenase

We have calculated redox potentials of the two metal clusters in Mo-nitrogenase with quantum mechanical (QM) calculations. We employ an approach calibrated for iron–sulfur clusters with 1–4 Fe ions, involving QM-cluster calculations in continuum solvent and large QM systems (400–500 atoms), based on structures from combined QM and molecular mechanics (QM/MM) geometry optimisations. Calculations on the P-cluster show that we can reproduce the experimental redox potentials within 0.33 V. This is similar to the accuracy obtained for the smaller clusters, although two of the redox reactions involve also proton transfer. The calculated P¹⁺/P^N redox potential is nearly the same independently of whether P¹⁺ is protonated or deprotonated, explaining why redox titrations do not show any pH dependence. For the FeMo cluster, the calculations clearly show that the formal oxidation state of the cluster in the resting E₀ state is MoIIIFe3IIFe4III , in agreement with previous experimental studies and QM calculations. Moreover, the redox potentials of the first five E₀–E₄ states are nearly constant, as is expected if the electrons are delivered by the same site (the P-cluster). However, the redox potentials are insensitive to the formal oxidation states of the Fe ion (i.e., whether the added protons bind to sulfide or Fe ions). Finally, we show that the later (E₄–E₈) states of the reaction mechanism have redox potential that are more positive (i.e., more exothermic) than that of the E₀/E₁ couple.     

Über die Reduktion und die Darstellungen positiver quaternärer quadratischer Formen

Ohne Zusammenfassung     

Verbesserung der Lithiumbestimmung in Mineralw?ssern und ein neuer Weg zur Lithiumbestimmung in Silicatgesteinen

Ohne Zusammenfassung