Size determination of metallic iron particles by temperature dependent Mössbauer and ferromagnetic resonance studies

The thermal decomposition products of NaX·Fe(CO)₅ after various steps of heat treatment, in vacuum as well as under gas atmosphere (Ar, H₂), have been investigated by temperature-dependent Mössbauer and ferromagnetic resonance measurements. The measured spectra of both techniques consistently are analyzed using superparamagnetic relaxation behavior of magnetic particles. From this analysis information concerning composition and size of magnetic iron particles is derived.     

Interaction of atomic hydrogen with metal surfaces

Received: 23 March 1998/Accepted: 21 August 1998     

Strict Kinetic Treatment of the Electron Response to Spatial Field Disturbances in Nonequilibrium Plasmas

In generalization of former approaches for the simplified solution of the inhomogeneous electron Boltzmann equation a higher order solution technique has been developed. This technique is based on a multi-term expansion of the electron velocity distribution function and allows a strict study of the electron kinetics in plasmas acted upon by space-dependent electric fields. This solution technique is used to investigate the response of the plasma electrons to spatially limited disturbances of the electric field in weakly ionized plasmas of helium and mercury. By solving the kinetic equation with increasing order of the multi-term expansion the convergent solution of the kinetic problem and thus the strict spatial behaviour of the velocity distribution and of significant macroscopic properties of the electrons has been determined and analysed. Furthermore, the impact of higher order terms of the expansion has been revealed and the falsification of the velocity distribution and of related macroscopic properties has been evaluated when instead of the multi-term solution the simpler two-term solution of the kinetic equation is used.     

A critical exponent in a degenerate parabolic equation

We consider positive solutions of the Cauchy problem in $\mathbb{R\,}^n$ for the equation $$u_t=u^p\,\Delta u+u^q,\quad p\geq1,\; q\geq 1$$\nopagenumbers\end and show that concerning global solvability, the number q = p + 1 appears as a critical growth exponent. Copyright © 2002 John Wiley & Sons, Ltd.     

Extremal cover times for random walks on trees

Let C_ν(T) denote the “cover time” of the tree T from the vertex v, that is, the expected number of steps before a random walk starting at v hits every vertex of T. Asymptotic lower bounds for C_ν(T) (for T a tree on n vertices) have been obtained recently by Kahn, Linial, Nisan and Saks, and by Devroye and Sbihi; here, we obtain the exact lower bound (approximately 2n In n) by showing that C_ν(T) is minimized when T is a star and v is one of its leaves. In addition, we show that the time to cover all vertices and then return to the starting point is minimized by a star (beginning at the center) and maximized by a path (beginning at one of the ends).     

The shadowgraph method in convection experiments

The shadowgraph method is applied to thermal convection experiments and electro-hydrodynamic convection (EHC) in nematic liquid crystals. In both cases convection leads to a spatially periodic field of the refractive index causing a spatially periodic intensity modulation of parallel light passing the cell. Close to the onset of convection the temperature or director field is given by linear stability analysis. Knowing these functions the determination of their amplitudes becomes possible by means of the shadowgraph method. The method is demostrated using various examples of thermal and EHC convection experiments.     

Total synthesis of pamamycin-649B

The first total synthesis of the macrodiolide antibiotic pamamycin-649B (1) was achieved by using sultone methodology. The diethyl substituted larger hydroxy acid fragment was constructed in a concise fashion through domino elimination/alkoxide-directed 1,6-additions of ethyllithium to sultones derived from intramolecular Diels-Alder reaction of furan-containing vinylsulfonates. Intermolecular Yamaguchi esterification of the two hydroxy acid building blocks and subsequent Yamaguchi cyclization eventually provided the target macrocycle 1. Since the final lactonization with formation of the ester linkage between C1' and the C8 oxygen proceeded with complete C2' epimerization, the more readily available C2' epimeric smaller fragment could be used to streamline the synthetic sequence.     

Core, shell, and surface-optimized dendrimers for blue light-emitting diodes

We present a novel core-shell-surface multifunctional structure for dendrimers using a blue fluorescent pyrene core with triphenylene dendrons and triphenylamine surface groups. We find efficient excitation energy transfer from the triphenylene shell to the pyrene core, substantially enhancing the quantum yield in solution and the solid state (4-fold) compared to dendrimers without a core emitter, while TPA groups facilitate the hole capturing and injection ability in the device applications. With a luminance of up to 1400 cd/m(2), a saturated blue emission CIE(xy) = (0.15, 0.17) and high operational stability, these dendrimers belong to the best reported fluorescence-based blue-emitting organic molecules.