The structure of a metallic glass system using EXELFS and EXAFS as complementary probes

The short-range atomic order around all three constituent atoms in a prototypical bulk metallic glass (BMG) system was probed in a complementary way, using extended X-ray absorption fine structure for neighborhood of the higher atomic number elements, and extended energy loss fine structure (EXELFS) for the lower atomic number ones. The Pd_xNi_(80−x)P₍₂₀₎ system is a prototype for a whole class of BMG formers which are 80% transition metal and 20% metalloid. We find that the structure of these BMGs could be explained in terms of those of glasses at the end of the BMG range, namely, Pd₆₀Ni₂₀P₂₀ and Pd₃₀Ni₅₀P₂₀. The binary phosphide crystals near x=0 and 80 are found to be simulate very well the local atomic structure of Pd₃₀Ni₅₀P₂₀ and Pd₆₀Ni₂₀P₂₀ glasses, respectively. The best glass former in this series, Pd₄₀Ni₄₀P₂₀, is best described by a weighted average of Pd₃₀Ni₅₀P₂₀ and Pd₆₀Ni₂₀P₂₀ structures.     

Copper-ion interaction with the 106-113 domain of the prion protein: a solution-equilibria study on model peptides

Prion diseases are characterized by a structural modification of the regular prion protein (PrP(C)) to its isoform, termed PrP(Sc)(scrapie). Such a modification involves the secondary and tertiary structure of the protein; the amino acidic sequence remains unchanged. PrP(Sc) is almost insoluble in non-denaturing solvents, resistant to proteases and it loses its redox activity. PrP(C) is able to bind copper and other metal ions: these complexes have been suggested to play an important role in the protein refolding leading to PrP(Sc). It is well-known that at least one relatively strong copper-binding site is located in the PrP(92--126) domain, where two His residues (96 and 111) are present. However, in the same domain, other amino acidic residues bear potentially donating atoms, i.e. Met, Asn and Lys residues. In order to shed light on the role of the side chains of such potentially tridentate amino acids on copper complexation, the polypeptide Ac-KTNMKHMA-NH(2), corresponding to the PrP(106--113) fragment, and some synthetic analogues have been investigated as ligands for the copper ion, by means of both thermodynamic and spectroscopic techniques. The pivotal role of imidazolic side chain of His in "anchoring" the metal ion has been confirmed. On the other hand, no clue was found on the participation of sulfur atom of Met or side amino-group of Lys residues to copper complex-formation.     

A density-functional theory based study on the 16O/18O-exchange reactions of the prototype iron-oxygen compounds FeO+ and FeOH+ with H2(18)O in the gas phase

The mechanism of the degenerate 16O/18O exchange in the reactions of FeO+ and FeOH+ with water is examined by density functional theory. Based on previous experimental work (Chem. Eur. J. 1999, 5, 1176), two possible reaction pathways are investigated for both systems. The first mechanism consists of one (for FeOH+ + H20) or two (for FeO+ + H20) 1,3-hydrogen migrations from one oxygen atom to the other; the iron atom is not directly involved in these OH bond activations. The second route comprises a series of two (for FeOH+ + H20) or four (for FeO+ + H20) 1,2-hydrogen migration steps which involve the intermediate formations of metal-hydrogen bonds. Both mechanisms are evaluated under consideration of the respective low- and high spin potential-energy surfaces. The computational results show a clear preference for the 1,3-routes occurring on the respective high-spin surfaces bypassing the intermediacy of high-valent iron compounds having FeH bonds.     

Linear and nonlinear time series analysis of the black hole candidate cygnus X-1

We analyze the variability in the x-ray lightcurves of the black hole candidate Cygnus X-1 by linear and nonlinear time series analysis methods. While a linear model describes the overall second order properties of the observed data well, surrogate data analysis reveals a significant deviation from linearity. We discuss the relation between shot noise models usually applied to analyze these data and linear stochastic autoregressive models. We debate statistical and interpretational issues of surrogate data testing for the present context. Finally, we suggest a combination of tools from linear and nonlinear time series analysis methods as a procedure to test the predictions of astrophysical models on observed data.     

Solving Second-Order Differential Equations with Lie Symmetries

Lie"s theory for solving second-order quasilinear differential equations based on its symmetries is discussed in detail. Great importance is attached to constructive procedures that may be applied for designing solution algorithms. To this end Lie"s original theory is supplemented by various results that have been obtained after his death one hundred years ago. This is true above all of Janet"s theory for systems of linear partial differential equations and of Loewy"s theory for decomposing linear differential equations into components of lowest order. These results allow it to formulate the equivalence problems connected with Lie symmetries more precisely. In particular, to determine the function field in which the transformation functions act is considered as part of the problem. The equation that originally has to be solved determines the base field, i.e. the smallest field containing its coefficients. Any other field occurring later on in the solution procedure is an extension of the base field and is determined explicitly. An equation with symmetries may be solved in closed form algorithmically if it may be transformed into a canonical form corresponding to its symmetry type by a transformation that is Liouvillian over the base field. For each symmetry type a solution algorithm is described, it is illustrated by several examples. Computer algebra software on top of the type system ALLTYPES has been made available in order to make it easier to apply these algorithms to concrete problems.     

Modulated beam relaxation spectrometry: Its application to the study of heterogeneous kinetics

The results obtained to date using modulated beam relaxation spectrometry (MBRS) to study heterogeneous kinetics are reviewed. The method of MBRS is described with particular emphasis on judicious use of appropriate experimental variables to infer the kinetics and mechanism of surface reactions. Recent calculations are presented for simulated MBRS studies of nonlinear reaction sequences, including coverage dependent adsorption-desorption phenomena and second order reactions. Linearization methods are discussed which enable the study of more complex reactions. Finally, the general applicability of MBRS to heterogeneous kinetics is discussed.     

Modifiziertes Statistisches Atommodell

Zusammenfassung Die statistische Atomtheorie nach Thomas und Fermi liefert ein fehlerhaftes Verhalten der Elektronendichte bei kleinen Kernabständen. Es wird eine modifizierte statistische Besetzungsvorschrift vorgeschlagen, die hier zu einer Verbesserung führt. Während die einfache Fermi-Statistik nur das Pauliprinzip berücksichtigt, befriedigt die modifizierte Statistik auch die Heisenbergsche Unschärfe-Relation, indem für stark lokalisierte Elektronen Phasenraum-Bereiche mit kleinen Impulsen verboten sind. Die modifizierte Statistik erfordert eine Neubehandlung des statistischen Austauschpotentials. Dichteverteilungen und verschiedene Erwartungswerte werden mit dem neuen Atommodell berechnet und mit exakten Werten verglichen.

---

Modified stasistical atom model

The statistical atom model of Thomas and Fermi leads to a wrong behaviour of electron density near the nucleus. A modified statistical occupation rule has been suggested, which improves this weakness of the model. Whereas the simple statistics of Fermi only satisfy the Pauli exclusion principle, the modified statistics allows for the Heisenberg uncertainty principle, too, by excluding small momentum domains of the phase space from occupation by spatially localized electrons. The modified statistics necessitate a redetermination of the statistical exchange potential. Atomic charge distributions and expectation values are calculated by the new model and compared with HF-results.

---

Résumé Le modèle statistique de l'atome de Thomas et Fermi conduit à un comportement erroné de la densité électronique au voisinage du noyau. On suggère une règle d'occupation modifiée qui remédie à cette faiblesse du modèle. Alors que la simple statistique de Fermi ne satisfait qu'au principe d'exclusion de Pauli, la statistique modifiée satisfait aussi au principe d'incertitude de Heisenberg, en excluant de l'occupation par des électrons localisés dans l'espace de petits domaines de moment dans l'espace des phases. La statistique modifiée nécessite une nouvelle évaluation du potentiel d'échange statistique. Les distributions de charge atomiques et les valeurs moyennes sont calculées à l'aide de ce nouveau modèle et comparées aux résultats Hartree-Fock.