Unraveling the symmetry of the hole states near the Fermi level in the MgB2 superconductor

We use x-ray absorption spectroscopy (XAS) and electron energy loss spectroscopy (EELS) to study the fine structure at the K edge of boron in MgB(2). We observe in XAS a peak of width 0.7 eV at the edge threshold, signaling a narrow energy region with empty boron p states near the Fermi level. The changes in the near edge structure observed in EELS with direction of the momentum transfer imply that these states have p(x)p(y) symmetry. Our observations are consistent with electronic structure calculations indicating a narrow energy window of empty p(x)p(y) states that falls to zero at 0.8 eV above the Fermi level. The disappearance of the p(x)p(y) feature in EELS at grain boundaries suggests that this signature may become powerful in probing superconductivity at nanoscale.     

Novel rearrangements and formation of 2,5-dihydro-1,2,5-oxoniastannaboratolesvia methanolysis of zwitterionic η2-(alkyn-1-yl-borate)alkenyltin compounds

Organo-substituted 2,5-dihydro-1,2,5-oxoniastannaboratoles3 were prepared by methanolysis of zwitterionic ²-(alkyn-l-ylborate)alkenyltin compounds1. Analogously, the bis[²-(alkyn-l-yl-borate)alkenyltin derivative2 reacts with an excess of methanol to give a dimeric MeO-bridged 2,5-dihydro-1,2,5-oxoniastannaboratole (10a)₂. Various intermediates could be identified by NMR spectroscopy, and the molecular structure of(10a) ₂ was determined by X-ray analysis. The structures of several products of the methanolysis indicate that protolysis of an Sn-C=bond occurs first, followed by an MeO/alkenyl exchange reaction.Dedicated to the 100th Anniversary of A. N. Nesmeyanov's birth.Laboratorium für Anorganische Chemie, Universität Bayreuth, D-95440 Bayreuth, Germany Institut für Anorganische Chemie, Universität-GH Essen, Universitätsstr. 5–7, D-45117 Essen, Germany. Published in Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1185–1196, September, 1999.     

Chemometric investigations on the differentiated evaluation of element trace analysis in river waters

The combination of sequential leaching methods for a first assessment of the kind of species in river sediments with multivariate-statistical methods (like factor analysis) for identifying anthropogenic and/or geogenic loading is useful for the differentiated characterization of the pollution state of a river. Electrochemical investigations, planned on the basis of statistical design and following empirical modelling, enables quantitative conclusions on the binding forms of heavy metals in river waters. Deposition-remobilisation effects of heavy metals in the complex system river water-river sediment can be described by PLS modelling.     

Unimolecular Fragmentation of Energy Selected Allene Molecular Cations

The unimolecular fragmentations of internal energy selected allene molecular ions ( 1 ⁺) have been investigated by photoelectron-photoion coincidence spectroscopy. The energy dependence of the branching ratio between the two most important fragmentation channels is reported, together with values for the kinetic energy release during fragmentation. Loss of an hydrogen atom from 1 ⁺ is interpreted to proceed via two paths, yielding different isomeric forms of C₃H with different kinetic energy release values.     

Metallation des halogenures d'aryle par le tributylstannyllithium dans le tetrahydrofuranne

The metallation of aryl halides with tributylstannyllithium in THF proceeds via at least three different mechanisms whose relative rates depend on the nature of the halogen, the substrate and the experimental conditions. In the case of aryl bromides, a halogenmetal exchange (major process) is competing with a radical mechanism. With aryl chlorides, an arynic process, a halogenmetal exchange and a radical mechanism are involved. Finally, with aryl fluorides, the arynic process is usual, but it may be strongly competed by a radical mechanism whenever suitable experimental conditions are arranged.     

Off-shell NN annihilation studied in π+p interactions at 8 and 16 GeV/c

Interactions in which the baryon is emitted forwards in the c.m.s. are studied in 8 and 16 GeV/cπ⁺p collisions. These interactions are interpreted as representing baryon exchange. The properties of such off-shell $\text{N}$N annihilation events are studied. It is found that the multiplicity distribution of the multi-pion system as a function of the effective mass for events with |u_pp| < 1 GeV² is the same as for real $\text{N}\text{N}$ annihilations.     

Randverteilungen holomorpher Funktionen und die Topologie vonD′

Ohne Zusammenfassung     

Optical properties and electronic structure of US

The optical properties of uranium sulphide single crystals have been determined for the first time. An excellent agreement is found between the structure in the optical spectrum and the results of a self-consistent cellular multiple scattering calculation. The results evidence that the 5? electrons form a resonance state at E_F and that the 5??6d coupling produces a dip in the 6d density of states near E_F which is responsible for many peculiar properties of US.