Untersuchungen zum Energiespektrum vonα,ω-substituierten Polymethinketten

Taking into account the electron correlation by the method of alternant molecular Orbitals the energy spectrum ofα,ω-substituted polyenes X-(CH)_2n -X with (2n+2)π-electrons has been studied. In reasonable agreement with values estimated from available experimental spectral data of homologous polyendiones the energy gap forn→ ∞ was found to be in the same order of magnitude as for unsubstituted polyenes. The effect of the bond alternation should be small.     

Role of framework sodium versus local framework structure in determining the hydrothermal stability of MCM-41 mesostructures.

Two mesostructured MCM-41 silicas that differ dramatically in hydrothermal stability have been examined by (29)Si MAS NMR spectroscopy and pair distribution function (PDF) analysis of synchrotron X-ray scattering data. The less stable mesostructure assembled from sodium silicate and the substantially more stable derivative made from fumed silica possess equivalent local framework wall structures, as judged by NMR and PDF methods. Approximately 80% of the SiO(4) tetrahedra are fully cross-linked as Q(4) (Si(OSi)(4)) units in both calcined samples. Additionally, the structural correlation distances for the two materials are nearly identical, having values of 1.62(1), approximately 2.60, and 3.09(1) A for the Si-O, O-O, and nearest neighbor Si-Si distances in the framework. Sodium ions in the framework play a crucial role in limiting the hydrothermal stability of the mesostructure. Residual sodium ( approximately 0.05-0.10% Na(2)O) is retained in the MCM-41 made from sodium silicate, even after two ion exchange reactions with ammonium ions in more than 300 - fold excess. The entrapped framework sodium ions catalyze the collapse of the mesopores upon exposure to 20% steam at 800 degrees C for 5 h. The sodium - free mesostructure assembled from fumed silica retains an open framework under the same hydrothermal conditions. The stability of the fumed silica derivative, however, is greatly compromised when doped with as little as 0.10% Na(2)O, thus confirming the deleterious effect of sodium on hydrothermal stability.     

High spin states in 128Ba

High spin states in ¹²⁸Ba have been investigated with the fusion–evaporation reaction ⁹⁶Zr(³⁶S,4n)¹²⁸Ba at a beam energy of 150 MeV with the GASP-spectrometer at INFN in Legnaro, Italy. The level scheme could be extended to higher spins and six-quasiparticle states have been observed. The deduced alignments are compared to the predictions of previously reported cranked shell model calculations. Received: 4 January 1999     

Spectroscopic studies of (AsSe)100−xAgx thin films

Thin (AsSe)_100−xAg_x films have been grown onto quartz substrates by vacuum thermal evaporation or pulsed laser deposition from the corresponding bulk materials. The amorphous character of the coatings was confirmed by X-ray diffraction investigations. Their transmission was measured within the wavelength range 400-2500 nm and the obtained spectra were analyzed by the Swanepoel method to derive the optical band gap E_g and the refractive index n. We found that both parameters are strongly influenced by the addition of silver to the glassy matrix: E_g decreases while n increases with Ag content. These variations are discussed in terms of the changes in the atomic and electronic structure of the materials as a result of silver incorporation.     

Multiple interactions of muons in BUST

Analysis of the experimental data for a long period of Baksan underground scintillation telescope (BUST) operation to search for very-high-energy muons is presented. The phenomenological parameters and the sensitivity of the method of multiple interactions, based on the concepts of the pair-meter technique, are discussed. The experimental distributions are compared with the results of calculations for a “conventional” muon spectrum, based on 3D simulation of muon passage through the telescope.     

Semiclassical Resolvent Estimates for Trapping Perturbations

We study the semiclassical estimates of the resolvent of a self-adjoint operator L(h) in the space of bounded operators . In the general case of long-range trapping “black-box” perturbations we prove that the estimate of the cut-off resolvent implies the estimate . Received: 15 November 1999 / Accepted: 1 March 2000     

Preparation of oriented mesoporous carbon nano-filaments within the pores of anodic alumina membranes

Oriented mesoporous carbon nano-filaments were prepared as replicas of Fe-containing silica mesostructures deposited inside the channels of anodic alumina membranes (AAMs). Due to the confinement imposed by the channels of the AAMs, rather unusual mesophase structures showing "circular" or "columnar" mesoscopic channels were formed that were replicated as pure carbon analogues after removal of the templating hosts. These structures are assembled in the forms of macroscopic membranes and are hierarchically ordered first due to the arrangement of the channels of the AAMs and second due to the mesophase counterparts.     

Stress investigations of thin amorphous Ge-Se-Ga films

Thin chalcogenide films from the systems (GeSe₄)_1-xGa_x and (GeSe₅)_1-xGa_x with gallium contents up to 20 at. % have been prepared by vacuum evaporation and their stress has been investigated by a cantilever technique. The addition of gallium to the Ge-Se matrix plays an important role in stress formation in the films: films without gallium possess negligible stress, while all gallium-containing films are under compressive stress. The increase of the gallium content leads to structural changes and an increase in the density, which results in higher stress values. For all films, stress reduction with time is observed due to spontaneous relaxation. Received: 2 October 2002 / Accepted: 22 November 2002 / Published online: 28 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-561/8044-136, E-mail: popov@schottky.physik.uni-kassel.de