Computer simulations of the X-ray diffraction patterns of imperfect Al/Nb superlattices

In order to obtain more structural details from X-ray diffraction (XRD) patterns of metallic multilayers we developed a simulation program for XRD patterns of Al/Nb multilayers. We followed the theory of an imperfect one-dimensional superlattice described by Z. Mitura and P. Mikolajczak. Computer simulated patterns are compared with experimentally obtained XRD spectra.     

SENSITIZED PHOTODEGRADATION OF CELLULOSE AND CELLULOSIC WASTES

Abstract— The photodegradation of cellulose and cellulose–containing waste sensitized by a variety of dyes was measured by means of viscosity, tensile strength, and scanning electron microscopy. Anthraquinone-2-sulfonate and proflavin dihydrochloride were both more effective than either rose bengal or methylene blue for degradation. Samples degraded by these dyes were similar in appearance to enzyme-degraded ones, but were less susceptible to further degradative action by enzymes.     

Electron energy loss spectroscopy and ultraviolet photoemission of TmSe

Intraionic Tm²⁺ and Tm³⁺ ? → d and p → d charge transfer transitions are identified in the energy loss spectra of 100eV electrons of TmSe by comparison with X-ray photoemission data. In contrast the ultraviolet photoemission spectra with photon energies up to 21.2 eV do not show any evidence for the intermediate valence character of TmSe due to matrix element effects.     

Three-pion interferometry results from central Pb+Pb collisions at 158A GeV/c

Three-particle correlations have been measured for identified pi(-) from central 158A GeV Pb+Pb collisions by the WA98 experiment at CERN. A substantial contribution of the genuine three-body correlation has been found as expected for a mainly chaotic and symmetric source.     

Oxygen nonstoichiometry and ionic transport properties of La0.4Sr0.6CoO3-delta

Homogeneous samples of La0.4Sr0.6CoO3-delta were obtained by the glycine nitrate process. The oxygen nonstoichiometry was determined from oxygen exchange measurements as a function of oxygen partial pressure (10(-4) bar < PO2 < 10(-2) bar) and temperature (300 degrees C < T< 900 degrees C). The chemical diffusion coefficient D was obtained from oxygen exchange measurements applying a stepwise variation of the oxygen partial pressure of the ambient atmosphere of a disk-shaped sample. The amount of oxygen absorbed or desorbed by the perovskite was analyzed as a function of time. Chemical diffusion data were evaluated using simplified and exact fitting procedures taking into account the surface exchange coefficient. Alternatively, galvanostatic polarization measurements were performed in a PO2-range between 10(-4) and 10(-2) bar to yield D and the ionic conductivity sigma(i) from the long time solution of the diffusion equation. Values for D from polarization measurements at T= 775 degrees C and from oxygen exchange measurements at T= 725 degrees C are in good agreement with each other. D and sigma(1) increase with increasing PO2 (10(-4) to 10(-2) bar). The ionic conductivity shows a maximum at 3-delta approximately 2.82 and decreases with decreasing oxygen content indicating the possible formation of vacancy ordered structures.     

Proportionality principle for the simplicial volume of families of \mathbb Q -rank 1 locally symmetric spaces

We establish the proportionality principle between the Riemannian volume and locally finite simplicial volume for $\mathbb Q $ -rank 1 locally symmetric spaces covered by products of hyperbolic spaces, giving the first examples for manifolds whose cusp groups are not necessarily amenable. Also, we give a simple direct proof of the proportionality principle for the locally finite simplicial volume and the relative simplicial volume of $\mathbb Q $ -rank $1$ locally symmetric spaces with amenable cusp groups established by Löh and Sauer [26].     

Transforming and separating rotating disk vibrations using a sensor array

This paper presents a method to separate the complex vibration pattern of a rotating disk into simpler entities. The decomposition transforms data measured by an array of sensors into time domain signals representing the contribution of individual modes of vibration. Having performed the decomposition with respect to wavelength, speed and direction of travel, the obtained measurements can be projected onto a rotating body experiencing variable rotational speed relative to the sensors. Unlike previous works, the vibrations here are decomposed into time domain signals that provide better insight into stress levels and fatigue than frequency domain based decompositions. Furthermore, the proposed method works under non-stationary conditions, e.g. under rapid angular acceleration and during transient motions. By exploiting the spatial deployment of sensors, the proposed transformation can produce information about the deformations in the body-fixed or material coordinates which is essential for stress analysis. The main feature of the method is the ability to separate modes of vibration that normally overlap in the frequency domain, to enable better insight into the sources of vibration. The method is demonstrated by analytical, numerical and experimental examples.     

On the Use of Accelerated Molecular Dynamics to Enhance Configurational Sampling in Ab Initio Simulations

We have implemented the accelerated molecular dynamics approach (Hamelberg, D.; Mongan, J.; McCammon, J. A. J. Chem. Phys. 2004, 120 (24), 11919) in the framework of ab initio MD (AIMD). Using three simple examples, we demonstrate that accelerated AIMD (A-AIMD) can be used to accelerate solvent relaxation in AIMD simulations and facilitate the detection of reaction coordinates: (i) We show, for one cyclohexane molecule in the gas phase, that the method can be used to accelerate the rate of the chair-to-chair interconversion by a factor of ～1 × 10(5), while allowing for the reconstruction of the correct canonical distribution of low-energy states; (ii) We then show, for a water box of 64 H(2)O molecules, that A-AIMD can also be used in the condensed phase to accelerate the sampling of water conformations, without affecting the structural properties of the solvent; and (iii) The method is then used to compute the potential of mean force (PMF) for the dissociation of Na-Cl in water, accelerating the convergence by a factor of ～3-4 compared to conventional AIMD simulations.(2) These results suggest that A-AIMD is a useful addition to existing methods for enhanced conformational and phase-space sampling in solution. While the method does not make the use of collective variables superfluous, it also does not require the user to define a set of collective variables that can capture all the low-energy minima on the potential energy surface. This property may prove very useful when dealing with highly complex multidimensional systems that require a quantum mechanical treatment.     

Nickel (II) tetraphenylporphyrin modified surfaces via electrografting of an aryldiazonium salt

We have prepared and isolated the monodiazonium salt of nickel (II) tetraphenylporphyrin and grafted the corresponding complex to glassy carbon, pyrolysed photoresist film, gold and indium tin oxide surfaces via reduction of the diazonium moiety. Characterisations of the films by voltammetry, UV–vis spectroscopy and atomic force microscopy depth profiling confirm that the metallated porphyrin is intact in the film and is stably attached to the surface with well-behaved, but highly solvent-dependent electrochemistry. Under the grafting conditions used, the films appear to have close to monolayer thickness with the porphyrin macrocycles oriented predominantly upright on the surface.     

Triplet excited states of cyclic disulfides and related compounds: electronic structures, geometries, energies, and decay

We have performed a computational study on the properties of a series of heterocycles bearing two adjacent heteroatoms, focusing on the structures and electronic properties of their first excited triplet states. If the heteroatoms are both heavy chalcogens (S, Se, or Te) or isoelectronic species, then the lowest excited triplet state usually has (π*, σ*) character. The triplet energies are fairly low (30-50 kcal mol(-1)). The (π*, σ*) triplet states are characterized by a significantly lengthened bond between the two heteroatoms. Thus, in 1,2-dithiolane (1b), the S-S bond length is calculated to be 2.088 ? in the singlet ground state and 2.568 ? in the first triplet excited state. The spin density is predicted to be localized almost exclusively on the sulfur atoms. Replacing one heavy chalcogen atom by an oxygen atom or an NR group results in a significant destabilization of the (π*, σ*) triplet excited state, which then no longer is lower in energy than an open-chain biradical. The size of the heterocyclic ring also contributes to the stability of the (π*, σ*) triplet state, with five-membered rings being more favorable than six-membered rings. Benzoannulation, finally, usually lowers the energy of the (π*, σ*) triplet excited states. If one of the heteroatoms is an oxygen or nitrogen atom, however, the corresponding lowest triplet states are better described as σ,π-biradicals.