Geometry of acetylene chemisorption on Ni(100) investigated by leed intensity analysis: The c(2 × 2) structure

Results from LEED dynamical calculations performed on the Ni(100) c(2 × 2)C₂H₂ structure, produced by adsorption of C₂H₂ at about 273 K and 1.5 L exposure, are reported. Among the several model geometries tried, the most favoured one appears to be a distorted acetylene molecule with the midpoint of the CC bond (1.20 Å long) placed above the fourfold hollow site at a vertical distance 2.02 Å from the topmost Ni layer and with the CC axis itself tilted by 50° with respect to the surface normal in the [011] direction. In this geometry the NiC distance is 2.2 Å.     

Cosmological black holes are coloured too

A new solution of the Einstein-Yang-Mills system with axial symmetry and cosmological constant is given here. This new metric, which in the absence of matter becomes the de Sitter universe, is the natural generalization of both a recent result of Perry for stellar (asymptotically flat) black holes and a well-knwn family of solutions of the Einstein-Maxwell system with nonvanishing cosmological constant given some time ago by Carter. Calculations have been carried out making extensive use of the coordinates found by Plebanski.     

Size-selected synthesis of PtRu nano-catalysts: reaction and size control mechanism

A rapid synthesis method for the preparation of PtRu colloids and their subsequent deposition on high surface area carbons is presented. The reaction mechanism is shown to involve the oxidation of the solvent, ethylene glycol, to mainly glycolic acid or, depending on the pH, its anion, glycolate, while the Pt(+IV) and Ru(+III) precursor salts are reduced. Glycolate acts as a stabilizer for the PtRu colloids and the glycolate concentration, and hence the size of the resulting noble metal colloids is controlled via the pH of the synthesis solution. Carbon-supported PtRu catalysts of controlled size can be prepared within the range of 0.7-4 nm. Slow scan X-ray diffraction and high-resolution transmission electron microscopy show the PtRu catalysts to be crystalline. The Ru is partly dissolved in the face-centered cubic Pt lattice, but the catalysts also consist of a separate, hexagonal Ru phase. The PtRu catalysts appear to be of the same composition independent of the catalyst size in the range of 1.2-4 nm. Particular PtRu catalysts prepared in this work display enhanced activities for the CH(3)OH electro-oxidation reaction when compared to two commercial catalysts.     

Single Particle Extinction and Scattering allows novel optical characterization of aerosols

We apply to aerosols the optical method of Single Particle Extinction and Scattering recently proposed for characterizing liquid suspensions and specifically adapted to the aim. It provides simultaneous measurements of the real and imaginary parts of the field scattered in the forward direction by single airborne particles passing through a tightly focused laser beam. The intensity of transmitted light is collected in the forward direction, thus realizing a self-reference interferometric scheme relying on the fundamentals of the optical theorem. A high frequency (20 MS/s), extended dynamics (12 bits) sampling is performed by a cheap segmented photodiode, and a specific pulse shape analysis is exploited to validate the signals against a precise mathematical model. We show that accessing two independent physical quantities allows to exploit physical models to recover the aerosol size distribution from the measurement of the refractive index, either real or even complex. Laboratory measurements have been performed with polydisperse aerosols made of water droplets and NaCl in the submicron range, and the system has been accurately characterized. Examples of measurements of graphite nanoparticles and Pyrethrum smoke are shown. Limitations are discussed.     

A simplified method for levoglucosan quantification in wintertime atmospheric particulate matter by high performance anion-exchange chromatography coupled with pulsed amperometric detection

Levoglucosan, a tracer for the assessment of the biomass burning contribution to atmospheric particulate matter (PM) concentrations, was determined by means of high-performance anion-exchange chromatography (HPAEC) with pulsed amperometric detection (PAD). In this work we propose a modification in the instrumental set-up aiming at an improvement in the detector response by adding NaOH after chromatographic separation to increase the pH. The comparison between this technique and the gas chromatography/mass spectrometry (GC/MS) method commonly used showed good agreement. Repeatability is 4.8% RSD, limits of detection for pevoglucosan, mannosan and galactosan are in the range 0.001–0.002 µg mL^?1 in solution, corresponding to 3–4 ng m^?3 for 24 m³ of air sampled. PM10 samples were characterised for levoglucosan and for organic and elemental carbon contents. The preliminary results reported here for five sites in the Lombardy region (Northern Italy) are, as far as we know, the first data on levoglucosan contribution to OC in Italy. The levoglucosan concentrations observed in Lombardy vary in the range 173–963 ng m^?3 with an average levoglucosan-C to OC ratio ranging from 1.5% to 2.5%.     

Three-dimensional atomic structure of metastable nanoclusters in doped semiconductors

Aberration-corrected scanning transmission electron microscopy is used to determine the atomic structure of nanoclusters of cerium dopant atoms embedded in silicon. By channeling electrons along two crystallographic orientations, we identify a characteristic zinc-blende chemical ordering within CeSi clusters coherent with the silicon host matrix. Strain energy limits the size of these ordered arrangements to just above 1 nm. With the local order identified, we then determine the atomic configuration of an individual subnanometer cluster by quantifying the scattering intensity under weak channeling condition in terms of the number of atoms. Analysis based on single-atom visualization also evidences the presence of split-vacancy impurity complexes, which supports the hypothesis of a vacancy-assisted formation of these metastable CeSi nanophases.