C-Fe chains due to segregated carbon impurities on Fe(1 0 0)

Bulk carbon impurities segregate at the Fe(1 0 0) surface and, upon thermal annealing, can form metastable surface phases with local and long range order and peculiar electronic properties. We present a surface science study of C-segregated Fe(1 0 0) with scanning tunneling microscopy, angle resolved photoemission, and ab initio calculations of the surface structure and electron states. In particular the c(3√2 × √2) structure, observed for 0.67 atomic layers of C segregated at the iron surface, is found to be due to self-organized carbon stripes made of zig-zag chains. The strong hybridization between C and Fe was observed in ARPES spectra.     

Mesoscopic chiral reshaping of the Ag(110) surface induced by the organic molecule PVBA

We report scanning tunneling microscopy observations on the restructuring of a Ag(110) surface induced by the molecule 4-[trans-2-(pyrid-4-yl-vinyl)]benzoic acid (PVBA). Our data reveal that the surface undergoes a mesoscopic step faceting following exposure to submonolayer coverages and thermal activation. A sawtooth arrangement evolves implying long-range mass transport of substrate atoms and forming a regular arrangement of kink sites. Its formation is associated with the molecules' functional headgroups forming carboxylates with [100] Ag microfacets at step edges, and eventually operating to reshape the surface morphology. Interestingly, the resulting microfacets act as chiral templates for the growth of supramolecular PVBA structures. Theoretical modeling based on ab initio results indicates that chiral recognition processes discriminating between the two enantiomers of adsorbed PVBA molecules occur in this process.     

Stereochemical effects in supramolecular self-assembly at surfaces: 1-D versus 2-D enantiomorphic ordering for PVBA and PEBA on Ag(111)

We present investigations on noncovalent bonding and supramolecular self-assembly of two related molecular building blocks at a noble metal surface: 4-[trans-2-(pyrid-4-yl-vinyl)]benzoic acid (PVBA) and 4-[(pyrid-4-yl-ethynyl)]benzoic acid (PEBA). These rigid, rodlike molecules comprising the same complementary moieties for hydrogen bond formation are comparable in shape and size. For PVBA, the ethenylene moiety accounts for two-dimensional (2-D) chirality upon confinement to a surface; PEBA is linear and thus 2-D achiral. Molecular films were deposited on a Ag(111) surface by organic molecular beam epitaxy and characterized by scanning tunneling microscopy. At low temperatures (around 150 K), both species form irregular networks of flat lying molecules linked via their endgroups in a diffusion-limited aggregation process. In the absence of kinetic limitations (adsorption or annealing at room temperature), hydrogen-bonded supramolecular assemblies form which are markedly different. With PVBA, enantiomorphic twin chains in two mirror-symmetric species running along a high-symmetry direction of the substrate lattice form by diastereoselective self-assembly of one enantiomer. The chirality signature is strictly correlated between neighboring twin chains. Enantiopure one-dimensional (1-D) supramolecular nanogratings with tunable periodicity evolve at intermediate coverages, reflecting chiral resolution in micrometer domains. In contrast, PEBA assembles in 2-D hydrogen-bonded islands, which are enantiomorphic because of the orientation of the supramolecular arrangements along low-symmetry directions of the substrate. Thus, for PVBA, chiral molecules form 1-D enantiomorphic supramolecular structures because of mesoscopic resolution of a 2-D chiral species, whereas with PEBA, the packing of an achiral species causes 2-D enantiomorphic arrangements. Model simulations of supramolecular ordering provide a deeper understanding of the stability of these systems.     

Vehicle emissions,urban air quality and current Italian legislation: the case of Messina. V

In the current program for the preparation of an urban air map, this paper reports data obtained by the five base stations in the period 1997-2000 regarding traditional pollutants and gives the first results obtained in determining continuously benzene, toulene and PM10. Additional measurements have been carried out by a mobile station at the falcate zone, never tested before, which has very low traffic but with the presence of some small metal industries and, under favourable meteorological conditions, the effects of the harbour. A statistical study on the hourly-trend of particulate suspended matter in two zones of the city which have different traffic volume for the period 1995-1997 is also reported. The ozone characteristic peak at 15.00 is fully confirmed in all stations and over all the period contrarily to most Italian cities, which present the peak about three hours earlier. PM10 annual mean value during 1999 was a little higher than the legal one. Base stations "Archimede" and "Minissale" still present some risk of a stop or limitation of traffic even with respect to traditional or "new" pollutants. Statistical analysis of particulate suspended matter confirm the very different characteristics of the base stations under consideration.     

Proton and α sequential emission in the 16O + 58Ni deep inelastic reaction: A semi-classical approach

The 132 MeV ¹⁶O + ⁵⁸Ni reaction has been experimentally investigated by using coincident charged particle techniques. A closed-form theoretical approach, describing in a simple picture the non-equilibrium component and the evaporative one of the angular correlation between light particles and reaction residues emitted in a peripheral heavy-ion collision, is applied — in the hypotesis of a sequential process — to the (C,N,O)-α and (C,N,O)-p differential multiplicities for the ¹⁶O + ⁵⁸Ni at 8.25 MeV/A deep inelastic collision. From this analysis some reaction mechanism information is deduced.     

LATERAL INTERACTIONS BETWEEN VERTEBRATE PHOTORECEPTORS

Vertebrate photoreceptors generate electrical signals across their cell membrane when they absorb light. Recent studies show, moreover, that the membrane potential of an individual photoreceptor may be also modified by illumination of its neighbors. Two types of lateral interactions have been described: a direct interaction mediated by electrical synapses between adjacent photoreceptors, and a recurrent interaction mediated by a feedback circuit involving horizontal cells. In the first mechanism, photoreceptors summate their responses over short retinal distances. In the feedback circuit photoreceptors may develop light responses of opposite polarity to those induced by direct illumination. In general, these 'antagonistic' responses are best elicited by large area retinal illumination. As a consequence of these photoreceptor interactions rather complex processing of visual information, in both spatial and chromatic domains, occurs at the first stage of the retinal network.     

Evidence for pair correlation effects in heavy ion reactions

The study of the ¹²C(¹⁴N,¹⁴N)¹²C reaction was performed at 28 and 35 MeV beam energies. The results were analyzed in the frame of the EFR-DWBA (Exact-Finite-Range Distorted Wave Born Approximation) assuming the simultaneous and sequential transfer of a np pair. The angular distributions, fairly reproduced in the first case, confirm the validity of the generalized BCS (Bardeen–Cooper–Schrieffer) theory to explain this behaviour. Moreover, this process could be regarded as a possible Nuclear Josephson Effect.     

Structure prediction and targeted synthesis: a new Na(n)N2 diazenide crystalline structure

Significant progress in theoretical and computational techniques for predicting stable crystal structures has recently begun to stimulate targeted synthesis of such predicted structures. Using a global space-group optimization (GSGO) approach that locates ground-state structures and stable stoichiometries from first-principles energy functionals by objectively starting from randomly selected lattice vectors and random atomic positions, we predict the first alkali diazenide compound Na(n)N(2), manifesting homopolar N-N bonds. The previously predicted Na(3)N structure manifests only heteropolar Na-N bonds and has positive formation enthalpy. It was calculated based on local Hartree-Fock relaxation of a fixed-structure type (Li(3)P-type) found by searching an electrostatic point-ion model. Synthesis attempts of this positive ΔH compound using activated nitrogen yielded another structure (anti-ReO(3)-type). The currently predicted (negative formation enthalpy) diazenide Na(2)N(2) completes the series of previously known BaN(2) and SrN(2) diazenides where the metal sublattice transfers charge into the empty N(2) Π(g) orbital. This points to a new class of alkali nitrides with fundamentally different bonding, i.e., homopolar rather than heteropolar bonds and, at the same time, illustrates some of the crucial subtleties and pitfalls involved in structure predictions versus planned synthesis. Attempts at synthesis of the stable Na(2)N(2) predicted here will be interesting.     

Influence of α-Tocopherol Load and Annealing Treatment on the Wear Resistance of Biomedical UHMWPE Irradiated with Electron Beam

A biomedical UHMWPE electron beam irradiated in air with doses raging between 25 and 100 KGy was doped with different amounts of α-tocopherol (0.1–0.7 wt.%) to prevent oxidative degradation. The polymer was annealed just after the irradiation at 110° for 24 h. Physical and mechanical tests were performed on the UHMWPE before and after the irradiation and on neat or doped material. Results showed that heat treatment after irradiation improves cross-links in the UHMWPE and α-tocopherol has good antioxidant action. The UHMWPE blended with 0.7% of antioxidant became harder and more wear resistant (+52%) after the irradiation with 100 KGy.     

Construction and solution of a Wannier-functions based Hamiltonian in the pseudopotential plane-wave framework for strongly correlated materials

Ab initio determination of model Hamiltonian parameters for strongly correlated materials is a key issue in applying many-particle theoretical tools to real narrow-band materials. We propose a selfcontained calculation scheme to construct, with an ab initio approach, and solve such a Hamiltonian. The scheme uses a Wannier-function-basis set, with the Coulomb interaction parameter U obtained specifically for theseWannier functions via constrained Density functional theory (DFT) calculations. The Hamiltonian is solved by Dynamical Mean-Field Theory (DMFT) with the effective impurity problem treated by the Quantum Monte Carlo (QMC) method. Our scheme is based on the pseudopotential plane-wave method, which makes it suitable for developments addressing the challenging problem of crystal structural relaxations and transformations due to correlation effects. We have applied our scheme to the “charge transfer insulator” material nickel oxide and demonstrate a good agreement with the experimental photoemission spectra.