Novel water overlayer growth on Pd(111) characterized with scanning tunneling microscopy and density functional theory

Scanning tunneling microscopy (STM) images of water submonolayers on Pd(111) reveal quasiperiodic and isolated adclusters with internal structure that would ordinarily be ascribed to icelike puckered hexagonal units. However, density functional theory and STM simulations contradict this conventional picture, showing instead that the water adlayers are composed mainly of flat-lying molecules arranged in planar water hexagons. A new rule for two dimensional (2D) water growth is offered that generates the structures observed experimentally from planar hexamer units.     

Atomic force microcopy study of the mechanical and electrical properties of monolayer films of molecules with aromatic end groups

The effect of intermolecular pi-pi stacking on the electrical and mechanical properties of monolayer film molecules containing aromatic groups was studied using atomic force microscopy. Two types of aromatic molecules, (4-mercaptophenyl) anthrylacetylene (MPAA) and (4-mercaptophenyl)-phenylacetylene (MPPA), were used as model systems with different pi-pi stacking strength. Monolayer films of these molecules on Au(111) surfaces exhibited conductivities differing by more than 1 order of magnitude, with MPAA being the most conductive and MPPA being the least conductive. The response to compressive loads by the AFM tip was also found to be very different for both molecules. In MPAA films, distinct molecular conductivity changes are observed upon mechanical perturbation. This effect, however, was not observed on the MPPA film, where intermolecular pi-pi interactions are likely weaker.     

STM studies on the growth of monolayers: Co on Cu(110) with one half monolayer of preadsorbed oxygen

The growth of the first cobalt monolayer (ML) on the Cu(110)-(2×1)O surface was studied by scanning tunneling microscopy. Extensive exchange of Cu and Co atoms takes place in the first stages of the deposition. The displaced Cu atoms form new Cu---O---Co mixed islands, with the same structure as those of the terrace surface. At 0.25 ML Co, a new structure nucleates, which contains three Cu atoms, four Co atoms and two O atoms per 2×2 cell. The structure consists of rows in the [ 10] direction with an internal periodicity of two lattice units. The rows are separated from one another by two lattice units along the [001] direction, and are found both in-phase and out-of-phase relative to one another. The result is a mixed p(2×2) and c(2×4) surface. The fraction of the surface covered by the new structure increases with Co coverage, and completely covers the surface at 1 ML Co.     

Chemical and electronic characterization of cobalt in a lanthanum perovskite. Effects of strontium substitution

Two different cobaltites, LaCoO₃ and La_0.5Sr_0.5CoO_3−δ, have been prepared and characterized by means of high energy Co K-edge and low energy O K-edge X-ray absorption spectroscopy (XAS). Even though half of the La(III) is substituted by Sr(II), little or no changes can be detected in the formal oxidation state of cobalt atoms. The presence of strontium cations induces two main effects in the chemical and electronic state of the perovskite. The charge balance with Sr(II) species is reached by the formation of oxygen vacancies throughout the network, which explains the well-known increase in the reactivity of this substituted perovskite. O K-edge XAS experiments show that the Sr(II) species induce the transitions of d electrons of cobalt cations from low to high spin configuration. We propose that this change in spin multiplicity is induced by two cooperative effects: the oxygen vacancies, creating five coordinated cobalt atoms, and the bigger size of Sr(II) cations, aligning the Co-O-Co atoms, and favoring the overlapping of π-symmetry cobalt and oxygen orbitals, reducing the splitting energy of e_g and t_2g levels.     

On a Fix-and-Relax Framework for a Class of Project Scheduling Problems

We consider the following problem: Given a set of projects to be executed along a multi-year time horizon, find a sequencing and scheduling feasible solution that optimizes a merit function. A feasible solution should satisfy availability of storable and non-storable resources with budget carrier and non-carrier periods, and precedence, exclusivity and implication relationships among the projects. This is an NP-hard problem. We present several Fix-and-Relax strategies which partition the set of binary variable into clusters in order to selectively explore the branch-and-bound tree. Computational performance is favorably compared with a state-of-the-art optimization engine over a set of real-life cases.     

K−π− elastic scattering cross section measured in 14.3 GeV/c K−p interactions

Results are presented on a measurement of the I = 3/2 Kπ elastic scattering cross section by applying the Chew-Low extrapolation method in the reaction K^?p → K^?π^?Δ⁺⁺at14.3GeV/c. The magnitude of σ(K^?π^? → K^?π^?) is found to be about 3 mb at threshold and slowly falling with energy to about 1.5 mb at 2.8 GeV. The off-mass-shell moments of the differential elastic cross section are given. The absolute value of δ₃⁰ is found to be increasing slowly from 0° to 20° between threshold and 1.2 GeV.     

Hydrogen and deuterium exchange on Pt(111) and its poisoning by carbon monoxide studied by surface sensitive high-pressure techniques

Catalytic hydrogen/deuterium exchange on a platinum (111) single crystal and its poisoning with carbon monoxide was studied using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), sum frequency generation vibrational spectroscopy (SFG), and mass spectrometry under reaction conditions at pressures in the mTorr to atmospheric range. At 298 K and in the presence of 200 mTorr of hydrogen and 20 mTorr of deuterium the surface is catalytically active, producing HD with activation energy of approximately 5.3 kcal/mol. Addition of 5 mTorr of CO stops the reaction completely. In situ STM images reveal an ordered surface structure of chemisorbed CO. At 353 K the addition of 5 mTorr of CO slows the reaction by 3 orders of magnitude, but HD production continues with an activation energy of 17.4 kcal/mol. Changes in coverage and adsorption site of CO were followed by XPS and SFG up to a temperature of 480 K. From these data, a CO dominated, mobile and catalytically active surface model is proposed.     

Velocity dependence of friction and hydrogen bonding effects

We show that the friction force varies with the sliding velocity in a manner that depends on the chemical nature of the interface. Surfaces terminated with the hydrogen acceptor and donor moieties capable of forming H-bond networks exhibit a friction that decreases with sliding velocity, a behavior that is opposite to that of surfaces where no such networks can form. We explain the results with a model where the domains of glassy H-bond networks are disrupted at a critical applied stress leading to slippage.     

When Langmuir is too simple: H2 dissociation on Pd(111) at high coverage

Recent experiments of H2 adsorption on Pd(111) [Nature (London) 422, 705 (2003)]] have questioned the classical Langmuir picture of second order adsorption kinetics at high surface coverage requiring pairs of empty sites for the dissociative chemisorption. Experiments find that at least three empty sites are needed. Through density functional theory, we find that H2 dissociation is favored on ensembles of sites that involve a Pd atom with no direct interaction with adsorbed hydrogen. Such active sites are formed by aggregation of at least 3 H-free sites revealing the complex structure of the "active sites."