Unidirectional adsorbate motion on a high-symmetry surface: "walking" molecules can stay the course

Step edges and low-symmetry faces of metal crystals can restrict the diffusive motion of adsorbates, yet they offer little flexibility with regards to the location and/or direction of the guided motion. We show inherently unidirectional motion of an organic molecule on a high-symmetry thermodynamic-equilibrium metal surface [Cu(111)]. Sequential placement of the substrate linkers of 9,10-dithioanthracene prevents it from rotating or veering off course. A combination of low temperature scanning tunneling microscopy and density functional theory simulations provide atomistic insight.     

Study at high frequencies of a stratified waveguide

A waveguide in integrated optics is defined by its refractiveindex. The guide is assumed to be invariant in the propagationdirection while in the transverse direction it is supposed tobe a compact perturbation of an unbounded stratified medium.We are interested in the high frequency modes guided by thisdevice. We consider the problem under the assumptions of weak guidance,so that it reduces to a two-dimensional eigenvalue problem fora scalar field. While a general study has been done in a previouspaper (Bonnet-BenDhia et al., IMAJAM 60, 1998), our goal hereis to present an asymptotic study at high frequencies, whichillustrates the dispersive character of the stratified guide.We will give the limit as the frequency tends to of the guidedmodes and characterize this limit as the solution of an eigenproblem.The technical difficulty lies in the stratified character ofthe unbounded reference medium.     

Nature of the binding of a c(2×2)-CO overlayer on Ag(001) and surface mediated intermolecular coupling

We present a first-principles study of the nature of the binding of a c(2×2)-CO overlayer on Ag(001) and of the origin of CO-CO interactions upon adsorption. Electronic structural changes induced by molecular adsorption provide an interpretation for earlier X-ray photoemission valence band spectra of CO/Ag(001). Our results establish that CO chemisorbs on clean Ag(001) and follows the Blyholder model of donation and back-donation between CO and metal orbitals. We analyze the origin of the dispersion of the C-O stretch mode and attest that it is caused by the metal-CO coupling. Specifically, the coupling of CO to Ag, although the weakest of those between it and transition and other noble metals, greatly enhances the intermolecular force constants. We also find that the response of the charge density around CO is much stronger and of longer range when the molecule stretches than when it rigidly vibrates against the surface. This difference explains why the C-O stretch mode disperses while the Ag-CO stretch mode does not.     

Pyrolysis Studies to Investigate Effects of Polymerization Techniques on Structure and Thermal Behavior of Poly(1,2-Epoxy-4-epoxyethylcyclohexanes)

Abstract

Thermal behaviors of poly(1,2-epoxy-4-epoxyethylcyclohexanes) (poly-EECH), polymerized by electroinitiation, radiation, and catalytic action of HClO₄ and BF₃-etharate, have been studied by direct pyrolysis MS and evolved gas analyses by mass and FT-IR spectrometric techniques. Direct pyrolysis experiments indicated that electroinitiated and catalytic polymerization of EECH with the use of BF₃-etharate produced the least stable polymers thermally. High mass peaks were related to the presence of blocks propagated through the 1,2-epoxy group, especially in the polymer produced by catalytic action of BF₃-etharate. Crosslinked structures were also justified by thermal analysis. Production of low molecular weight compounds such as CO, CO₂, CH₃OH, ethylene oxide, methylpropionate, and cyclohexane was investigated by evolved gas analysis.     

First principles study of the electronic and geometric structure of Cu(5 3 2)

Using density functional theory with the generalized gradient approximation and ultra-soft pseudo-potentials, we have calculated structural relaxations of the Cu(5 3 2) surface which contains steps and kinks. We find the relaxation pattern to oscillate dramatically for atoms in the first 10 layers before decaying rapidly in the bulk. The most striking feature is an outward expansion of the relative interlayer separation d₁₂ of 25%. We also find serious discrepancies with relaxation pattern and relaxation amplitudes calculated using embedded atom method potentials that may reflect the limitation of these potentials to accurately describe systems with complex geometries. Full potential calculations reveal a dispersionless surface state along a high symmetry direction in the surface Brillouin zone. Valence charge density along several planes show excess of charge around kink atom and the nature of bonding with other (5 3 2) atoms.     

The role of van der Waals interaction in the tilted binding of amine molecules to the Au(111) surface

We present the results of ab initio electronic structure calculations for the adsorption characteristics of three amine molecules on Au(111), which show that the inclusion of van der Waals interactions between the isolated molecule and the surface leads in general to good agreement with experimental data on the binding energies. Each molecule, however, adsorbs with a small tilt angle (between -5 and 9°). For the specific case of 1,4-diaminobenzene (BDA) our calculations reproduce the larger tilt angle (close to 24°) measured by photoemission experiments, when intermolecular (van der Waals) interactions (for about 8% coverage) are included. These results point not only to the important contribution of van der Waals interactions to molecule-surface binding energy, but also that of intermolecular interactions, often considered secondary to that between the molecule and the surface, in determining the adsorption geometry and pattern formation.     

Non-polynomial quintic spline solution for the system of third order boundary-value problems

In this paper we develop a non-polynomial quintic spline function to approximate the solution of third order linear and non-linear boundary value problems associated with odd-order obstacle problems. Such problems arise in physical oceanography and can be studied in the framework of variational inequality theory. The class of methods are second and fourth order convergent. End equations of the splines are derived and truncation error is obtained. Two numerical examples are given to illustrate the applicability and efficiency of proposed method. It is shown that the new method gives approximations, which are better than those produced by other methods.     

Off-lattice pattern recognition scheme for kinetic Monte Carlo simulations

We report the development of a pattern-recognition scheme for the off-lattice self-learning kinetic Monte Carlo (KMC) method, one that is simple and flexible enough that it can be applied to all types of surfaces. In this scheme, to uniquely identify the local environment and associated processes involving three-dimensional (3D) motion of an atom or atoms, space around a central atom is divided into 3D rectangular boxes. The dimensions and the number of 3D boxes are determined by the accuracy with which a process needs to be identified and a process is described as the central atom moving to a neighboring vacant box accompanied by the motion of any other atom or atoms in its surrounding boxes. As a test of this method to we apply it to examine the decay of 3D Cu islands on the Cu(100) and to the surface diffusion of a Cu monomer and a dimer on Cu(111) and compare the results and computational efficiency to those available in the literature.