Pedro L. de Andrés

Weakly interacting molecular layer of spinning C60 molecules on TiO2 (110) surfaces.

The adsorption of C(60), a typical acceptor organic molecule, on a TiO(2) (110) surface has been investigated by a multitechnique combination, including van der Waals density functional calculations. It is shown that the adsorbed molecules form a weakly interacting molecular layer, which sits on the fivefold-coordinated Ti that is confined between the prominent bridging oxygen rows (see figure).

Hydrogen Embrittlement of High Strength Steels

Hydrogen embrittlement is believed to be one of the main reasons for cracking of structures under stress. High strength steels in these structures often include a ferritic core made of alpha-iron (body centered cubic lattice). We compute the interaction of atomic hydrogen with iron using first principles. The interstitial hydrogen can be placed in two high symmetry positions: octahedral and tetrahedral sites. Our calculations provide diffusion barriers between these sites. These barriers have been analyzed to understand the propagation of hydrogen through the iron lattice. We analyze how these barriers can be modified by the hydrogen concentration. The results show the main site for high and low hydrogen density and they show the diffusion coefficient variation by the hydrogen density.

Patterson function from low-energy electron diffraction measured intensities and structural discrimination

Surface patterson functions have been derived by direct inversion of experimental low-energy electron diffraction I-V spectra measured at multiple incident angles. The direct inversion is computationally simple and can beused to discriminate between different structural models. (1×1) YSi 2 epitaxial layers grown on Si(111) have been used to illustrate the analysis. We introduce a suitable R factor for the Patterson function to make the structural discrimination as objective as possible. From six competing models needed to complete the geometrical search, four could easily be discarded, achieving a very significant and useful reduction in the parameter space to be explored by standard dynamical low-energy electron diffraction methods. The amount and quality of data needed for this analysis is discussed.

Advances in direct methods in LEED: the diffuse LEED pattern as a hologram

Abstract The direct analysis of the standard diffraction techniques used routinely to determine surface structures is one of the key points for their future. We emphasize that a diffuse low-energy electron diffraction (DLEED) pattern may not only be used in a conventional way, but may also be reinterpreted as a hologram. This provides a rapid new direct method in LEED, and allows to get directly geometrical information without the necessity of an expensive trial-and-error search. We discuss how different parameters may influence the holographic reconstruction process.

A FORTRAN-90 Low-Energy Electron Diffraction program (LEED90 v1.1)

We describe a FORTRAN-90 program to compute low-energy electron diffraction I(V) curves. Plane-waves and layer doubling are used to compute the inter-layer multiple-scattering, while the intra-layer multiple-scattering is computed in the standard way expanding the wavefield on a basis of spherical waves. The program is kept as general as possible, in order to allow testing different parts of multiple-scattering calculations. In particular, it can handle non-diagonal t-matrices describing the scattering of non-spherical potentials, anisotropic vibrations, anharmonicity, etc. The program does not use old FORTRAN flavours, and has been written keeping in mind the advantage for parallelism brought forward by FORTRAN-90.

Hydrogen induced changes in structural properties of iron: Ab initio calculations

We have used Ab initio calculations to study the structural changes produced by the inclusion of light impurities into pure bcc iron. In order to obtain a clear picture of the mechanics of the phase changes Bain’s pathway was studied in detail for pure iron. The position that hydrogen atoms tend to occupy at high densities favours octahedral sites inside the bcc matrix, producing an internal stress field that suggests a deformation that matches the prediction of martensitic transformation predicted by Bain’s pathway. We have used Density Functional Theory in order to optimize the structures studied, obtaining the enthalpy of the configuration as a function of c/a, allowing a better understanding of the dynamics of the process of phase changes.

Molecular t-matrices for Low-Energy Electron Diffraction (TMOL v1.1) ☆

We describe a FORTRAN-90 program that computes scattering t-matrices for a molecule. These can be used in a Low-Energy Electron Diffraction program to solve the molecular structural problem very efficiently. The intramolecular multiple scattering is computed within a Dyson-like approach, using free space Green propagators in a basis of spherical waves. The advantage of this approach is related to exploiting the chemical identity of the molecule, and to the simplicity to translate and rotate these t-matrices without performing a new multiple-scattering calculation for each configuration. FORTRAN-90 routines for rotating the resulting t-matrices using Wigner matrices are also provided.

How Au Outperforms Pt in the Catalytic Reduction of Methane Towards Ethane and Molecular Hydrogen

Within the context of a “hydrogen economy”, it is paramount to guarantee a stable supply of molecular hydrogen to devices such as fuel cells. Besides, catalytic conversion of the environmentally harmful methane into ethane, which has a significantly lower Global Warming Potential, is an important endeavour. Herein we propose a novel proof-of-concept mechanism to accomplish both tasks simultaneously. We provide transition-state barriers and reaction Helmholtz free energies obtained from first-principles Density Functional Theory by taking account vibrations for

On-surface azo-coupling reaction of p-aminophenol on Cu(110)

2\hbox {CH}_4(\hbox {g}) \rightarrow \hbox {C}_2\hbox {H}_6(\hbox {g}) + \hbox {H}_2(\hbox {g})