Competition between electron and phonon excitations in the scattering of nitrogen atoms and molecules off tungsten and silver surfaces

We investigate the role played by electron-hole pair and phonon excitations in the interaction of reactive gas molecules and atoms with metal surfaces. We present a theoretical framework that allows us to evaluate within a full-dimensional dynamics the combined contribution of both excitation mechanisms while the gas particle-surface interaction is described by an ab-initio potential energy surface. The model is applied to study energy dissipation in the scattering of N$_2$ on W(110) and N on Ag(111). Our results show that phonon excitation is the dominant energy loss channel whereas electron-hole pair excitations represent a minor contribution. We substantiate that, even when the energy dissipated is quantitatively significant, important aspects of the scattering dynamics are well captured by the adiabatic approximation.Comment: 4pages and 3 figure

Density functional theory calculations of nitrogen adsorption features on Fe(111) surfaces

Trabajo presentado al Workshop on Controlled Atomic Dynamics on Solid Surfaces: Atom an Molecular Manipulation, celebrado en Donostia-San sebastián (España) del 13 al 16 de Mayo de 2013.The interaction of nitrogen with metal surfaces has been one of the most popular topics of research in surface science for the last decades. This is due in part to the industrial importance of ammonia synthesis, typically obtained from nitrogen and hydrogen catalyzed over iron-based compounds. The rate limiting step in ammonia synthesis is the adsorption and dissociation of nitrogen on the catalyst surface. In Fe surfaces, the reactivity of the process depends on the face, the Fe(111) and Fe(211) surfaces being the most reactive ones. Although Fe(111) is the most reactive iron face for N2 dissociation, the dynamics of such process has not been analyzed in detail. In this work we present exhaustive calculations of the interaction of nitrogen atoms and molecules with the Fe(111) surface. These calculations set the basis for subsequent analysis of the N2 dissociation dynamics. We perform Density functional Theory spin-polarized calculations using VASP code. We first study the relaxation of the Fe(111) surface, which was a matter of controversy in the past. From here, we calculate the interaction energy of nitrogen atoms and molecules when approaching the Fe(111) surface. Our results show the preferred adsorption paths and sites for nitrogen adsorption, as well as the adsorption energies. We finally discuss the dynamics of the dissociation process and make the link with the high reactivity properties of the surface.Peer reviewe

Influence of the van der Waals interaction in the dissociation dynamics of N2 on W(110) from first principles

Under the terms of the Creative Commons Attribution (CC BY) license.Using ab initio molecular dynamics (AIMD) calculations, we investigate the role of the van der Waals (vdW) interaction in the dissociative adsorption of N2 on W(110). Hitherto, existing classical dynamics calculations performed on six-dimensional potential energy surfaces based on density functional theory (DFT), and the semi-local PW91 and RPBE [Hammer et al. Phys. Rev. B 59, 7413 (1999)] exchange-correlation functionals were unable to fully describe the dependence of the initial sticking coefficient on the molecular beam incidence conditions as found in experiments. N2 dissociation on W(110) was shown to be very sensitive not only to short molecule-surface distances but also to large distances where the vdW interaction, not included in semilocal-DFT, should dominate. In this work, we perform a systematic study on the dissociative adsorption using a selection of existing non-local functionals that include the vdW interaction (vdW-functionals). Clearly, the inclusion of the non-local correlation term contributes in all cases to correct the unrealistic energy barriers that were identified in the RPBE at large molecule-surface distances. Among the tested vdW-functionals, the original vdW-DF by Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)] and the ulterior vdW-DF2 give also an adequate description of the N2 adsorption energy and energy barrier at the transition state, i.e., of the properties that are commonly used to verify the quality of any exchange-correlation functional. However, the results of our AIMD calculations, which are performed at different incidence conditions and hence extensively probe the multi-configurational potential energy surface of the system, do not seem as satisfactory as the preliminary static analysis suggested. When comparing the obtained dissociation probabilities with existing experimental data, none of the used vdW-functionals seems to provide altogether an adequate description of the N2/W(110) interaction at short and large distances.The authors acknowledge financial support by the Gobierno Vasco-UPV/EHU project IT756-13 and the Spanish Ministerio de Economía y Competitividad (Grant No. FIS2013-48286-C02-02-P).Peer Reviewe

La función cultural de las especies arvenses en los huertos domésticos de dos barrios periféricos de la ciudad de Santa Rosa (La Pampa) = The cultural role of weeds in home gardens of two outlying neighborhoods of Santa Rosa City (La Pampa)

Este trabajo pretende comprender cuales son los conocimientos, usos y significados que conservan los horticultores con el entorno vegetal, más precisamente con las arvenses, definidas como especies vegetales espontáneas que crecen junto a los cultivos de interés. La investigación se realizó en la ciudad de Santa Rosa en los barrios Zona Norte y Villa Germinal. El tema de estudio fue abordado desde la etnobotánica y agroecología. Para la recolección de los datos se utilizaron herramientas etnobotánicas. Se colectaron e identificaron 57 especies. Aproximadamente la mitad de los informantes elimina las plantas arvenses sin hacer uso alguno de ellas. Los informantes restantes las emplean principalmente como abono para sus huertas y como alimento para sus animales domésticos. En cuanto a la bibliografía consultada, los usos más frecuentes se remiten a preparados para fines medicinales. Cynodon dactylon (“gramilla”) fue la especie con mayor frecuencia de aparición en las huertas siendo también una de las consideradas como “plaga”. Los resultados de esta investigación, establecen elementos de base para futuras líneas de investigación relacionadas con la agroecología, agricultura familiar y agricultura urbana, así como para trabajos de extensión en agencias gubernamentales y ONGs. Por último, esta clase de estudios pueden ser útiles en la visualización de demandas de la población que aún permanecen insatisfechas.The present work was aimed to examine knowledge, practices and significations kept by vegetable gardeners regarding plant communities, and particularly weeds as species growing spontaneously in cropping lands. This research was carried out in the “Zona Norte” and “Villa Germinal” neighborhoods of Santa Rosa city, La Pampa. The topic of study was approached under ethnobotany and agroecology criteria. Collection of data using ethnobotanical tools, allowed the identification of 57 weed species. After performing a survey in both neighborhoods, it was revealed that about half of gardeners eliminate weeds without making use of them. The other half did mention using weeds mainly as fertilizer for gardens, but also as food for own domestic animals. In this study, the grass Cynodon dactylon was the most frequently found weed in gardens, being commonly considered a pest for that reason. The results of this work provide basic information for future research lines focusing on agroecology, small-scale agriculture and urban farming, as well as extension works involving government agencies and non-governmental organizations. This kind of studies can be useful to identify people's unsatisfied demands.EEA AnguilFil: Fernández, L. Universidad Nacional de La Pampa. Facultad de Ciencias Exactas y Naturales; ArgentinaFil: Muiño, Walter Alejandro. Universidad Nacional de La Pampa. Facultad de Ciencias Exactas y Naturales; ArgentinaFil: Ermini, Pablo. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Anguil; Argentin

Interactions of Ar(9+) and metastable Ar(8+) with a Si(100) surface at velocities near the image acceleration limit

Auger LMM spectra and preliminary model simulations of Ar(9+) and metastable Ar(8+) ions interacting with a clean monocrystalline n-doped Si(100) surface are presented. By varying the experimental parameters, several yet undiscovered spectroscopic features have been observed providing valuable hints for the development of an adequate interaction model. On our apparatus the ion beam energy can be lowered to almost mere image charge attraction. High data acquisition rates could still be maintained yielding an unprecedented statistical quality of the Auger spectra.Comment: 34 pages, 11 figures, http://pikp28.uni-muenster.de/~ducree

Non-adiabatic effects during the dissociative adsorption of O2 at Ag(111)? A first-principles divide and conquer study

We study the gas-surface dynamics of O2 at Ag(111) with the particular objective to unravel whether electronic non-adiabatic effects are contributing to the experimentally established inertness of the surface with respect to oxygen uptake. We employ a first-principles divide and conquer approach based on an extensive density-functional theory mapping of the adiabatic potential energy surface (PES) along the six O2 molecular degrees of freedom. Neural networks are subsequently used to interpolate this grid data to a continuous representation. The low computational cost with which forces are available from this PES representation allows then for a sufficiently large number of molecular dynamics trajectories to quantitatively determine the very low initial dissociative sticking coefficient at this surface. Already these adiabatic calculations yield dissociation probabilities close to the scattered experimental data. Our analysis shows that this low reactivity is governed by large energy barriers in excess of 1.1 eV very close to the surface. Unfortunately, these adiabatic PES characteristics render the dissociative sticking a rather insensitive quantity with respect to a potential spin or charge non-adiabaticity in the O2-Ag(111) interaction. We correspondingly attribute the remaining deviations between the computed and measured dissociation probabilities primarily to unresolved experimental issues with respect to surface imperfections.Comment: 18 pages including 6 figure

Contribuciones al conocimiento de la flora de Malargüe. Nuevas citas para la provincia de Mendoza (Argentina) Parte II.

During botanical explorations carried out in Malargüe, between 2003 and 2005, 940 specimens were collected, and 39 taxa are recorded for the first time for Mendoza province, Argentina. Characteristics of habitat and life form are given for each taxon. Common names of some of these taxa are given, based on the oral information obtained from the local people.Este trabajo es producto de exploraciones botánicas llevadas a cabo, entre los años 2003 y 2005. en el sur de la provincia de Mendoza, lo cual permitió la colección de 940 especímenes en el área en estudio, de los cuales, 39 taxones se citan por primera vez para la flora de la provincia. Se describen características de hábitat y forma de vida al que corresponde cada taxón, y asimismo se dan nombres vulgares sobre la base de la información oral obtenida de los pobladores de las áreas estudiadas

Sugar composition and sugar-related parameters of honeys from the northern Iberian Plateau

This research was aimed to study the sugar composition of fifty-four representative artisanal honeys from the northern Iberian plateau. Moisture, specific rotation, and crystallization indexes were also determined. Sugars were analyzed by gas chromatography-flame ionization detector (GC-FID) after Pourtallier´s derivatization procedure. Fourteen carbohydrates were reliably quantified: two monosaccharides, five disaccharides, six trisaccharides and one tetrasaccharide. Honeydew honeys showed the highest disaccharides (6.71%) and trisaccharides (1.81%) averages and the lowest monosaccharides (63.10%) average, in contrast to heather honeys that had the lowest disaccharides (4.93%) and trisaccharides (0.69%) averages and the highest monosaccharides (70.96%). Chestnut honeys possessed low concentrations of monosaccharides, sucrose, trehalose, and trisaccharides. Clover and lavender honeys possessed high monosaccharide and disaccharide percentages. As expected, lavender samples showed the highest sucrose concentrations (0.05–5.18%). Isomaltose contents were particularly high in honeydew (1.17–2.49%) and chestnut (1.34–1.74%) samples, and low in lavender (0.6–1.16%) honeys, the latter also being low in raffinose (0.01‒0.05%). Moisture percentages (14.4–18.6%) indicated optimum honey ripeness. Averages for all groups of samples were levorotatory. In contrast to honeydew and chestnut honeys, lavender samples showed the fastest granulation tendency. In the analyzed honeys, the higher the percentage of isomaltose was, the lower the crystallization tendency the honeys exhibited.Junta de Castilla y León” (Spain) for supporting this research under the project BU007A0

Contribuciones al conocimiento de Ia flora de Malargüe. Nuevas citas para la provincia de Mendoza (Argentina). Parte I

In botanical explorations carried out in Malargüe, 601 specific and infraespecifics taxa were collected, 76 of those are recorded for the first time for Mendoza province, Argentina, Herbarium specimens as a reference, characteristics of habitat and life form are given for each taxon. Common names of some of these taxa based on the oral information obtained from the residents in the studied areas are given.En exploraciones botánicas llevadas a cabo en el Departamento Malargüe, se colectaron 601 taxones específicos e infraespecíficos, 76 de Ios cuales se citan por primera vez para la provincia de Mendoza, Argentina. Se proporciona al menos un ejemplar de herbario como referencia para cada taxón y se informa sobre características de hábitat y forma de vida al que corresponda cada uno. asimismo, se dan nombres vulgares sobre la base de la información oral obtenida de los pobladores de las áreas estudiadas