105,410 research outputs found
7D Quantum Dynamicsof H2 scattering form Cu(111): the accuracy of the phonon sudden approximation
No full textThe correct prediction of elementary processes occurring when H 2 scatters from a metal surface is one of the main challenges of surface science. In the field, the scattering of H 2 from Cu(111) has been considered a prototype system for activated dissociative chemisorption. Experimental and theoretical work suggested that a proper description of some scattering experiments on this system might require going beyond the static surface approximation, to consider how the motion of the Cu atoms affects the scattering event. Previous work suggested that important effects of phonons on the dynamics can be incorporated in the Potential Energy Surface (PES) by including four degrees of freedom, that have approximately additive couplings with the hydrogen molecule: the 3 dimensional motion of the nearest 1st layer copper atom and the displacement of the nearest 2nd layer copper atom along the direction perpendicular to the surface [3]. In the present work, we extend the 6D dynamical model by including the perpendicular motion of the 2nd layer surface atom and we study this novel dynamical model with two techniques: an approximate method based on the Phonon Sudden Approximation (PSA) and an exact description using 7D wavepacket quantum dynamics. We consider how the inclusion and the excitation of the lattice degree of freedom affect some relevant processes: dissociative chemisorption, vibrational excitation of H2 and state-to-state scattering probabilities fully resolved with respect to the vibrational states of the surface. We show that the PSA works in an excellent way for the system, thereby suggesting that this might be a viable way to study higher dimensional quantum models, incorporating four surface degrees of freedom that appear to be most relevant for H2 scattering.Fil: Bonfanti, Matteo. Leiden University; Países BajosFil: Somers, Mark F.. Leiden University; Países BajosFil: Díaz, Cristina. Universidad Autónoma de Madrid; EspañaFil: Busnengo, Heriberto Fabio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); Argentina. Universidad Nacional de Rosario. Facultad de Ciencias Exactas, Ingeniería y Agrimensura; ArgentinaFil: Kroes, Geert Jan. Leiden University; Países Bajo
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Toward detection of electron-hole pair excitation in H-atom collisions with Au(111): Adiabatic molecular dynamics with a semi-empirical full-dimensional potential energy surface.
No full textWe report an analytic potential energy surface (PES) based on several hundred DFT energies for H interacting with a Au(111) surface. Effective medium theory is used to fit the DFT data, which were obtained for the Au atoms held at their equilibrium positions. This procedure also provides an adequate treatment of the PES for displacements of Au atoms that occur during scattering of H atoms. The fitted PES is compared to DFT energies obtained from ab initio molecular dynamics trajectories. We present molecular dynamics simulations of energy and angle resolved scattering probabilities at five incidence angles at an incidence energy, Ei = 5 eV, and at a surface temperature, TS = 10 K. Simple single bounce trajectories are important at all incidence conditions explored here. Double bounce events also make up a significant fraction of the scattering. A qualitative analysis of the double-bounce events reveals that most occur as collisions of an H-atom with two neighboring surface gold atoms. The energy losses observed are consistent with a simple binary collision model, transferring typically less than 150 meV to the solid per bounce
Appropriate Similarity Measures for Author Cocitation Analysis
No full textWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Surface Temperature Effects on Dissociative Chemisorption of H2 on Cu(100)
No full textAt present, much remains unknown about the effect surface phonons and surface temperature may have on the reactivity of molecules at surfaces. Here, this problem is addressed for the dissociation of H2 on copper, which is a benchmark system for activated dissociative chemisorption on a metal surface. Ab Initio Molecular Dynamics (AIMD) calculations, quantum dynamics calculations using a static surface model, and experiments are reported and compared on the effects of surface temperature (Ts) on the initial state-selected reaction of (v = 0, j = 8) and (v = 1, j = 4) H2 scattering from Cu(100) and the orientational dependence of this process, at Ts = 1030 K. In the theory, the specific reaction parameter approach to density functional theory (SRP-DFT) was used. The rotational quadrupole alignment parameters computed for H2 reacting on the hot Cu(100) surface (1030 K) are smaller than the values computed with a static surface model. The initial state-selected reaction probabilities computed with AIMD for the hot surface are shifted to lower energies, by 40–60 meV, and broadened with respect to static surface quantum dynamics results. The rotational quadrupole alignment parameters computed with AIMD are in good agreement with experiment if the experimental results are shifted to lower energies by 100–150 meV. The AIMD average desorption energies underestimate the experimental results by 150–180 meV. Our study shows that the H2 + Cu(100) system presents a useful benchmark for the simultaneously accurate description of dissociative chemisorption and surface thermal effects on reaction because surface temperature effects on the (100) surface are much more pronounced than on the Cu(111) surface, while the (100) face does not yet show surface reconstruction at temperatures of interest to associative desorption experiment
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Hydrogen dissociation on Cu(111) : the influence of lattice motion. Part 1
No full textAbstract: We have studied the effect of lattice displacement on the interaction of H(2) with the Cu(111) surface using the Specific Reaction Parameter (SRP) approach to Density Functional Theory (DFT). We have systematically investigated how the motion of the surface atoms affects some features of the Potential Energy Surface (PES), such as the dissociation barrier height and the barrier geometry corresponding to some representative reaction pathways, and the anisotropy of the potential at these geometries. This analysis has allowed us to identify the surface degrees of freedom that are likely to be most relevant for H(2) dissociation. In particular, we have found that the lattice coordinate displacements that have the largest effect on the H(2)/Cu(111) DFT-SRP barrier heights and locations concern the motion of the 1st layer and 2nd layer Cu atoms in the Z direction, and motion of the 1st layer atoms in the directions parallel to the surface. Whereas the first degree of freedom mostly affects the barrier geometry, the second and third motions can lower or raise the barrier height. The latter effect cannot be described with the usual surface oscillator dynamical models employed in the past to include surface motion, and its dynamical influence on the dissociative adsorption needs to be further investigated
The construction of Karen Karnak: The multi-author-function
No full textThis thesis is situated within the comparatively recent developments of Web 2.0 and the emergence of interactive WikiMedia, and explores the mode of authorship within a Read/Write culture compared to that of a Read/Only tradition. The hypothesis of this study is that the role of the audience has become merged with the author, and as such, represents new functions and attributes, distinct from a more conventional concept of authorship, in which the roles of audience and author are more separate. Read/Write and participatory culture, as defined by this study, is focused on collaboration, and includes the influences of D.I.Y. culture, Open-Source practices and the production of text by multiple authors. Multi-authorship presents a re-thinking of several concepts which support the notion of the individual author, since the focus of multi-authorship is not on attribution and ownership of a finished text, but on the continued malleability of a text. Modes of multi-authorship, demonstrated in the use of the pseudonyms Alan Smithee and Karen Eliot, represent declarative authors whose names signify multiple origins, whilst concurrently indicating a distinct body of work. The function of these names form an important context to this study, since primary research involves the construction of an experimental mode of multi-authorship utilising WikiMedia technology and the interaction of thirty nine participants, who are invited to create a body of work under the collective pseudonym Karen Karnak. The data generated by this experiment is analysed using aspects of Michel Foucault's author-function to identify and determine power structures inherent in the WikiMedia context. The interplay of power structures, including concepts such as identity, ownership and the body of work, affect the resulting mode of authorship and contribute to the construction of Karen Karnak, suggesting further areas of research into the emerging multi-author
Contribution of Information and Communication Technology (ICT) in Country’S H-Index
No full textThe aim of this study is to examine the effect of Information and Communication Technology (ICT) development on country’s scientific ranking as measured by H-index. Moreover, this study applies ICT development sub-indices including ICT Use, ICT Access and ICT skill to find the distinct effect of these sub-indices on country’s H-index. To this purpose, required data for the panel of 14 Middle East countries over the period 1995 to 2009 is collected. Findings of the current study show that ICT development increases the H-index of the sample countries. The results also indicate that ICT Use and ICT Skill sub-indices positively contribute to higher H-index but the effect of ICT access on country’s H-index is not clear
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