Ab initio Quantum and ab initio Molecular Dynamics of the Dissociative Adsorption of Hydrogen on Pd(100)

The dissociative adsorption of hydrogen on Pd(100) has been studied by ab initio quantum dynamics and ab initio molecular dynamics calculations. Treating all hydrogen degrees of freedom as dynamical coordinates implies a high dimensionality and requires statistical averages over thousands of trajectories. An efficient and accurate treatment of such extensive statistics is achieved in two steps: In a first step we evaluate the ab initio potential energy surface (PES) and determine an analytical representation. Then, in an independent second step dynamical calculations are performed on the analytical representation of the PES. Thus the dissociation dynamics is investigated without any crucial assumption except for the Born-Oppenheimer approximation which is anyhow employed when density-functional theory calculations are performed. The ab initio molecular dynamics is compared to detailed quantum dynamical calculations on exactly the same ab initio PES. The occurence of quantum oscillations in the sticking probability as a function of kinetic energy is addressed. They turn out to be very sensitive to the symmetry of the initial conditions. At low kinetic energies sticking is dominated by the steering effect which is illustrated using classical trajectories. The steering effects depends on the kinetic energy, but not on the mass of the molecules. Zero-point effects lead to strong differences between quantum and classical calculations of the sticking probability. The dependence of the sticking probability on the angle of incidence is analysed; it is found to be in good agreement with experimental data. The results show that the determination of the potential energy surface combined with high-dimensional dynamical calculations, in which all relevant degrees of freedon are taken into account, leads to a detailed understanding of the dissociation dynamics of hydrogen at a transition metal surface.Comment: 15 pages, 9 figures, subm. to Phys. Rev.

Brane cosmology with curvature corrections

We study the cosmology of the Randall-Sundrum brane-world where the Einstein-Hilbert action is modified by curvature correction terms: a four-dimensional scalar curvature from induced gravity on the brane, and a five-dimensional Gauss-Bonnet curvature term. The combined effect of these curvature corrections to the action removes the infinite-density big bang singularity, although the curvature can still diverge for some parameter values. A radiation brane undergoes accelerated expansion near the minimal scale factor, for a range of parameters. This acceleration is driven by the geometric effects, without an inflaton field or negative pressures. At late times, conventional cosmology is recovered.Comment: RevTex4, 8 pages, no figures, minor change

Fit for what?: towards explaining Battlegroup inaction

The thrust of this paper concerns the case of the European Battlegroup (BG) non-deployment in late 2008, when the United Nations requested European military support for the United Nations Organisation Mission peacekeeping force in the Democratic Republic of the Congo (DRC). The argument is built on the fact that when, in official documents, the EU approaches the European security and ESDP/CSDP's military crisis management policy and interventions, it makes strong references to the United Nations and the UN Charter Chapter VII's mandate of restoring international peace and security. Such references make it seem that supporting the UN when it deals with threats and crises is a primary concern of the EU and the member states. These allusions lead to the main contention of this paper, that there is much ambivalence in these indications. The paper develops its argument from one key hypothesis; namely, that the non-deployment of a European BG in the DRC, at the end of 2008, constitutes a useful case study for detecting a number of ambiguities of the EU in respect of its declarations in the official documents establishing the European military crisis management intervention structure

Broadband observations of the afterglow of GRB 000926: Observing the effect of inverse Compton scattering

GRB 000926 has one of the best-studied afterglows to date, with multiple X-ray observations, as well as extensive multifrequency optical and radio coverage. Broadband afterglow observations, spanning from X-ray to radio frequencies, provide a probe of the density structure of the circumburst medium, as well as of the ejecta energetics, geometry, and physical parameters of the relativistic blast wave resulting from the explosion. We present an analysis of Chandra X-Ray Observatory observations of this event, along with Hubble Space Telescope and radio monitoring data. We combine these data with ground-based optical and IR observations and fit the synthesized afterglow light curve using models where collimated ejecta expand into a surrounding medium. We find that we can explain the broadband light curve with reasonable physical parameters if the cooling is dominated by inverse Compton scattering. For this model, an excess due to inverse Compton scattering appears above the best-fit synchrotron spectrum in the X-ray band. No previous bursts have exhibited this component, and its observation would imply that the GRB exploded in a moderately dense (n ∼ 30 cm-3) medium, consistent with a diffuse interstellar cloud environment

GRB 010921: Strong limits on an underlying supernova from the Hubble Space Telescope

GRB 010921 was the first HETE-2 gamma-ray burst (GRB) to be localized via its afterglow emission. The low redshift of the host galaxy, z = 0.451, prompted us to undertake intensive multicolor observations with the Hubble Space Telescope with the goal of searching for an underlying supernova (SN) component. We do not detect any coincident SN to a limit 1.33 mag fainter than SN 1998bw at 99.7% confidence, making this one of the most sensitive searches for an underlying SN. Analysis of the afterglow data allows us to infer that the GRB was situated behind a net extinction (Milky Way and the host galaxy) of Av ∼ 1.8 mag in the observer frame. Thus, had it not been for such heavy extinction, our data would have allowed us to probe for an underlying SN with brightness approaching those of more typical Type Ib/c SNe

Common variants near MC4R are associated with fat mass, weight and risk of obesity.

To identify common variants influencing body mass index (BMI), we analyzed genome-wide association data from 16,876 individuals of European descent. After previously reported variants in FTO, the strongest association signal (rs17782313, P = 2.9 x 10(-6)) mapped 188 kb downstream of MC4R (melanocortin-4 receptor), mutations of which are the leading cause of monogenic severe childhood-onset obesity. We confirmed the BMI association in 60,352 adults (per-allele effect = 0.05 Z-score units; P = 2.8 x 10(-15)) and 5,988 children aged 7-11 (0.13 Z-score units; P = 1.5 x 10(-8)). In case-control analyses (n = 10,583), the odds for severe childhood obesity reached 1.30 (P = 8.0 x 10(-11)). Furthermore, we observed overtransmission of the risk allele to obese offspring in 660 families (P (pedigree disequilibrium test average; PDT-avg) = 2.4 x 10(-4)). The SNP location and patterns of phenotypic associations are consistent with effects mediated through altered MC4R function. Our findings establish that common variants near MC4R influence fat mass, weight and obesity risk at the population level and reinforce the need for large-scale data integration to identify variants influencing continuous biomedical traits