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.

Deep exclusive π+\pi^+ electroproduction off the proton at CLAS

The exclusive electroproduction of $\pi^+$ above the resonance region was studied using the $\rm{CEBAF}$ Large Acceptance Spectrometer ($\rm{CLAS}$) at Jefferson Laboratory by scattering a 6 GeV continuous electron beam off a hydrogen target. The large acceptance and good resolution of $\rm{CLAS}$, together with the high luminosity, allowed us to measure the cross section for the $\gamma^* p \to n \pi^+$ process in 140 ($Q^2$, $x_B$, $t$) bins: $0.16<x_B<0.58$, 1.6 GeV$^2<$$Q^2$$<4.5$ GeV$^2$ and 0.1 GeV$^2<$$-t$$<5.3$ GeV$^2$. For most bins, the statistical accuracy is on the order of a few percent. Differential cross sections are compared to two theoretical models, based either on hadronic (Regge phenomenology) or on partonic (handbag diagram) degrees of freedom. Both can describe the gross features of the data reasonably well, but differ strongly in their ingredients. If the handbag approach can be validated in this kinematical region, our data contain the interesting potential to experimentally access transversity Generalized Parton Distributions.Comment: 18pages, 21figures,2table

Fish remains, mostly otoliths, from the non-marine early Miocene of Otago, New Zealand

Fish remains described from the early Miocene lacustrine Bannockburn Formation of Central Otago, New Zealand, consist of several thousand otoliths and one skeleton plus another disintegrated skull. One species, Mataichthys bictenatus Schwarzhans, Scofield, Tennyson, and T. Worthy gen. et sp. nov., an eleotrid, is established on a skeleton with otoliths in situ. The soft embedding rock and delicate, three−dimensionally preserved fish bones were studied by CT−scanning technology rather than physical preparation, except where needed to extract the otolith. Fourteen species of fishes are described, 12 new to science and two in open nomenclature, representing the families Galaxiidae (Galaxias angustiventris, G. bobmcdowalli, G. brevicauda, G. papilionis, G. parvirostris, G. tabidus), Retropinnidae (Prototroctes modestus, P. vertex), and Eleotridae (Mataichthys bictenatus, M. procerus, M. rhinoceros, M. taurinus). These findings prove that most of the current endemic New Zealand/southern Australia freshwater fish fauna was firmly established in New Zealand as early as 19–16 Ma ago. Most fish species indicate the presence of large fishes, in some cases larger than Recent species of related taxa, for instance in the eleotrid genus Mataichthys when compared to the extant Gobiomorphus. The finding of a few otoliths from marine fishes corroborates the age determination of the Bannockburn Formation as the Altonian stage of the New Zealand marine Tertiary stratigraphy.Werner Schwarzhans, R. Paul Scofield, Alan J.D. Tennyson, Jennifer P. Worthy, and Trevor H. Worth

Orogenic versus extensional settings for regional metamorphism: Knoydartian events in the Moine Supergroup revisited

© 2009 Geological Society of LondonConsiderable debate exists over the tectonic regimes associated with mid-Neoproterozoic metamorphism of the Moine Supergroup, NW Scotland. Published pressure conditions imply burial to 35-40 km, a potential doubling of crustal thickness, and hence a substantial collisional event. Re-evaluation using updated thermodynamic software suggests more modest peak pressures of c. 7.5 kbar implying burial to c. 21 km. The revised P-T path has a comparatively flat clockwise evolution from early high geothermal gradient conditions. The revised P-T conditions suggest that c. 800 Ma crustal thickening within the Moine Supergroup was less significant than previously envisaged and possibly preceded by extension. © 2009 Geological Society of London.K.A. Cutts, M. Hand, D.E. Kelsey and R.A. Stracha

<em>P</em>–<em>T</em> constraints and timing of Barrovian metamorphism in the Shetland Islands, Scottish Caledonides: implications for the structural setting of the Unst ophiolite

<p>An integrated <em>in situ</em> monazite laser-ablation inductively coupled plasma mass spectrometry and metamorphic equilibria study is used to establish the <em>P</em>–<em>T</em> conditions and timing of Barrovian metamorphism in the Shetland Islands, Scottish Caledonides. The results have implications for the structural setting of the Unst ophiolite, which was obducted onto metasedimentary rocks of the Dalradian Supergroup. Metapelites in the footwall of the ophiolite yield U–Pb ages between 462 and 451 Ma with <em>P</em>–<em>T</em> conditions varying from <em>c</em>. 7.5 kbar and 550 °C directly below the ophiolite to <em>c</em>. 10 kbar and 775 °C at structurally deeper levels. The timing of peak metamorphism corresponds closely to that of Grampian (<em>c</em>. 450–470 Ma) metamorphism in mainland Scotland and Ireland, and Taconic (<em>c</em>. 450–460 Ma) metamorphism in the Appalachians, thus confirming the near-synchroneity of this important arc accretion event along the Laurentian margin. There is a significant metamorphic contrast between the low-grade rocks associated with the Unst ophiolite and the <em>P</em>–<em>T</em> conditions recorded in its footwall. If published K–Ar ages of <em>c.</em> 470 Ma broadly record obduction of the ophiolite, its present basal contact is probably a younger tectonic break that was associated with the excision of at least <em>c</em>. 10 km of crustal section. </p

Numerical Studies of Three and Four HAWT Array Using LES and IDDES

The use of wind farms has been growing significantly in the past decades. However, harvesting the wind energy, from wind turbine farms poses significant challenges associated with the complex fluid dynamics such as wind turbine wake flow, wind-tower interactions, blade-wake interactions, etc. The present studies concern the aerodynamics and aeroacoustics studies of HAWT arranged in a 3 and 4 four array configuration. The computational studies are performed using the large-eddy simulation (LES) and improved delayed detached eddy simulation (IDDES) approach. The simulations were performed for a Reynolds number, Re = 1.2 x 105 based on the NACA0012 airfoil chord and free-stream velocity. The results show that the LES and IDDES approaches capture very well the flow physics associated with the wind turbine

P-T constraints and timing of Barrovian metamorphism in the Shetland Islands, Scottish Caledonides: implications for the structural setting of the Unst ophiolite

An integrated in situ monazite laser-ablation inductively coupled plasma mass spectrometry and metamorphic equilibria study is used to establish the P–T conditions and timing of Barrovian metamorphism in the Shetland Islands, Scottish Caledonides. The results have implications for the structural setting of the Unst ophiolite, which was obducted onto metasedimentary rocks of the Dalradian Supergroup. Metapelites in the footwall of the ophiolite yield U–Pb ages between 462 and 451 Ma with P–T conditions varying from c. 7.5 kbar and 550 °C directly below the ophiolite to c. 10 kbar and 775 °C at structurally deeper levels. The timing of peak metamorphism corresponds closely to that of Grampian (c. 450–470 Ma) metamorphism in mainland Scotland and Ireland, and Taconic (c. 450–460 Ma) metamorphism in the Appalachians, thus confirming the near-synchroneity of this important arc accretion event along the Laurentian margin. There is a significant metamorphic contrast between the low-grade rocks associated with the Unst ophiolite and the P–T conditions recorded in its footwall. If published K–Ar ages of c. 470 Ma broadly record obduction of the ophiolite, its present basal contact is probably a younger tectonic break that was associated with the excision of at least c. 10 km of crustal section.K. A. Cutts, M. Hand, D. E. Kelsey and R. A. Stracha