77 research outputs found
Klimaat Respons Database 2.0
Met de ‘Klimaat Respons Database’ kunnen niet alleen wetenschappers maar ook beleidsmakers, planologen, natuurbeheerders en andere geïnteresseerden zelf in kaart brengen welke gevolgen klimaatverandering op flora en fauna kan hebben. Zo is te zien dat voor de Cetti’s zanger, een vogeltje uit zuidelijke streken, de geschikte klimaatzone langzaam opschuift naar het noorden, naar Nederland. En de soort wordt inderdaad al regelmatig in Nederland aangetroffen
Dynamical Quantum Processes of Molecular Beams at Surfaces: Dissociative Adsorption of Hydrogen on Metal Surfaces
Due to the improvement of computer power and the development of efficient
algorithms it is now possible to combine high-dimensional quantum dynamical
calculations of the dissociative adsorption of molecular beams with reliable
ab-initio potential energy surfaces (PES). In this brief review two recent
examples of such studies of the systems H_2/Cu(111), where adsorption is
hindered by a noticeable energy barrier, and H_2/Pd(100), where activated as
well as non-activated paths to adsorption exist, are presented. The effect of
lateral surface corrugations on the sticking probability in the tunneling and
the classical regime and the role of additional parallel momentum are discussed
in the context of the H_2/Cu(111) results. For the system H_2/Pd(100) it is
shown that the initial decrease of the sticking probability with increasing
kinetic energy, which is usually attributed to a precursor mechanism, can be
explained by dynamical steering. In addition, the influence of rotation on the
adsorption and desorption dynamics is examined.Comment: RevTeX, 22 pages, 6 figure
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.
Decline in Surface Melt Duration on Larsen C Ice Shelf Revealed by ASCAT Scatterometer
Surface melting has been contributing to the surface lowering and loss of firn air content on Larsen C Ice Shelf since at least the mid-1990s. Where the amount of melting and refreezing is significant, the firn can become impermeable and begin to support ponds of surface meltwater such as have been implicated in ice shelf collapse. Although meteorological station data indicated an increase in melt on the Antarctic Peninsula over the second half of the 20th century, the existing Ku-band Quick Scatterometer (QuikSCAT) time series is too short (1999–2009) to detect any significant 21st century trends. Here we investigate a longer 21st century period by extending the time series to 2017 using the C-band Advanced Scatterometer (ASCAT). We validate our recent observations with in situ weather station data and, using a firn percolation model, explore the sensitivity of scatterometry to water at varying depths in the firn. We find that active microwave C-band (5.6-cm wavelength) instruments can detect water at depths of up to 0.75 m below a frozen firn layer. Our longer scatterometry time series reveals that Larsen C Ice Shelf has experienced a decrease in melt season length of 1–2 days per year over the past 18 years consistent with decreasing summer air temperatures. Only in western inlets, where föhn winds drive melt, has the annual melt duration increased during this period
Adsorption of CO on a Platinum (111) surface - a study within a four-component relativistic density functional approach
We report on results of a theoretical study of the adsorption process of a
single carbon oxide molecule on a Platinum (111) surface. A four-component
relativistic density functional method was applied to account for a proper
description of the strong relativistic effects. A limited number of atoms in
the framework of a cluster approach is used to describe the surface. Different
adsorption sites are investigated. We found that CO is preferably adsorbed at
the top position.Comment: 23 Pages with 4 figure
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