733 research outputs found
Origin of power laws for reactions at metal surfaces mediated by hot electrons
A wide range of experiments have established that certain chemical reactions
at metal surfaces can be driven by multiple hot electron mediated excitations
of adsorbates. A high transient density of hot electrons is obtained by means
of femtosecond laser pulses and a characteristic feature of such experiments is
the emergence of a power law dependence of the reaction yield on the laser
fluence . We propose a model of multiple inelastic scattering by hot
electrons, which reproduces this power law and the experimentally found
exponents of several experiments. All parameters are calculated within Density
Functional Theory and the Delta Self-Consistent Field method. With a simplified
assumption, the power law becomes exact and we obtain a simple and very useful
physical interpretation of the exponent , which represents the number of
adsorbate vibrational states participating in the reaction
Memory effects in non-adiabatic molecular dynamics at metal surfaces
We study the effect of temporal correlation in a Langevin equation describing
non-adiabatic dynamics at metal surfaces. For a harmonic oscillator the
Langevin equation preserves the quantum dynamics exactly and it is demonstrated
that memory effects are needed in order to conserve the ground state energy of
the oscillator. We then compare the result of Langevin dynamics in a harmonic
potential with a perturbative master equation approach and show that the
Langevin equation gives a better description in the non-perturbative range of
high temperatures and large friction. Unlike the master equation, this approach
is readily extended to anharmonic potentials. Using density functional theory
we calculate representative Langevin trajectories for associative desorption of
N from Ru(0001) and find that memory effects lowers the dissipation of
energy. Finally, we propose an ab-initio scheme to calculate the temporal
correlation function and dynamical friction within density functional theory
Vibrationally Mediated Control of Single Electron Transmission in Weakly Coupled Molecule-Metal Junctions
We propose a mechanism which allows one to control the transmission of single
electrons through a molecular junction. The principle utilizes the emergence of
transmission sidebands when molecular vibrational modes are coupled to the
electronic state mediating the transmission. We will show that if a
molecule-metal junction is biased just below a molecular resonance one may
induce the transmission of a single electron by externally exciting a
vibrational mode of the molecule. The analysis is quite general but requires
that the molecular orbital does not hybridize strongly with the metallic
states. As an example we perform a density functional theory (DFT) analysis of
a benzene molecule between two Au(111) contacts and show that exciting a
particular vibrational mode can give rise to transmission of a single electro
Hot electron mediated desorption rates calculated from excited state potential energy surfaces
We present a model for Desorption Induce by (Multiple) Electronic Transitions
(DIET/DIMET) based on potential energy surfaces calculated with the Delta
Self-Consistent Field extension of Density Functional Theory. We calculate
potential energy surfaces of CO and NO molecules adsorbed on various transition
metal surfaces, and show that classical nuclear dynamics does not suffice for
propagation in the excited state. We present a simple Hamiltonian describing
the system, with parameters obtained from the excited state potential energy
surface, and show that this model can describe desorption dynamics in both the
DIET and DIMET regime, and reproduce the power law behavior observed
experimentally. We observe that the internal stretch degree of freedom in the
molecules is crucial for the energy transfer between the hot electrons and the
molecule when the coupling to the surface is strong.Comment: Typos corrected. Comment on thermal ensemble Green function added in
appendix
Dislocation nucleation and vacancy formation during high-speed deformation of fcc metals
Recently, a dislocation free deformation mechanism was proposed by Kiritani
et al., based on a series of experiments where thin foils of fcc metals were
deformed at very high strain rates. In the experimental study, they observed a
large density of stacking fault tetrahedra, but very low dislocation densities
in the foils after deformation. This was interpreted as evidence for a new
dislocation-free deformation mechanism, resulting in a very high vacancy
production rate.
In this paper we investigate this proposition using large-scale computer
simulations of bulk and thin films of copper. To favour such a dislocation-free
deformation mechanism, we have made dislocation nucleation very difficult by
not introducing any potential dislocation sources in the initial configuration.
Nevertheless, we observe the nucleation of dislocation loops, and the
deformation is carried by dislocations. The dislocations are nucleated as
single Shockley partials.
The large stresses required before dislocations are nucleated result in a
very high dislocation density, and therefore in many inelastic interactions
between the dislocations. These interactions create vacancies, and a very large
vacancy concentration is quickly reached.Comment: LaTeX2e, 8 pages, PostScript figures included. Minor modifications
only. Final version, to appear in Philos. Mag. Let
Modelling of dislocation generation and interaction during high-speed deformation of metals
Recent experiments by Kiritani et al. have revealed a surprisingly high rate
of vacancy production during high-speed deformation of thin foils of fcc
metals. Virtually no dislocations are seen after the deformation. This is
interpreted as evidence for a dislocation-free deformation mechanism at very
high strain rates.
We have used molecular-dynamics simulations to investigate high-speed
deformation of copper crystals. Even though no pre-existing dislocation sources
are present in the initial system, dislocations are quickly nucleated and a
very high dislocation density is reached during the deformation.
Due to the high density of dislocations, many inelastic interactions occur
between dislocations, resulting in the generation of vacancies. After the
deformation, a very high density of vacancies is observed, in agreement with
the experimental observations. The processes responsible for the generation of
vacancies are investigated. The main process is found to be incomplete
annihilation of segments of edge dislocations on adjacent slip planes. The
dislocations are also seen to be participating in complicated dislocation
reactions, where sessile dislocation segments are constantly formed and
destroyed.Comment: 8 pages, LaTeX2e + PS figures. Presented at the Third Workshop on
High-speed Plastic Deformation, Hiroshima, August 200
Quantum corrected Langevin dynamics for adsorbates on metal surfaces interacting with hot electrons
We investigate the importance of including quantized initial conditions in
Langevin dynamics for adsorbates interacting with a thermal reservoir of
electrons. For quadratic potentials the time evolution is exactly described by
a classical Langevin equation and it is shown how to rigorously obtain quantum
mechanical probabilities from the classical phase space distributions resulting
from the dynamics. At short time scales, classical and quasiclassical initial
conditions lead to wrong results and only correctly quantized initial
conditions give a close agreement with an inherently quantum mechanical master
equation approach. With CO on Cu(100) as an example, we demonstrate the effect
for a system with ab initio frictional tensor and potential energy surfaces and
show that quantizing the initial conditions can have a large impact on both the
desorption probability and the distribution of molecular vibrational states
Simulation of Cu-Mg metallic glass: Thermodynamics and Structure
We have obtained effective medium theory (EMT) interatomic potential
parameters suitable for studying Cu-Mg metallic glasses. We present
thermodynamic and structural results from simulations of such glasses over a
range of compositions. We have produced low-temperature configurations by
cooling from the melt at as slow a rate as practical, using constant
temperature and pressure molecular dynamics. During the cooling process we have
carried out thermodynamic analyses based on the temperature dependence of the
enthalpy and its derivative, the specific heat, from which the glass transition
temperature may be determined. We have also carried out structural analyses
using the radial distribution function (RDF) and common neighbor analysis
(CNA). Our analysis suggests that the splitting of the second peak, commonly
associated with metallic glasses, in fact has little to do with the glass
transition itself, but is simply a consequence of the narrowing of peaks
associated with structural features present in the liquid state. In fact the
splitting temperature for the Cu-Cu RDF is well above . The CNA also
highlights a strong similarity between the structure of the intermetallic
alloys and the amorphous alloys of similar composition. We have also
investigated the diffusivity in the supercooled regime. Its temperature
dependence indicates fragile-liquid behavior, typical of binary metallic
glasses. On the other hand, the relatively low specific heat jump of around
indicates apparent strong-liquid behavior, but this can
be explained by the width of the transition due to the high cooling rates.Comment: 12 pages (revtex, two-column), 12 figures, submitted to Phys. Rev.
Rich Ground State Chemical Ordering in Nanoparticles: Exact Solution of a Model for Ag-Au Clusters
We show that nanoparticles can have very rich ground state chemical order.
This is illustrated by determining the chemical ordering of Ag-Au 309-atom
Mackay icosahedral nanoparticles. The energy of the nanoparticles is described
using a cluster expansion model, and a Mixed Integer Programming (MIP) approach
is used to find the exact ground state configurations for all stoichiometries.
The chemical ordering varies widely between the different stoichiometries, and
display a rich zoo of structures with non-trivial ordering.Comment: Revised version. New figure added, discussion expanded, some material
moved into supplementary fil
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