305 research outputs found
On the role of physisorption states in molecular scattering: A semi-local density-functional theory study on O2/Ag(111)
We simulate the scattering of O from Ag(111) with classical dynamics
simulations performed on a six-dimensional potential energy surface calculated
within semi-local density-functional theory (DFT). The enigmatic experimental
trends that originally required the conjecture of two types of repulsive walls,
arising from a physisorption and chemisorption part of the interaction
potential, are fully reproduced. Given the inadequate description of the
physisorption properties in semi-local DFT, our work casts severe doubts on the
prevalent notion to use molecular scattering data as indirect evidence for the
existence of such states.Comment: 5 pages, 4 figure
Ultrafast Transient Dynamics of Adsorbates on Surfaces Deciphered: The Case of CO on Cu(100)
Time-resolved vibrational spectroscopy constitutes an invaluable experimental
tool for monitoring hot-carrier induced surface reactions. However, the absence
of a full understanding on the precise microscopic mechanisms causing the
transient spectral changes has been limiting its applicability. Here we
introduce a robust first-principles theoretical framework that successfully
explains both the nonthermal frequency and linewidth changes of the CO internal
stretch mode on Cu(100) induced by femtosecond laser pulses. Two distinct
processes engender the changes: electron-hole pair excitations underlie the
nonthermal frequency shifts, while electron-mediated vibrational mode coupling
gives rise to linewidth changes. Furthermore, the origin and precise sequence
of coupling events are finally identified.Comment: Article as accepted for publication in Physical Review Letters; 5
pages, 2 figures, 1 tabl
Femtosecond laser driven molecular dynamics on surfaces: Photodesorption of molecular oxygen from Ag(110)
We simulate the femtosecond laser induced desorption dynamics of a diatomic
molecule from a metal surface by including the effect of the electron and
phonon excitations created by the laser pulse. Following previous models, the
laser induced surface excitation is treated through the two temperature model,
while the multidimensional dynamics of the molecule is described by a classical
Langevin equation, in which the friction and random forces account for the
action of the heated electrons. In this work, we propose the additional use of
the generalized Langevin oscillator model to also include the effect of the
energy exchange between the molecule and the heated surface lattice in the
desorption dynamics. The model is applied to study the laser induced desorption
of O from the Ag(110) surface, making use of a six-dimensional potential
energy surface calculated within density functional theory. Our results reveal
the importance of the phonon mediated process and show that, depending on the
value of the electronic density in the surroundings of the molecule adsorption
site, its inclusion can significantly enhance or reduce the desorption
probabilities.Comment: 11 pages, 8 figure
Diffusion of Hydrogen in Pd Assisted by Inelastic Ballistic Hot Electrons
Sykes {\it et al.} [Proc. Natl. Acad. Sci. {\bf 102}, 17907 (2005)] have
reported how electrons injected from a scanning tunneling microscope modify the
diffusion rates of H buried beneath Pd(111). A key point in that experiment is
the symmetry between positive and negative voltages for H extraction, which is
difficult to explain in view of the large asymmetry in Pd between the electron
and hole densities of states. Combining concepts from the theory of ballistic
electron microscopy and electron-phonon scattering we show that H diffusion is
driven by the -band electrons only, which explains the observed symmetry.Comment: 5 pages and 4 figure
Trajectory-dependent energy loss for swift He atoms axially scattered off a silver surface
Angle- and energy-loss- resolved distributions of helium atoms grazingly
scattered from a Ag(110) surface along low indexed crystallographic directions
are investigated considering impact energies in the few keV range. Final
projectile distributions are evaluated within a semi-classical formalism that
includes dissipative effects due to electron-hole excitations through a
friction force. For mono-energetic beams impinging along the ,
and directions, the model predicts the presence of
multiple peak structures in energy-loss spectra. Such structures provide
detailed information about the trajectory-dependent energy loss. However, when
the experimental dispersion of the incident beam is taken into account, these
energy-loss peaks are completely washed out, giving rise to a smooth
energy-loss distribution, in fairly good agreement with available experimental
data
Competition between electron and phonon excitations in the scattering of nitrogen atoms and molecules off tungsten and silver surfaces
We investigate the role played by electron-hole pair and phonon excitations
in the interaction of reactive gas molecules and atoms with metal surfaces. We
present a theoretical framework that allows us to evaluate within a
full-dimensional dynamics the combined contribution of both excitation
mechanisms while the gas particle-surface interaction is described by an
ab-initio potential energy surface. The model is applied to study energy
dissipation in the scattering of N on W(110) and N on Ag(111). Our results
show that phonon excitation is the dominant energy loss channel whereas
electron-hole pair excitations represent a minor contribution. We substantiate
that, even when the energy dissipated is quantitatively significant, important
aspects of the scattering dynamics are well captured by the adiabatic
approximation.Comment: 4pages and 3 figure
Non-adiabatic effects during the dissociative adsorption of O2 at Ag(111)? A first-principles divide and conquer study
We study the gas-surface dynamics of O2 at Ag(111) with the particular
objective to unravel whether electronic non-adiabatic effects are contributing
to the experimentally established inertness of the surface with respect to
oxygen uptake. We employ a first-principles divide and conquer approach based
on an extensive density-functional theory mapping of the adiabatic potential
energy surface (PES) along the six O2 molecular degrees of freedom. Neural
networks are subsequently used to interpolate this grid data to a continuous
representation. The low computational cost with which forces are available from
this PES representation allows then for a sufficiently large number of
molecular dynamics trajectories to quantitatively determine the very low
initial dissociative sticking coefficient at this surface. Already these
adiabatic calculations yield dissociation probabilities close to the scattered
experimental data. Our analysis shows that this low reactivity is governed by
large energy barriers in excess of 1.1 eV very close to the surface.
Unfortunately, these adiabatic PES characteristics render the dissociative
sticking a rather insensitive quantity with respect to a potential spin or
charge non-adiabaticity in the O2-Ag(111) interaction. We correspondingly
attribute the remaining deviations between the computed and measured
dissociation probabilities primarily to unresolved experimental issues with
respect to surface imperfections.Comment: 18 pages including 6 figure
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