144 research outputs found
Role of molecular electronic structure in inelastic electron tunneling spectroscopy: O2 on Ag(110)
10 páginas, 8 figuras, 2 tablas.-- PACS number(s): 68.37.-d, 72.10.-d, 72.25.-b, 79.20.RfDensity-functional theory (DFT) simulations corrected by the intramolecular Coulomb repulsion U are performed to evaluate the vibrational inelastic electron tunneling spectroscopy (IETS) of O2 on Ag(110). In contrast to DFT calculations that predict a spinless adsorbed molecule, the inclusion of the U correction leads to the polarization of the molecule by shifting a spin-polarized molecular orbital toward the Fermi level. Hence, DFT+U characterizes O2 on Ag(110) as a mixed-valent system. This has an important implication in IETS because a molecular resonance at the Fermi level can imply a decrease in conductance while in the off-resonance case, an increase in conductance is the expected IETS signal. We use the lowest-order expansion on the electron-vibration coupling in order to evaluate the magnitude and spatial distribution of the inelastic signal. The final IET spectra are evaluated with the help of the self-consistent Born approximation and the effect of temperature and modulation-voltage broadening are explored. Our simulations reproduce the experimental data of O2 on Ag(110) [ J. R. Hahn, H. J. Lee and W. Ho Phys. Rev. Lett. 85 1914 (2000)] and give extra insight of the electronic and vibrational symmetries at play. This ensemble of results reveals that the IETS of O2 is more complicated that a simple decrease in conductance and cannot be ascribed to the effect of a single molecular-orbital resonance.We acknowledge financial support from the Spanish
MICINN (Grants No. FIS2007-066711-CO2-00 and No.
FIS2009-12721-C04-01), and the Basque Government-UPV/
EHU (Grant No. IT-366-07).Peer reviewe
Photo-Induced CO Desorption Dominates over Oxidation on Different O+CO Covered Ru(0001) Surfaces
The photo-induced desorption and oxidation of CO on Ru(0001) is simulated
using ab initio molecular dynamics with electronic friction that accounts for
the non-equilibrated excited electrons and phonons. Different (O,CO) coverages
are considered, the experimental room temperature coverage consisting in
0.5ML-O+0.25ML- CO (low coverage), the saturation coverage achieved
experimentally at low temperatures (0.5ML-O+0.375ML-CO, intermediate coverage),
and the equally mixed monolayer that is stable according to our calculations
but not experimentally observed yet (0.5ML-O+0.5ML-CO, high coverage). The
results of our simulations for the three coverages are consistent with
femtosecond laser experiments showing that the CO photo-desorption largely
dominates over CO photo-oxidation. These results cannot be explained in terms
of the distinct activation energies calculated for the relaxed surfaces.
Different (dynamical) factors such as the coupling to the laser-excited
electrons and, more importantly, the interadsorbate energy exchange and the
strong surface distortions induced in the more crowded surfaces are fundamental
to understand the competition between these two processes under the extremely
non-equilibrated conditions created by the laser
Insights into the Coadsorption and Reactivity of O and CO on Ru(0001) and Their Coverage Dependence
Using density functional theory and an exchange-correlationfunctional that
includes the van der Waals interaction, we study the coadsorption of CO on
Ru(0001) saturated with 0.5 ML of oxygen. Different coexisting CO coverages are
considered that are experimentally motivated, the room temperaturecoverage
consisting of 0.5 ML-O + 0.25 ML-CO (low coverage), the saturation coverage
achieved at low temperatures (0.5 ML-O + 0.375 ML-CO, intermediate coverage),
and the equally mixed monolayer that is stable according to our calculations
but not experimentally observed yet (0.5 ML-O + 0.5 ML-CO, high coverage). For
each coverage, we study the competition between the desorption and oxidation of
CO on the corresponding optimized structure by analyzing their reaction
energies and minimum energy reaction paths. The desorption process is
endothermic at all coverages, although the desorption energy decreases as the
CO coverage increases. The process itself (and also the reverted adsorption)
becomes more involved at the intermediate and high coverages because of the
appearance of a physisorption well and concomitant energy barrier separating it
from the chemisorbed state. Remarkably, the oxidation of CO, which is
endothermic at low coverages, turns exothermic at the intermediate and high
coverages. In all cases, the minimum reaction path for oxidation, which
involves the chemisorbed and physisorbed CO2, is ruled by one of the large
energy barriers that protect these molecular states. Altogether, the larger
activation energies for oxidation as compared to those for desorption and the
extreme complexity of the oxidation against the desorption paths explain that
CO desorption dominates over the oxidation in experiments
Ready, set and no action: A static perspective on potential energy surfaces commonly used in gas-surface dynamics
In honoring the seminal contribution of Henry Eyring and Michael Polanyi who
first introduced the concept of potential energy surfaces (PESs) to describe
chemical reactions in gas-phase [Z. Phys. Chem. 12, 279-311, (1931)], this work
comes to review and assess state-of-the-art approaches towards first-principle
based modeling in the field of gas-surface dynamics. Within the
Born-Oppenheimer and frozen surface approximations, the O2-Ag(100) interaction
energetics are used as a showcase system to accentuate the complex landscape
exhibited by the PESs employed to describe the impingement of diatomics on
metal substrates and draw attention to the far-from-trivial task of
continuously representing them within all six molecular degrees of freedom. To
this end, the same set of ab initio reference data obtained within Density
Functional Theory (DFT) are continuously represented by two different
state-of-the-art high-dimensional approaches, namely the Corrugation-Reducing
Procedure and Neural Networks. Exploiting the numerically undemanding nature of
the resulting representations, a detailed static evaluation is performed on
both PESs based on an extensive global minima search. The latter proved
particularly illuminating in revealing representation deficiencies which affect
the dynamical picture yet go otherwise unnoticed within the so-called
"divide-and-conquer" approach.Comment: 27 pages including 8 figures; related publications can be found at
http://www.th4.ch.tum.de/index.php?id=167 [corrected typos, formulae,
references, modified a few formulations based on added supporting material
Mixed-valency signature in vibrational inelastic electron tunneling spectroscopy
4 páginas, 3 figuras, 1 tabla.-- PACS numbers: 68.37.Ef, 72.10.-d, 72.25.-b, 79.20.RfDensity functional theory simulations of the vibrational inelastic electron tunneling spectroscopy (IETS) of O2 on Ag(110) permits us to solve its unexplained IETS data [ Hahn et al. Phys. Rev. Lett. 85 1914 (2000)]. When semilocal density functional theory is corrected by including static intra-atomic correlations, the IETS simulations are in excellent agreement with the experiment. The unforeseen consequence of our calculations is that when adsorbed along the [001] direction, molecular O2 on Ag(110) is a mixed-valent system. This analysis of IETS unambiguously reveals the paramagnetic nature of O2 on Ag(110).We acknowledge financial support from the Spanish
MICINN (No. FIS2007-066711-CO2-00 and
No. FIS2009-12721-C04-01), and the Basque
Government—UPV/EHU (Grant No. IT-366-07).Peer reviewe
Dissociative adsorption of N2 on W(110): Theoretical study of the dependence on the incidence angle
5 páginas, 2 figuras.-- El Pdf del artÃculo es la versión post-print.The dissociative adsorption of N2 on W(110) is studied using classical dynamics on
a six-dimensional potential energy surface obtained from density functional theory
calculations. Two distinct channels are identified in the dissociation process: a direct
one and an indirect one. It is shown that the direct channel is inhibited for low energy
molecules (Ei < 400 meV) and low incidence angles. The indirect channel includes
long-lasting dynamic trapping of the molecule at the surface before dissociation.
The dependence of the sticking coefficient on the initial incidence angle is analyzed.
The theoretical results compare well with values measured using molecular beam
techniques.We acknowledge partial support by the University of
the Basque Country UPV/EHU (Grant No. 9/UPV
00206.215-13639/2001), and the Spanish MCyT (Grant
No. FIS2004-06490-CO3-00). M.A. acknowledges financial
support by the Gipuzkoako Foru Aldundia, and H.F.B.
and A.S. by the DIPC.Peer reviewe
Low sticking probability in the nonactivated dissociation of N2 molecules on W(110)
The six-dimensional potential energy surface for the dissociation of N2 molecules on the W(110) surface has been determined by density functional calculations and interpolated using the corrugation reducing procedure. Examination of the resulting six-dimensional potential energy surface shows that nonactivated paths are available for dissociation. In spite of this, the dissociation probability goes to a very small value when the impact energy goes to zero and increases with increasing energy, a behavior usually associated with activated systems. Statistics on the dynamics indicate that this unconventional result is a consequence of the characteristics of the potential energy surface at long distances. Furthermore, two distinct channels are identified in the dissociation process, namely, a direct one and an indirect one. The former is responsible for dissociation at high energies. The latter, which includes long-lasting dynamic trapping in the vicinity of a potential well above the W top position, is the leading mechanism at low and intermediate energies.© 2006 American Institute of Physics.The authors acknowledge partial support by the Basque Departamento de Educación, Universidades e Investigación, the University of the Basque Country UPV/EHU (Grant No. 9/UPV 00206.215-13639/2001), and the Spanish MCyT (Grant No. FIS2004-06490-CO3-00). One of the authors (M.A.) acknowledges financial support by the Gipuzkoako Foru Aldundia.Peer Reviewe
Anomalous transient blueshift in the internal stretch mode of CO/Pd(111)
In time-resolved pump-probe vibrational spectroscopy the internal stretch
mode of polar molecules is utilized as a key observable to characterize the
ultrafast dynamics of adsorbates on surfaces. The adsorbates non-adiabatic
intermode couplings are the commonly accepted mechanisms behind the observed
transient frequency shifts. Here, we study the CO/Pd(111) system with a robust
theoretical framework that includes electron-hole pair excitations and
electron-mediated coupling between the vibrational modes. A mechanism is
revealed that screens the electron-phonon interaction and originates a
blueshift under ultrafast non-equilibrium conditions. The results are explained
in terms of the abrupt change in the density of states around the Fermi level,
and are instrumental for understanding dynamics at multi-component surfaces
involving localized and standard or states.Comment: 3 figures, 5 page
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