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De la misma manera que al entrar en una habitación a oscuras buscamos automáticamente con la mano el interruptor que nos permite iluminar el entorno, muchos físicos recurren de forma casi instintiva a la luz para orientarse en el mundo microscópico y entender cómo funciona la materia en la escala atómica. Este texto hace un breve repaso de la información que podemos adquirir al hacer incidir luz sobre sistemas de muy distinto tamaño y recoger los electrones que son emitidos en procesos de fotoemisión.Peer reviewe

Dynamic screening and energy loss of antiprotons colliding with excited Al clusters

We use time-dependent density functional theory to calculate the energy loss of an antiproton colliding with a small Al cluster previously excited. The velocity of the antiproton is such that non-linear effects in the electronic response of the Al cluster are relevant. We obtain that an antiproton penetrating an excited cluster transfers less energy to the cluster than an antiproton penetrating a ground state cluster. We quantify this difference and analyze it in terms of the cluster excitation spectrum.Comment: 23 pages, 4 figures, to be published in Nuclear Instruments and Methods B as a proceeding of the IISC-19 Workshop on Inelastic Ion-Surface Collision

Diffraction of swift atoms after grazing scattering from metal surfaces: N/Ag(111) system

6 páginas, 6 figuras.-- PACS number(s): 79.20.Rf, 79.60.Bm, 34.20.Cf, 61.85.+pDiffraction patterns produced by grazing scattering of fast N atoms from a Ag(111) surface are investigated by employing the surface eikonal approximation. This method is a distorted-wave theory that takes into account the coherent addition of contributions coming from different projectile paths. In the model the projectile-surface potential is obtained from an accurate density-functional theory calculation. The dependence of the scattered projectile spectra on impact energy and incidence channel is analyzed, and possible incident direction and energy range for the observation of the interference patterns are predicted. In addition, it is found that as a result of the high reactivity of N atoms, asymmetries of the surface potential might be detected through their effects on diffraction patterns.M.S.G. acknowledges financial support from CONICET, UBA, and ANPCyT of Argentina.Peer reviewe

Ab Initio Molecular Dynamics of Hydrogen on Tungsten Surfaces

The dissociation process of hydrogen molecules on W(110) was studied using density functional theory and classical molecular dynamics. We have calculated the dissociation probability for molecules with energies below 300 meV and analyzed the dynamics of the adsorption process. Our results show that the fate of each trajectory is determined at distances relatively far from the surface, at roughly 2-2.5 angstrom. This distance varies slightly with the initial kinetic energy of the molecule. Part of our simulations include van der Waals dispersion effects in the interaction between molecule and surface. We present a comparison between these results and other theoretical and experimental results previously published. The inclusion of the van der Waals term provokes an increase in the far-distance attraction that is compensated by a stronger repulsion at short distances. The combination of both effects appreciably decreases the value of the dissociation probability. The successful comparison of our results with experimental information confirms that the methodology employed can be considered as a rich and accurate instrument to study the dissociation of hydrogen on surfaces.A. R. F. acknowledges financial support by the University of Bordeaux. This work was conducted in the scope of the transborder joint Laboratory “QuantumChemPhys: Theoretical Chemistry and Physics at the Quantum Scale” (ANR-10-IDEX-03-02). This work has been supported in part by the Basque Departamento de Educación, Universidades e Investigación, the University of the Basque Country UPV/EHU (Grant No. IT1246-19) and the Spanish Ministerio de Ciencia e Innovación (PID2019-107396GB-I00/AEI/10.13039/501100011033)

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

Dissociative adsorption of N2 on W(110): Theoretical study of the dependence on the incidence angle

5 páginas, 2 figuras.-- El Pdf del artículo 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

Density functional theory calculations of nitrogen adsorption features on Fe(111) surfaces

Trabajo presentado al Workshop on Controlled Atomic Dynamics on Solid Surfaces: Atom an Molecular Manipulation, celebrado en Donostia-San sebastián (España) del 13 al 16 de Mayo de 2013.The interaction of nitrogen with metal surfaces has been one of the most popular topics of research in surface science for the last decades. This is due in part to the industrial importance of ammonia synthesis, typically obtained from nitrogen and hydrogen catalyzed over iron-based compounds. The rate limiting step in ammonia synthesis is the adsorption and dissociation of nitrogen on the catalyst surface. In Fe surfaces, the reactivity of the process depends on the face, the Fe(111) and Fe(211) surfaces being the most reactive ones. Although Fe(111) is the most reactive iron face for N2 dissociation, the dynamics of such process has not been analyzed in detail. In this work we present exhaustive calculations of the interaction of nitrogen atoms and molecules with the Fe(111) surface. These calculations set the basis for subsequent analysis of the N2 dissociation dynamics. We perform Density functional Theory spin-polarized calculations using VASP code. We first study the relaxation of the Fe(111) surface, which was a matter of controversy in the past. From here, we calculate the interaction energy of nitrogen atoms and molecules when approaching the Fe(111) surface. Our results show the preferred adsorption paths and sites for nitrogen adsorption, as well as the adsorption energies. We finally discuss the dynamics of the dissociation process and make the link with the high reactivity properties of the surface.Peer reviewe

Non-reactive scattering of N2 from the W(110) surface studied with different exchange–correlation functionals

8 páginas, 8 figuras, 1 tabla.-- El Pdf del artículo es la versión post-print.The non-reactive scattering of N2 from the W(110) surface is studied with six dimensional (6D) classical dynamics and two distinct potential energy surfaces (PES). Here, we use the PESs calculated with density functional theory and two different exchange–correlation functionals, the PW91 [J. E. Perdew et al., Phys. Rev. B, 1992, 46, 6671] and the RPBE [B. Hammer et al., Phys. Rev. B, 1999, 59, 7413]. By analyzing the final rotational state and angular distributions, we extract information on the characteristics of the two PESs in the 6D configurational space. Comparison of the theoretical results with the available experimental data provides detailed information on the validity of each functional. In general, the PW91 PES is more corrugated than the RPBE one in all the configurational space, meaning that there is a stronger dependence of the potential energy on the molecular orientation and position over the surface unit cell. Furthermore, we find that the larger corrugation and the less repulsive character exhibited by the PW91 PES seems to be realistic at distances above the chemisorption well. In contrast, the less corrugated RPBE PES performs better in the region below the chemisorption well.This work has been supported in part by the Basque Departamento de Educación, Universidades e Investigación, the University of the Basque Country UPV/EHU (Grant No. IT-366-07) and the Spanish Ministerio de Ciencia e Innovación (Grant No. FIS2010-19609-C02-02).Peer reviewe

Lifetime of electronic excitations in metal nanoparticles

9 páginas, 4 figuras.-- Trabajo presentado al "6th International Meeting on Photodynamics" celebrado en La Habana (Cuba) del 1 al 5 de febrero de 2010.Electronic excitations in metal particles with sizes up to a few nanometers are shown to have a one-electron character when a laser pulse is applied off the plasmon resonance. The calculated lifetimes of these excitations are in the femtosecond timescale but their values are substantially different from those in bulk. This deviation can be explained from the large weight of the excitation wave function in the nanoparticle surface region, where dynamic screening is significantly reduced. The well-known quadratic dependence of the lifetime with the excitation energy in bulk breaks down in these finite-size systems.We acknowledge the partial support of the Basque Government, the University of the Basque Country UPV/EHU (grant no. 9/UPV 00206.215-13639/2001) and the Spanish MEC (grants FIS2007-66711-C02-02 and MAT2008-06843-C03-01). JAA acknowledges the financial support of Ikerbasque.Peer reviewe

Lifetime of electronic excitations in metal nanoparticles

9 páginas, 4 figuras.-- Trabajo presentado al "6th International Meeting on Photodynamics" celebrado en La Habana (Cuba) del 1 al 5 de febrero de 2010.Electronic excitations in metal particles with sizes up to a few nanometers are shown to have a one-electron character when a laser pulse is applied off the plasmon resonance. The calculated lifetimes of these excitations are in the femtosecond timescale but their values are substantially different from those in bulk. This deviation can be explained from the large weight of the excitation wave function in the nanoparticle surface region, where dynamic screening is significantly reduced. The well-known quadratic dependence of the lifetime with the excitation energy in bulk breaks down in these finite-size systems.We acknowledge the partial support of the Basque Government, the University of the Basque Country UPV/EHU (grant no. 9/UPV 00206.215-13639/2001) and the Spanish MEC (grants FIS2007-66711-C02-02 and MAT2008-06843-C03-01). JAA acknowledges the financial support of Ikerbasque.Peer reviewe