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

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

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

Energy-loss contribution to grazing scattering of fast He atoms from a silver surface

The energy lost by helium atoms axially scattered from a Ag(110) surface is studied in order to investigate the influence of dissipative processes on fast atom diffraction spectra. In this work inelastic projectile distributions are evaluated within a semiclassical formalism that includes dissipative effects due to electron-hole excitations through a friction force. For incidence along the [11̄2] and [11̄0] directions the model predicts the presence of multiple peaks in the energy-loss spectrum for a given impact energy. But these structures are completely washed out when the experimental dispersion of the incident beam is taken into account, giving rise to a smooth energy-loss distribution. Simulations including the experimental energy spread are in fairly good agreement with available experimental data for the [11̄2] channel. In addition, our results suggest that inelastic processes produce an almost constant background in the transverse momentum distribution, except in the extremes of the momentum range where classical rainbow maxima appear. By adding elastic and inelastic contributions, experimental diffraction patterns are well reproduced. © 2014 American Physical Society.C.R.R and M.S.G. acknowledge financial support from CONICET, UBA, and ANPCyT of Argentina. G.A.B. acknowledges financial support by ANPCyT. J.I.J. acknowledges financial support by the Basque Departamento de Educación, Universidades, e Investigación, the University of the Basque Country UPV/EHU (Grant No. IT-756-13), and the Spanish Ministerio de Ciencia e Innovación (Grant No. FIS2010-19609-C02-02).Peer Reviewe

The role of exchange-correlation functionals in the potential energy surface and dynamics of N2 dissociation on W surfaces

8 pages.-- PACS nrs.: 68.43.Mn, 82.65.+r, 82.30.Lp.We study the dissociative adsorption of N2 on W(100) and W(110) by means of density functional theory and classical dynamics. Working with a full six-dimensional adiabatic potential energy surface (PES), we find that the theoretical results of the dynamical problem strongly depend on the choice of approximate exchange-correlation functional for the determination of the PES. We consider the Perdew-Wang-91 [Perdew et al., Phys. Rev. B 46, 6671 (1992)] and Perdew-Burke-Ernzerhof (RPBE) [Hammer et al., Phys. Rev. B 59, 7413 (1999)] functionals and carry out a systematic comparison between the dynamics determined by the respective PESs. Even though it has been shown in earlier works that the RPBE may provide better values for the chemisorption energies, our study brings evidence that it gives rise to a PES with excessive repulsion far from the surface.We acknowledge partial support by the Spanish Ministerio de Educación y Ciencia (Grant No. FIS2007-66711-C02-02). Computational resources were provided by the SGI/IZO-SGIker at the UPV/EHU (supported by the Spanish Ministerio de Educación y Ciencia and the European Social Fund) and the DIPC computer center.Peer reviewe

Energy dissipation channels in the adsorption of N on Ag(111)

6 páginas, 6 figuras.We theoretically study the competition between different energydissipationchannels in the adsorption of N atoms on Ag(1 1 1) surfaces. The three-dimensional potential energy surface that describes the interaction between the N atoms and the metal surface is built from density functional theory calculations. Classical dynamics simulations are subsequently performed to evaluate the adsorption probabilities. The contribution of electron–hole pairs excited in the surface during the adsorption process is included in the simulation by an electronic friction coefficient. Phonon excitations are also considered through the Generalized Langevin Oscillator model. We show that the role of the two channels during the adsorption dynamics is very different: phonons are responsible for determining the adsorption probability but electronic excitations are relevant at a later stage to fix the N atoms to the adsorption positions. We conclude that a theoretical model that intrinsically combines both energydissipationchannels is necessary to properly describe the full dynamics of the process.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

Dissociative adsorption dynamics of nitrogen on a Fe(111) surface

We study the dissociative adsorption dynamics of N2 on clean bcc Fe(111) surfaces. We base our theoretical analysis on a multidimensional potential energy surface built from density functional theory. The dissociative sticking probability is computed by means of quasi-classical trajectory calculations. For normal incidence and impact energies of the order of a few eV, our theoretical results agree well with existing experimental values. For these energies, the dynamics of the dissociated molecules shows that dissociation is a direct process that follows narrow paths in the multidimensional space. For lower energies of the beam, this direct process is not enough to explain the measured values. A better agreement with the experiment is obtained if we increase the surface temperature to promote the transfer to dissociation of molecules previously trapped. Most of the molecules dissociate very close to the Fe(111) third layer atoms and with an orientation parallel to the surface. A comparison between the dissociation of N2 on Fe(111) and Fe(110) highlights the role of the different energy barriers in both surfaces.This work was partially supported by the Basque Departamento de Educacion, Universidades e Investigacion, the University of the Basque Country UPV/EHU (Grant No. IT-756-13) and the Spanish Ministerio de Economía y Competitividad (Grants No. FIS2013-48286-C02-02-P and FIS2016-76471-P). M. A. N. acknowledges financial support by the Ministerio de Economia y Competitividad (Grant No. BES-2011-045536).Peer Reviewe

Adsorption dynamics of molecular nitrogen at an Fe(111) surface

We present an extensive theoretical study of N adsorption mechanisms on an Fe(111) surface. We combine the static analysis of a six-dimensional potential energy surface (6D-PES), based on ab initio density functional theory (DFT) calculations for the system, with quasi-classical trajectory (QCT) calculations to simulate the adsorption dynamics. There are four molecular adsorption states, usually called γ, δ, α, and ε, arising from our DFT calculations. We find that N adsorption in the γ-state is non-activated, while the threshold energy is associated with the entrance channel for the other three adsorption states. Our QCT calculations confirm that there are activated and nonactivated paths for the adsorption of N on the Fe(111) surface, which is in agreement with previous experimental investigations. Molecular dynamics at a surface temperature T = 300 K and impact energies E in the 0-5 eV range show the relative occupancy of the γ, δ, α, and ε states. The δ-state, however, is only marginally populated despite its adsorption energy being very similar to that of the γ-state. Our QCT calculations trace the dependence of molecular trapping on the surface temperature T and initial impact energy E and quantify the rates of the different competitive channels that eventually lead to molecular adsorption.This work was partially supported by the Basque Departamento de Educación, Universidades e Investigación, the University of the Basque Country UPV/EHU (Grant No. IT-756-13) and the Spanish Ministerio de Economía y Competitividad (Grant No. FIS2016-76471-P). M. A. N. acknowledges financial support from the Ministerio de Economía y Competitividad (Grant No. BES-2011-045536).Peer Reviewe

Efficient N2 formation on Ag(111) by eley-rideal recombination of hyperthermal atoms

Using molecular dynamics simulations and potential energy surfaces of ab initio quality, we show that direct pickup of N adsorbates by gas-phase N is a highly efficient channel for N2 formation on Ag(111). This recombination process, called Eley-Rideal, was traditionally associated with lighter projectiles and regarded as marginal, but here, we obtain reactivities for N of ≳35% even at incident energies of a few eV. The good agreement found between simulated and published experimental energies of the desorbed N2 is a fingerprint of this otherwise elusive recombination. © 2013 American Chemical Society.Work 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). M.B.-R. acknowledges the Gipuzkoako Foru Aldundia and the European Commission (Grant No. FP7-PEOPLE-2010-RG276921). G.A.B. acknowledges ANPCYT (Grant No. PICT 2010-1558).Peer Reviewe

Adsorption dynamics of molecular nitrogen at an Fe(111) surface

We present an extensive theoretical study of N2 adsorption mechanisms on an Fe(111) surface. We combine the static analysis of a six-dimensional potential energy surface (6D-PES), based on ab initio density functional theory (DFT) calculations for the system, with quasi-classical trajectory (QCT) calculations to simulate the adsorption dynamics. There are four molecular adsorption states, usually called γ, δ, α, and ε, arising from our DFT calculations. We find that N2 adsorption in the γ-state is non-activated, while the threshold energy is associated with the entrance channel for the other three adsorption states. Our QCT calculations confirm that there are activated and nonactivated paths for the adsorption of N2 on the Fe(111) surface, which is in agreement with previous experimental investigations. Molecular dynamics at a surface temperature Ts = 300 K and impact energies Ei in the 0-5 eV range show the relative occupancy of the γ, δ, α, and ε states. The δ-state, however, is only marginally populated despite its adsorption energy being very similar to that of the γ-state. Our QCT calculations trace the dependence of molecular trapping on the surface temperature Ts and initial impact energy Ei and quantify the rates of the different competitive channels that eventually lead to molecular adsorption.Fil: Nosir, M. A.. Consejo Superior de Investigaciones Científicas; España. Donostia International Physics Center; EspañaFil: Martin Gondre, L.. Université Bourgogne Franche; FranciaFil: Bocan, Gisela Anahí. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Díez Muiño, R.. Consejo Superior de Investigaciones Científicas; España. Donostia International Physics Center; Españ