Crystal effects in the Neutralization of He+ ions in the low energy ion scattering regime

4 pages.-- PACS nrs.: 34.35.+a, 68.47.De, 68.49.Sf, 79.20.Rf.Investigating possible crystal effects in ion scattering from elemental surfaces, measurements of the positive ion fraction P+ are reported for He+ ions scattered from single and polycrystalline Cu surfaces. In the Auger neutralization regime, the ion yield is determined by scattering from the outermost atomic layer. For Cu(110) P+ exceeds that for polycrystalline Cu by up to a factor of 2.5, thus exhibiting a strong crystal effect. It is much less pronounced at higher energies, i.e., in the reionization regime. However, there a completely different angular dependence of the ion yield is observed for poly- and single crystals, due to massive subsurface contributions in nonchanneling directions.This work was partially supported by the Austrian Science Fund FWF, project number P16469.Peer reviewe

Crystal effects in the Neutralization of He+ ions in the low energy ion scattering regime

4 pages.-- PACS nrs.: 34.35.+a, 68.47.De, 68.49.Sf, 79.20.Rf.Investigating possible crystal effects in ion scattering from elemental surfaces, measurements of the positive ion fraction P+ are reported for He+ ions scattered from single and polycrystalline Cu surfaces. In the Auger neutralization regime, the ion yield is determined by scattering from the outermost atomic layer. For Cu(110) P+ exceeds that for polycrystalline Cu by up to a factor of 2.5, thus exhibiting a strong crystal effect. It is much less pronounced at higher energies, i.e., in the reionization regime. However, there a completely different angular dependence of the ion yield is observed for poly- and single crystals, due to massive subsurface contributions in nonchanneling directions.This work was partially supported by the Austrian Science Fund FWF, project number P16469.Peer reviewe

Why Ultrafast Photoinduced CO Desorption Dominates over Oxidation on Ru(0001)

CO oxidation on Ru(0001) is a long-standing example of a reaction that, being thermally forbidden in ultrahigh vacuum, can be activated by femtosecond laser pulses. In spite of its relevance, the precise dynamics of the photoinduced oxidation process as well as the reasons behind the dominant role of the competing CO photodesorption remain unclear. Here we use ab initio molecular dynamics with electronic friction that account for the highly excited and nonequilibrated system created by the laser to investigate both reactions. Our simulations successfully reproduce the main experimental findings: the existence of photoinduced oxidation and desorption, the large desorption to oxidation branching ratio, and the changes in the O K-edge X-ray absorption spectra attributed to the initial stage of the oxidation process. Now, we are able to monitor in detail the ultrafast CO desorption and CO oxidation occurring in the highly excited system and to disentangle what causes the unexpected inertness to the otherwise energetically favored oxidation.A.T., J.I.J., and M.A. acknowledge financial support by the Gobierno Vasco-UPV/EHU [Project No. IT1569-22] and by the Spanish MCIN/AEI/10.13039/501100011033 [Grant No. PID2019-107396GB-I00]. P.S. acknowledges support by the Deutsche Forschungsgemeinschaft (DFG), through project Sa 547-18. C.E. acknowledges the Klaus Tschira Foundation for financial support. This research was conducted in the scope of the Transnational Common Laboratory (LTC) "Quantum-ChemPhys-Theoretical Chemistry and Physics at the Quantum Scale". Computational resources were provided by the DIPC computing center

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

Vibrational lifetimes of hydrogen on lead films: An ab initio molecular dynamics with electronic friction (AIMDEF) study

Using density functional theory and Ab Initio Molecular Dynamics with Electronic Friction (AIMDEF), we study the adsorption and dissipative vibrational dynamics of hydrogen atoms chemisorbed on free-standing lead films of increasing thickness. Lead films are known for their oscillatory behaviour of certain properties with increasing thickness, e.g., energy and electron spillout change in discontinuous manner, due to quantum size effects [G. Materzanini, P. Saalfrank, and P. J. D. Lindan, Phys. Rev. B 63, 235405 (2001)]. Here, we demonstrate that oscillatory features arise also for hydrogen when chemisorbed on lead films. Besides stationary properties of the adsorbate, we concentrate on finite vibrational lifetimes of H-surface vibrations. As shown by AIMDEF, the damping via vibration-electron hole pair coupling dominates clearly over the vibration-phonon channel, in particular for high-frequency modes. Vibrational relaxation times are a characteristic function of layer thickness due to the oscillating behaviour of the embedding surface electronic density. Implications derived from AIMDEF for frictional many-atom dynamics, and physisorbed species will also be given.He acknowledges support by the Deutsche Forschungsgemeinschaft (DFG) through project Sa 547/8-2. J.I.J., M.A., M.B., and R.D.M. acknowledge 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. FIS2013-48286-C2-2-P).Peer Reviewe

Influence of the van der Waals interaction in the dissociation dynamics of N2 on W(110) from first principles

Under the terms of the Creative Commons Attribution (CC BY) license.Using ab initio molecular dynamics (AIMD) calculations, we investigate the role of the van der Waals (vdW) interaction in the dissociative adsorption of N2 on W(110). Hitherto, existing classical dynamics calculations performed on six-dimensional potential energy surfaces based on density functional theory (DFT), and the semi-local PW91 and RPBE [Hammer et al. Phys. Rev. B 59, 7413 (1999)] exchange-correlation functionals were unable to fully describe the dependence of the initial sticking coefficient on the molecular beam incidence conditions as found in experiments. N2 dissociation on W(110) was shown to be very sensitive not only to short molecule-surface distances but also to large distances where the vdW interaction, not included in semilocal-DFT, should dominate. In this work, we perform a systematic study on the dissociative adsorption using a selection of existing non-local functionals that include the vdW interaction (vdW-functionals). Clearly, the inclusion of the non-local correlation term contributes in all cases to correct the unrealistic energy barriers that were identified in the RPBE at large molecule-surface distances. Among the tested vdW-functionals, the original vdW-DF by Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)] and the ulterior vdW-DF2 give also an adequate description of the N2 adsorption energy and energy barrier at the transition state, i.e., of the properties that are commonly used to verify the quality of any exchange-correlation functional. However, the results of our AIMD calculations, which are performed at different incidence conditions and hence extensively probe the multi-configurational potential energy surface of the system, do not seem as satisfactory as the preliminary static analysis suggested. When comparing the obtained dissociation probabilities with existing experimental data, none of the used vdW-functionals seems to provide altogether an adequate description of the N2/W(110) interaction at short and large distances.The authors acknowledge financial support by the Gobierno Vasco-UPV/EHU project IT756-13 and the Spanish Ministerio de Economía y Competitividad (Grant No. FIS2013-48286-C02-02-P).Peer Reviewe

Photochemistry and spectroscopy of molecules at surfaces: Insights from ab initio molecular dynamics

Resumen del trabajo presentado al 2nd CECAM Workshop: "Challenges in reaction dynamics of gas-­surface interactions and methodological advances in dissipative and non­adiabatic processes", celebrado en Toulouse (France) del 27 al 30 de septiembre de 2021.Peer reviewe

Ab initio molecular dynamics with electronic friction (AIMDEF): Application to the relaxation of hot atoms and molecules at metal surfaces

Resumen del trabajo presentado al CECAM workshop “Challenges in reaction dynamics of gas-surface interactions and methodological advances in dissipative and non-adiabatic processes”, celebrado en Albi (Francia) del 26 al 29 de junio de 2017.In dynamic gas-surface environments, where gas-phase atomic and molecular species impinge on the surface at energies of the order of hundreds of meV up to a few eV, energy dissipation occurs by the excitation of electron-hole (e-h) pairs and the excitation of lattice vibrations, i.e., phonons. While searching for an accurate and joint description of both energy dissipation channels, the recently developed abinitio molecular dynamics with electronic friction (AIMDEF) method that is based on the local density friction approximation (LDFA), constitutes a promising tool to meet this goal. Since the LDFA requires the knowledge at each integration step of the bare surface electronic density, an important ingredient of the AIMDEF method is the evaluation on the fly of the surface electronic density when it is modified due to large surface atom displacements. In this talk I will review the basis of the AIMDEF method and show different efficient ways to obtain at each integration step the electronic density of the distorted surface. Examples of the application of the AIMDEF methodology to the study of the relaxation of different hot adsorbates on metal surface, namely, dissociated H2 on Pd(100), N on Ag(111) and N2 on Fe(110) will be provided.Peer reviewe

Energy dissipation in the scattering of N2 from W(110)

Resumen del trabajo presentado al 75th Annual Meeting of the DPG and DPG Spring Meeting, celebrado en Dresden (Alemania) del 13 al 18 de marzo de 2011.Motivated by the measurements reported in, we study the scattering of N2 from W(110) with classical dynamics calculations on a multi-dimensional potential energy surface (PES). In this talk, I will show the results of our simulations for the rotational state distributions and discuss the possible mechanisms involved in the energy dissipation measured in the experiments. The potential energy surface is obtained by the interpolation of an extended ab-initio data set calculated within density functional theory over a broad six-dimensional configuration space. Regarding the energy dissipation mechanisms we analyze the relative importance of electron-hole and phonon excitations. The former are incorporated through a friction force with the aid of a model developed in our group. Briefly, the friction coefficient is calculated within a local density approach, for an ion embedded in an electron gas. Phonon excitations are included via a Generalized Langevin Oscillator model

Charge-state dependence of kinetic electron emission induced by slow ions in metals

A calculation is performed in order to analyze the charge-state dependence of the kinetic electron emission induced by slow ions in metals. All stages of the emission process are included: the excitation of the electrons, the neutralization of the projectile during its passage through the solid, and the transport of the excited electrons from where they are created to the surface. It is shown that the number of excited electrons depends strongly on the ion charge state. Nevertheless, due to the fast neutralization of the ions within the escape depth of the excited electrons, no significant initial charge-state dependence is expected in the kinetic electron yield. This result is consistent with available experimental data.J.I.J. and R.D.M. acknowledge partial support from the Basque Departamento de Educación, Universidades e Investigación, the University of Basque Country UPV/EHU (Grant No. 9/UPV 00206.215-13639/2001), and the Spanish Ministerio de Ciencia y Tecnología (Grant Nos. BSM2001- 0076 and MAT2001-0946). R.D.M. acknowledges partial support by the Gipuzkoako Foru Aldundia.Peer Reviewe