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(2) 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.

We study the dissociative adsorption of N 2 on W͑100͒ and W͑110͒ by means of density functional theory and classical dynamics. Working with a full six-dimensional adiabatic potential energy surface ͑PES͒, 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 ͑1992͔͒ and Perdew-Burke-Ernzerhof ͑RPBE͒ ͓Hammer et al., Phys. Rev. B 59, 7413 ͑1999͔͒ 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 present theoretical and experimental evidence of an anomalous surface corrugation behavior in He-KCl(001) for incidence along 110 . When the He normal energy decreases below 100 meV, i.e. He-surface distances Z > 2Å, the corrugation unexpectedly increases up to an impressive > ∼ 85%. This is not due to van der Waals interactions but to the combination of soft potential effects and the evolution of He-cation and He-anion interactions with Z. This feature, not previously analyzed on alkali-halide surfaces, may favor the alignement properties of weakly-interacting overlayers.

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In this work we address, both experimentally and theoretically, the very grazing scattering of He atoms off KCl(001) with incidence along the 100 channel. Our theoretical model combines a semiquantum description of the scattering dynamics and a high-precision interaction potential. By means of a thorough analysis of the quantum phase for in-plane scattering and rainbow trajectories, we are able to connect the presence of the physisorption well with the significant enhancements of the corrugation and rainbow angle, relative to the hard corrugated wall predictions. We trace this connection to dynamical effects on the incident and scattered beams due to their traversing of the physisorption well. Finally, we show that the inclusion of vdW interactions in the potential improves the theoretical accord with experiments for both the corrugation and the rainbow angle.

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Experimental diffraction patterns produced by grazing scattering of fast helium atoms from a Ag(110) surface are used as a sensitive tool to test an ab initio potential model derived from accurate density-functional theory (DFT) calculations. The scattering process is described by means of the surface eikonal approximation, which is a distorted-wave method that includes the quantum interference between contributions coming from different projectile paths, taking into account the complete corrugation of the three-dimensional projectile-surface potential. A fairly good agreement between the theoretical and experimental momentum distributions is found for incidence along different low-indexed crystallographic directions. This agreement is indicative of the quality of the DFT potential. The effective corrugation of the interaction potential across the incidence channel is also investigated.

Using molecular dynamics simulations and potential energy surfaces of ab-initio quality, we show that direct pick-up of N adsorbates by gas-phase N is a highly efficient channel for N 2 formation on Ag(111). This recombination process, called Eley-Rideal, was traditionally associated to lighter projectiles and regarded as marginal, but here we obtain reactivities for * To whom correspondence should be addressed TOC GraphicKeywords: Gas-surface interactions, Eley-Rideal reactions, scattering, potential energy surfaces, molecular dynamics. 2Among the various elementary processes that occur in everyday gas-surface reactions, those resulting in the recombination and desorption of molecules deserve a special attention, as they regulate the replenishment of the surface active sites. One of these processes, the direct Eley-Rideal (ER) recombination has been found to be a marginal phenomenon, only observed experimentally for incoming light atoms (H,D). [1][2][3][4][5][6][7][8][9] Originally postulated as an atom exchange, 10-12 the ER process is nowadays assigned to the direct recombination of an incoming gas-phase species that picks up a surface adsorbate to conform a molecular compound that desorbs during the collision. Even if the initial measurements pointed to ER cross sections of about 2 − 5 Å 2 , 4,6,9 subsequent theoretical studies demonstrated that they were around one order of magnitude smaller because most of the recombination events proceed after few collisions of the incoming (H,D) atom with the surface, i.e., through the slower hot-atom (HA) process. [13][14][15][16][17][18] In the scope of reactive ion scattering (RIS), C abstraction from a graphite surface by N + ions has been characterised by experiments, and found to happen after several collisions with the substrate 19 Recombination of neutral atomic species heavier than (H,D) has been experimentally studied in the context of CO formation and CO oxidation on metal surfaces. In all cases, CO and CO 2 formation is dominated by the Langmuir-Hinshelwood (LH) recombination in which the species involved in the reaction are fully thermalized with the surface. [20][21][22] Also in these systems, the direct ER pick-up events are accepted to play a very minor role. Within this context, the recent experiments by Ueta et al. suggesting the existence of direct recombination on the N-covered Ag(111) surface between gas-phase and adsorbed N atoms are certainly intriguing. 23,24 All the more so when recent simulations of N 2 formation on the W(100) surface also exhibit low ER probabilities. 25 In this letter we demonstrate that thermal and hyperthermal gas-phase N scattered off Ncovered Ag(111) is indeed a prototype system that fulfills all the requirements to observe a highly efficient direct ER recombination. This outcome poses a new scenario for the study of gas-surface interactions, as it shows that fast direct recombination channels may be active in contrast to the usual assumption of neglecting them, specially for large atomic projectiles. 3The comple...