Obtaining quantitative agreement between theory and experiment for dissociative adsorption of hydrogen on and associative desorption of hydrogen from Cu(111) remains challenging. Particularly troubling is the fact that theory gives values for the high energy limit to the dissociative adsorption probability that is as much as two times larger than experiment. In the present work we approach this discrepancy in three ways. First, we carry out a new analysis of the raw experimental data for D2 associatively desorbing from Cu(111). We also perform new ab initio molecular dynamics (AIMD) calculations that include effects of surface atom motion. Finally, we simulate time-of-flight (TOF) spectra from the theoretical reaction probability curves and we directly compare them to the raw experimental data. The results show that the use of more flexible functional forms for fitting the raw TOF spectra gives fits that are in slightly better agreement with the raw data and in considerably better agreement with theory, even though the theoretical reaction probabilities still achieve higher values at high energies. The mean absolute error (MAE) for the energy E0 at which the reaction probability equals half the experimental saturation value is now lower than 1 kcal/mol, the limit that defines chemical accuracy, while a MAE of 1.5 kcal/mol was previously obtained. The new AIMD results are only slightly different from the previous static surface results and in slightly better agreement with experiment.

We have studied survival and rotational excitation probabilities of H 2 (v i = 1, J i = 1) and D 2 (v i = 1, J i = 2) upon scattering from Cu(111) using six-dimensional (6D) adiabatic (quantum and quasi-classical) and non-adiabatic (quasi-classical) dynamics. Non-adiabatic dynamics, based on a friction model, has been used to analyze the role of electron-hole pair excitations. Comparison between adiabatic and non-adiabatic calculations reveals a smaller influence of non-adiabatic effects on the energy dependence of the vibrational deexcitation mechanism than previously suggested by low-dimensional dynamics calculations. Specifically, we show that 6D adiabatic dynamics can account for the increase of vibrational deexcitation as a function of the incidence energy, as well as for the isotope effect observed experimentally in the energy dependence for H 2 (D 2 )/Cu(100). Furthermore, a detailed analysis, based on classical trajectories, reveals that in trajectories leading to vibrational deexcitation, the minimum classical turning point is close to the top site, reflecting the multidimensionally of this mechanism. On this site, the reaction path curvature favors vibrational inelastic scattering. Finally, we show that the probability for a molecule to get close to the top site is higher for H 2 than for D 2 , which explains the isotope effect found experimentally.

International audienceDescribing diffraction of atomic and molecular projectiles at fast grazing incidence presents a realchallenge for quantum theoretical simulations due to the high incidence energy (100 eV–1 keV) usedin experiments. This is one of the main reasons why most theoretical simulations performed to dateare based on reduced dimensional models. Here we analyze two alternatives to reduce the computationaleffort, while preserving the real dimensionality of the system. First, we show that grazing incidenceconditions are already fulfilled for incidence angles 6 5 degrees, i.e., incidence angles higher than those typicallyused in experiments. Thus, accurate comparisons with experiment can be performed considering diffrac-tion at grazing incidence, but with smaller total incidence energies, whilst keeping the same experimen-tal normal energy in the calculations. Second, we show that diffraction probabilities obtained at fastgrazing incidence are fairly well reproduced by simulations performed at slow normal incidence. Thislatter approach would allow one to simulate several experimental spectra, measured at the same normalincidence energy for several incidence crystallographic directions, with only one calculation. Thisapproach requires to keep the full dimensionality of the system

Atomic diffraction by surfaces under fast grazing incidence conditions has been used for almost a decade to characterize surface properties with more accuracy than with more traditional atomic diffraction methods. From six-dimensional solutions of the time-dependent Schrödinger equation, we show that diffraction of H molecules under fast grazing incidence conditions could be even more informative for the characterization of ionic surfaces, due to the large anisotropic electrostatic interaction between the quadrupole moment of the molecule and the electric field created by the ionic crystal. Using the LiF(001) surface as a benchmark, we show that fast grazing incidence diffraction of H strongly depends on the initial rotational state of the molecule, while rotationally inelastic processes are irrelevant. We demonstrate that, as a result of the anisotropy of the impinging projectile, initial rotational excitation leads to an increase in intensity of high-order diffraction peaks at incidence directions that satisfy precise symmetry constraints, thus providing a more detailed information on the surface characteristics than that obtained from low-order atomic diffraction peaks under fast grazing incidence conditions. As quadrupole-ion surface potentials are expected to accurately represent the interaction between H and any surface with a marked ionic character, our analysis should be of general applicability to any of such surfaces. Finally, we show that a density functional theory description of the molecule-ion surface potential catches the main features observed experimentally.

The role of van der Waals (vdW) forces in the description of scattering processes of noble gases from metal surfaces is currently under debate. Although features of the potential energy surface such as anticorrugation or adsorption energies are sometimes found to be well described by standard density functional theory (DFT), the performance of DFT to describe diffraction spectra may rely on the accuracy of the vdW functionals used. To analyze the precise role of these vdW forces in noble gas diffraction by metal surfaces, we have thoroughly studied the case of Ne/Ru(0001), for which accurate experimental results are available. We have carried out classical and quantum dynamics calculations by using DFT-based potentials that account for the effect of vdW interactions at different levels of accuracy. From the comparison of our results with experimental data, we conclude that the inclusion of vdW effects is crucial to properly describe diffraction of noble gases from metal surfaces. We show that among the vdW-DFT functionals available in the literature, not all of them can be used to accurately describe this process. DOI: 10.1103/PhysRevB.93.060301The diffraction of noble gases is largely used in surface science as a nondestructive analytical tool to investigate, for example, surface morphology and surface phonons (see Refs. [1,2] and refs. therein). Furthermore, this tool can also be used to study the dynamics of adsorbate/surface systems [3,4]. In order to extract the maximum amount of information from experimental diffraction spectra, a detailed comparison with theoretical simulations is often desirable. However, from a theoretical point of view, the description of the electronic structure of noble-gas atom/surface systems, in particular when metal surfaces are involved, is not a trivial matter due to the possible prominent role of van der Waals (vdW) interactions. The first theoretical approach to treat these kind of systems was reported in the early 80's by Esbejerg and Nørskov [5], who proposed the use of an interaction potential proportional to the unperturbed electron density of the substrate at the position of the atomic projectile. But this approach was questioned only two years later by experimental results showing anticorrugation effects in He scattering from Ni(110) [6] that could not be reproduced with this simple model. Later on, in the 90's, first principles calculations were performed using a jellium model to describe the substrate [7,8]. Although this simple model is good enough to reproduce some general properties, to take into account the lattice structure is essential to analyze many other properties such as the corrugation amplitudes of the system, which are responsible for diffraction scattering phenomena.The periodic lattice structure of a noble-gas atom/surface system can be well described by density functional theory (DFT) with periodic boundary conditions. However, standard DFT functionals do not include, per se, the effect of the van * fernando.martin@uam.es † cristina.diaz@uam.es der W...