Abstract

The diffraction of noble gases is largely used in surface science as a nondestructive analytical tool, to investigate, for example, surfaces morphology, surface phonons or the dynamics of adsorbate/surface systems [1,2,3]. However, in order to extract the maximum amount of information from experimental diffraction spectra, a detailed comparison with theoretical simulations is often desirable. The periodic lattice structure of a noble-gas atom/surface system could be, in principle, 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 der Waals (vdW) forces, which are expected to be the main responsible forces driving the interaction of noble-gas atoms with surfaces. In the early 2000s, it was suggested, for example, that standard DFT could provide an appropriate description of scattering of Ne and He from a metal surface, due to the small interatomic distances at the classical turning points [4]. However, this hypothesis was never tested through a direct comparison with experimental diffraction spectra. We have carried out this test using as benchmark systems Ne/Ru(0001) [5]. We have performed a detailed theoretical study of diffraction of Ne/Ru(0001) using standard DFT and state-of-the-art DFT with explicit inclusion of vdW effects. Our study reveals that taking into account vdW forces is essential to accurately reproduce experimental results. Furthermore, our results also suggest that only methods that go beyond pairwise-based corrections are able to accurately describe diffraction of noble-gas atoms from surfaces. On the other hand, despite the promising results obtained for Ne, He diffraction remains a challenge.

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