Refraction of Fast Ne Atoms in the Attractive Well of a LiF(001) Surface

International audience; Ne atoms with energies up to 3 keV are diffracted under grazing angles of incidence from a LiF(100) surface. For small momentum component of the incident beam perpendicular to the surface, we observe an increase of the elastic rainbow angle together with a broadening of the inelastic scattering profile. We interpret these two effects as the refraction of the atomic wave in the attractive part of the surface potential. Quantitative agreement between our data and full quantum scattering simulations is achieved for up to ten diffraction orders and allows us to extract a potential well depth of 10.4 meV. Our results set a benchmark for more refined surface potential models which include the weak Van der Waals region, a long-standing challenge in the study of atom-surface interactions. Refraction is a well known phenomenon occurring when light is deflected at an interface between two transparent media of different refractive indices. In the early days of quantum mechanics, refraction of matter-wave was first observed with electrons [1, 2] and was explained by considering the beam to be refracted by the inner potential of the material [3]. Refraction has been also reported for neutrons [4], as well as for molecular and atomic projectiles scattered from surfaces at thermal energies [5]. In this latter case the refraction is produced by an attractive Van der Waals (VdW) part of the projectile-surface interaction potential [6]. The physisorption region , dominated by weak VdW polarisation forces, is of paramount importance for the surface self-assembly [7] as well as for adhesion and cohesion of less dense systems, for example, liquid crystals, polymers and biomolecular surfaces [8]. In this respect, collision experiments between neutral atoms and surfaces represent an ideal platform for characterization of the VdW potential [9]. In the past, atom scattering studies of the attractive part of the surface potential have been performed studying diffraction and refraction of the projectile beams at thermal energies [5, 10, 11]. Recent observation of the surface diffraction of fast atoms of keV energies under grazing incidence (GIFAD or FAD) [12-14] offered yet unexplored possibilities of the surface analysis. Grazing incidence conditions imply slow motion of the projectile perpendicular to the surface at fast motion parallel to it. GIFAD thus combines the sensitivity of thermal atoms with the geometry of reflection high energy electron diffraction, that allows one to record the full diffrac-tion pattern at once using an imaging detector (Fig.1). Fast atom diffraction has already proven to be sensitive to surface rumpling at the pm scale [15] and to the VdW potential [16, 17], in particular with the observation of bound state resonances [18]. Similar to the thermal atom diffraction, most of the GIFAD studies have been performed with light (He, H, H 2) projectiles. The diffrac-tion experiments with heavier Ne atoms are challenging because of enhanced inelastic scattering [19-22].-15-10-5 0 5 10 15 a) b) k y in diffraction orders FIG. 1: a) Diffraction pattern in the (ky, kz) plane recorded for 500 eV Ne atoms scattered off LiF(100) along the 110 direction. The Laue circle is shown with a dashed white line and corresponds to an incidence angle θ = 0.42 •. φr indicates the classical rainbow angle. b) Projected experimental intensity along the Laue circle (blue circles). The red line corresponds to a fit of the diffracted intensity using a Lorentzian line-shape for each peak (green line). In this Letter we show the diffraction of fast Ne atoms on a LiF surface. We fully exploit the shorter wavelength of Ne atoms so that many diffraction peaks can be observed offering a surface probe with a resolution not reachable with light projectiles [23]. In particular, we reveal the refraction of the matter wave at surface resulting in a shift of the elastic rainbow angle, and the broadening of the inelastic profile. Rich diffraction pattern measured in our experiments allows to gain further insights on the projectile-surface interaction from a detailed comparison with quantum simulation. Our work, provides an important basis for a characterization of the physisorption region in the regime of low energies dominated by weak VdW forces.