  Ne atoms with energies up to 3 keV are diffracted under grazing angles of
incidence from a LiF(001) surface. For a 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. We use a fast, rigorous dynamical
diffraction calculation to find a projectile-surface potential model that
enables a quantitative reproduction of the experimental data for up to ten
diffraction orders. This allows us to extract an attractive 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.
