Role of the He-atom surface potential on the Cu(001) phonon determination

We present new measurements of He atom inelastic scattering from the (001) surface of Copper. Our results confirmed the recent data obtained in Gottingen and show that at large momentum transfer in the GM direction the intensity of the longitudinal resonance is larger than that of the Rayleigh wave. We interpret these results by evaluating the reflection coefficient in the Distorted Wave Born Approximation. By taking the He surface interaction potential as a sum of nonspherical pairwise potential we are able to explain quantitatively the behavior of the experimental spectra