SCATTERING OF ELEMENTARY EXCITATIONS AT THE SURFACE OF SUPERFLUID HE-4

We study the scattering of atoms, rotons and phonons at the free surfa ce of superfluid He-4. We calculate the evaporation, condensation and reflection probabilities in the framework of linearized time dependent density functional theory accounting for elastic one-to-one processes . We explore the angular dependence of the scattering probabilities sh owing that most results are independent of the incidence angle and can be expressed in terms of a single energy dependent parameter.