Quasiparticle scattering at helium surfaces: A microscopic theory

We study the scattering of quasiparticles and atoms at the free surface of superfluid He-4 at T=0 K. As a starting point we take Beliaev's formalism i n a real-space formulation and derive equations of motion for the elementar y excitations valid in hulk helium, through the surface and the vacuum. We solve the equations numerically for the wave functions, from which we calcu late the currents associated with the atoms, phonons, and rotons. These are then used to calculate the probabilities of all the possible one-to-one su rface scattering processes. Results are presented for a wide range of energ ies, both for normal incidence and oblique incidence. The quantum evaporati on results for fixed angles of incidence are used in experimental simulatio ns which calculate the angular dependence of evaporated atom signals and th ese are compared with experimental data. [S0163-1829(99)05929-9].