Quantum evaporation from superfluid He-4: Normal incidence

We study the scattering of atoms and quasiparticles at the surface of He-4 at T=0 K. Using the microscopic theory of Beliaev in areal-space formulation we derive an equation of motion for the quasiparticles valid in bulk helium, through the surface and in the vacuum. Assuming normal incidence, we solve the equation and calculate probabilities for the various surface scattering processes for a range of energies (between 7.5 and 15 K). Some of the scattering rates obtained are markedly different from those reported recently.