The effect of roton backflow on quantum evaporation from superfluid He-4

We investigate the effect of roton backflow on the scattering of atoms, rot ons and phonons at the free surface of superfluid He-4 at T = 0 K by includ ing backflow semi-phenomenologically in the form of a backflow potential in the theory of Sobnack ct al, [M.B. Sobnack, J. C. Inkson, and J. C.H. Fung , Phys. Rev. B 60, 3465 (1999)]. We assume that all the surface scattering processes are elastic and that the quasiparticles and atoms are incident ob liquely to the free surface. We calcalate probabilities for the various one -to-one surface scattering processes allowed for a range of energies and co mpare the scattering rates with those obtained when backflow is neglected.