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

We investigate the effect of roton backflow on the scattering of atoms, rotons and phonons at the free surface of superfluid He-4 at T = 0 K by including 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 obliquely to the free surface. We calcalate probabilities for the various one-to-one surface scattering processes allowed for a range of energies and compare the scattering rates with those obtained when backflow is neglected.