  The probability of evaporation induced by $R^+$ and $R^-$ rotons at the
surface of superfluid helium is calculated using time dependent density
functional theory. We consider excitation energies and incident angles such
that phonons do not take part in the scattering process. We predict sizable
evaporation rates, which originate entirely from quantum effects. Results for
the atomic reflectivity and for the probability of the roton change-mode
reflection are also presented.
