A study of the evaporation of superfluid $^{4}\mathrm{He}$, using the heat-pulse technique, is presented; working at low temperature, $0.1lTl0.6\ifmmode^\circ\else\textdegree\fi{}$K, the phonon and the roton fluids are decoupled. We observe atoms evaporated by a phonon second-sound pulse between 0.4 and 0.6 \ifmmode^\circ\else\textdegree\fi{}K. The temperature dependence of the signal is interpreted by a simple model where one phonon of energy $E$ emits one atom of energy $E\ensuremath{-}{E}_{0}$ (${E}_{0}=7.15\ifmmode^\circ\else\textdegree\fi{}$K is the atomic binding energy in the liquid). At lower temperature, down to 0.1 \ifmmode^\circ\else\textdegree\fi{}K, a ballistic-phonon regime is observed, associated with no detected evaporation. Concerning rotons, we observe well-defined signals due to atoms evaporated by them. Analyzing the arrival time as a function of the liquid path, we propose an evaporation process such as one roton of energy $E$ emits one atom of energy $E\ensuremath{-}{E}_{0}$. This leads to a minimum kinetic energy of 1.5 \ifmmode^\circ\else\textdegree\fi{}K for the evaporated atoms, effectively observed. An estimation of the roton mean free path is deduced and a maximum roton velocity of 160 \ifmmode\pm\else\textpm\fi{} 10 m ${\mathrm{sec}}^{\ensuremath{-}1}$ is observed.