Experiments on superfluidHe4evaporation

A study of the evaporation of superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, using the heat-pulse technique, is presented; working at low temperature, $0.1<T<\mathrm{}0.6\mathrm{}°$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 °K. The temperature dependence of the signal is interpreted by a simple model where one phonon of energy $E$ emits one atom of energy $E-E_0$ ($E_0=7.15\mathrm{}°$K is the atomic binding energy in the liquid). At lower temperature, down to 0.1 °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 $E-E_0$. This leads to a minimum kinetic energy of 1.5 °K for the evaporated atoms, effectively observed. An estimation of the roton mean free path is deduced and a maximum roton velocity of 160 ± 10 m ${\sec }^{-1\mathrm{}}$ is observed.