EVAPORATION FROM SUPERFLUID HELIUM

A simple theory of evaporation from liquid He II is constructed, and it is used to analyze the experimental results of Johnston and King on the velocity distribution of atoms evaporating from He II at low temperatures. The theory suggests that rotons are responsible for shifting the velocity distribution to energies 1.5°K, in good agreement with experiment. However, multiexcitation processes are needed to explain the complete velocity distribution at high energies. We propose two new experiments that would test our model and provide new information concerning excitations in liquid helium. © 1969 The American Physical Society.