Velocity Spectrum of Atoms Evaporating from a Liquid He Surface at Low Temperatures

Using the heat-pulse technique, we have measured the velocity spectrum of atoms evaporating from a free liquid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ surface. In the ballistic regime (at sufficiently low ambient temperatures) this spectrum is found to be Maxwell-Boltzmann in nature with no effects evident owing to the quasiparticle nature of the liquid. At higher vapor pressures (higher temperatures) the transition from ballistic atomic propagation to collective adiabatic sound in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ has been studied.