General Physics I. Molecular Beams

1. Introduction Since the first measurements of the velocity distribution of atoms evaporating from the surface of bulk liquid helium by Johnston and King, several theories have been advanced to explain the helium evaporation process. In these theories, the excita-tions in liquid He II, phonons and rotons, play an important role in the evaporation mechanism. In this report, we discuss an experiment designed to study the evaporation mechanism and describe an apparatus built to carry out this experiment. In the Johnston-King evaporation experiment the velocity distribution of the evaporation of atoms from bulk liquid helium was measured and they found that the distribution was a Maxwell-Boltzmann distribution characterized by an effective temperature approximately 1°K hotter than the source. Attempts to explain this unusual result centered on the part that phonons and rotons play in the conversion of internal energy of the bulk helium into free particles in the vapor. Phonon conversion at the surface would give a broad low-energy spectrum of particles in the vapor, while roton conversion would give a high-energy tail starting at A-~i = 1. 5 ° K, where A is the roton minimum equal to 8. 68°K, and 4 is the latent heat of evaporation equal to 7. 18 0 K per atom. This combination of conversion processes could explain the higher energy velocity distribution of the vapor. When McWane 3 and Tinker 4 repeated the experiment under better experimental conditions , however, they found that the temperature shift in the vapor was not greater than 0. 3 0 K, and in all probability that the temperature of the evaporating atoms was in equilibrium with the bulk fluid. This shows that we still do not understand the processes that are important in the creation of evaporation atoms from the surface of the bulk fluid and, in particular, how rotons and phonons interact with the surface and how energy is transported across the surface. One way of investigating the surface interactions of phonons and rotons is to direct a beam of these excitations at the surface. These beams of phonons and rotons are the well-known phenomena of second sound in liquid He II. 5 It was demonstrated early in the investigation of second sound, that second-sound pulses incident on the surface of the bulk liquid could be detected by the density variation in the vapor. The density variations in the vapor are due to the heating of …