2 - Supersonic Beam Sources

TLDR: In this paper, the authors discuss the supersonic expansions from circular nozzles and plane jets, and show that collisions occur with sufficient frequency for equilibrium to be maintained throughout the expansion process, but the idealized continuum model will break down when the expanding gas becomes sufficiently rarefied that its density approaches that of the background gases.

Abstract: The supersonic expansions provide a means of preparing a molecular beam with a well-defined kinetic energy, which is particularly useful for crossed-beam and beam-surface scattering experiments This chapter discusses the supersonic expansions from circular nozzles The idealized continuum model assumes that collisions occur with sufficient frequency for equilibrium to be maintained throughout the expansion process This chapter also describes the interaction with background gases In any real apparatus, the idealized continuum model will also break down when the expanding gas becomes sufficiently rarefied that its density approaches that of the background gases This chapter also focuses on the supersonic expansions from slit nozzles (plane jets) In an idealized continuum slit supersonic expansion, it is still assumed that the gas flows without heat transfer to the walls and without viscosity, and an adiabatic, isentropic expansion is obtained