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Cooled molecular beams are created upon adiabatic expansion of high pressure gas through a nozzle into a low pressure background.
Upon supersonic expansion, random motion of a static gas is converted into directed mass flow in the expanding jet. If the system is isentropic during expansion, then the temperature of the gas is reduced.
A skimmer is positioned in front of the expansion to extract the
centre of the beam (from the `zone of silence' where the gas speed is
greater than the local speed of sound) and avoid shock waves which inevitably occur during supersonic expansion.
Tim Gibbon
1999-09-06