Properties of a supersonic jet

11 Spectroscopy of molecules in supersonic jets 9.3.11.1 Properties of a supersonic Jet 

In Section 2.3.5.1 we have seen that pumping of atoms or molecules through a narrow slit, or small pinhole, whose width (or diameter) is about 20 pm at a pressure of a few torr on the high-pressure side of the aperture produces an efllisive beam. Removal of pressure 

In 1951, Kantrowitz and Grey suggested that if conditions are changed such that 

Why are fhese beams, or jefs, distinguished from effusive beams by fheir description as supersonic In some ways fhis description is rafher misleading, firsf because particles in an effusive beam may well be fravelling af supersonic velocities and, second, because fhe name implies fhaf somefhing special happens when fhe particle velocities become supersonic whereas fhis is nof fhe case. Whaf supersonic is meanf fo imply is fhaf fhe particles may have very high Mach numbers (of fhe order of f 00). The Mach number M is defined as 

If a particularly parallel beam is required in the chamber into which it is flowing the beam may be skimmed in the region of hydrodynamic flow. A skimmer is a collimator which is specially constructed in order to avoid shockwaves travelling back into the gas and increasing 7). The gas that has been skimmed away may be pumped off in a separate vacuum chamber. Further collimation may be carried out in the region of molecular flow and a so-called supersonic beam results. When a skimmer is not used, a supersonic jet results this may or may not be collimated. 

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