Velocity Distributions in Molecular Beams from Nozzle Sources

The velocity distributions in molecular beams of argon produced by the Kantrowitz-Grey supersonic nozzle technique have been investigated using a time-of-flight method. It is shown that the transition from continuum to free molecular flow in free jet expansion limits the minimum width of the velocity distribution which may be obtained. The Mach number corresponding to the axial velocity distribution of the expanding gas approaches a limit which varies inversely with the two-fifths power of the Knudsen number at the nozzle. With argon at room temperature the narrowest velocity distribution obtained corresponds to a Mach number of 23 (90% of the molecules within 5.4% of the most probable velocity).