Molecular beams with extremely narrow velocity spreads from free jet expansions

Velocity distributions in molecular beams from free jets of light gases (He, H$sub 2$ and mixtures of 1% Xe in H$sub 2$) were studied as a function of the source Knudsen number. They were found to be narrower than what would be expected from previous studies. Mach numbers of 50 were observed for helium at a source temperature of T=85 deg K. The speed ratio for sources at room temperature was found to be S=29 for pure H$sub 2$ and 35 for a 1% Xe-99% H$sub 2$ mixture. (FR)