The effect of cluster formation on mass separation in binary molecular beams

The downstream composition of a skimmed supersonic binary molecular beam originally consisting of a 20% neon/80% xenon mixture before expansion has been studied as a function of nozzle stagnation pressure. We have found that the neon to xenon ratio dropped dramatically as the stagnation pressure was increased at low nozzle temperature (303 K), a drop which cannot be well described by existing theory. Time-of-flight (TOF) measurements indicate that Xe clustering occurs as the stagnation pressure is increased. This clustering coincides with the additional Ne depletion we observe. At a higher nozzle temperature where Xe clustering does not occur (573 K), this measured mass separation phenomenon is absent. Similar experiments have been done for another binary mixture, 20% $O2/80%$ Xe. Similar anomalous mass separation is observed with this mixture, confirming the attribution of this phenomenon to clustering of the more massive component of the mixture. These findings have implications for novel methods of gas-dynamics-based mass separation potentially including isotope enrichment.