Condensation embryos in an expanding gas beam

Abstract

Polymeric molecules have been observed in the mass spectra of several gases expanding isentropically from a high-pressure reservoir into a vacuum. The possible origins of these complex molecules have been examined and it is concluded from the experimental evidence that they are condensation 'embryos' forming in the cooled, supersaturated gas. Some interesting features of the condensation process are highlighted; in particular the formation of mixed polymers, such as Ar. CO$_2$ in expanding gas mixtures, and the preferential formation of even-numbered polymers, (H$_2$O)$_2$ and (H$_2$O)$_4$, in condensing steam.