On the formation mechanism of impurity–helium solids: evidence for extensive clustering

Optical emission studies of a discharged nitrogen–helium gas injected into superfluid helium near 1.5 K are described. Analysis of atomic (α group) and molecular Vegard–Kaplan transitions clearly indicates that the emitting species are embedded inside nitrogen clusters. Cluster formation is most efficient in the crater formed on the liquid surface. Model calculations based on the classical bubble model and density functional theory suggest that under the experimental conditions only clusters consisting of more than 1000 molecules have sufficient kinetic energy for stable cavity formation to occur inside liquid helium. The results obtained suggest that impurity–helium solids formation is a consequence of extensive clustering in the gas jet.