Ionization-driven collapse of xenon and argon foams in superfluid helium droplets

Abstract

The electron diffraction studies of neutral and ionic rare gas clusters formed within superfluid helium droplets reveal several notable findings. Under various doping conditions and droplet sizes ranging from 104 to 106 helium atoms, both neutral argon and xenon clusters can exhibit foam-like structures, in which helium atoms reside between rare gas atoms and inhibit the formation of a fully bound structure. Under comparable doping conditions and droplet sizes, argon clusters exhibit even fewer bound structures, best described by more dispersed distributions compared to xenon clusters. Under low doping conditions—despite differences in droplet sizes (104 vs 106 helium atoms per droplet)—argon clusters yield broad, Gaussian distance distributions. Moreover, increasing the doping pressure while maintaining the same droplet conditions promotes the formation of more tightly bound clusters, with a greater contribution from the van der Waals distance in the overall pair-distance distributions. Finally, electron impact ionization of both rare gas clusters triggers a collapse of the foam-like structures, resulting in clusters that are most accurately described by a single dominant interatomic distance.