Structural studies of impurity-helium solids

We have used x-ray diffraction and ultrasound techniques to study the structure of mesoporous impurity--helium solids created after the injection of impurity particles $({\mathrm{D}}_{2},$ Ne, ${\mathrm{N}}_{2},$ Kr) into a volume of superfluid ${}^{4}\mathrm{He}.$ Clusters of impurities with size of order $50\ifmmode\pm\else\textpm\fi{}20 \AA{}$ and density $\ensuremath{\sim}{10}^{20} \mathrm{impurities}/{\mathrm{cm}}^{3}$ were observed by x-ray diffraction. The presence of a wide distribution of pore sizes in Im--He solids was revealed by ultrasound (80 to $8600 \AA{})$ and by small-angle x-ray scattering (80 to $g400 \AA{}).$ Both x-ray and ultrasound methods detected irreversible structural changes when samples were warmed above ${T}_{\ensuremath{\lambda}}=2.17 \mathrm{K}.$ This is ascribed to the aggregation of small clusters caused by thermally activated diffusion. In addition to being of fundamental interest, the properties of the unique porous media studied in this work may be relevant to investigations of low temperature chemical reactions, storage of free radicals, matrix isolation spectroscopy, and superfluid ${}^{4}\mathrm{He}$ contained in the pores of an extremely compliant medium.