Impurity-helium condensates (IHCs) containing krypton and hydrogen atoms immersed in superfluid ${}^4\text{H}\text{e}$ have been studied via cw electron-spin-resonance (ESR) techniques. The IHCs are gel-like aggregates of nanoclusters composed of krypton and hydrogen atoms. We have found that very high average $\left(\sim {10}^{19}{\text{cm}}^{-3}\right)$ and local $\left(\sim 6\times {10}^{19}{\text{cm}}^{-3}\right)$ concentrations of hydrogen atoms can be stabilized in these samples. The analysis of ESR line positions and shapes shows that most of the H atom population resides in solid ${\text{H}}_2$ layers on the krypton nanocluster surfaces. High concentrations of atomic hydrogen achieved in IHCs provide an important step in the search for collective magnetic and quantum statistical phenomena at lower temperatures.

• Received 2 April 2009

DOI:https://doi.org/10.1103/PhysRevB.79.180506