The structure and energetics of He-3 and He-4 nanodroplets doped with alkaline earth atoms

We present systematic results, based on density functional calculations, for the structure and energetics of He-3 and He-4 nanodroplets doped with alkaline earth atoms. We predict that alkaline earth atoms from Mg to Ba go to the center of He-3 drops, whereas Ca, Sr, and Ba reside in a deep dimple at the surface of He-4 drops, and Mg is at their center. For Ca and Sr, the structure of the dimples is shown to be very sensitive to the He-alkaline earth pair potentials used in the calculations. The 5s5p <- 5s(2) transition of strontium atoms attached to helium nanodroplets of either isotope has been probed in absorption experiments. The spectra show that strontium is solvated inside He-3 nanodroplets, supporting the calculations. In the light of our findings, we emphasize the relevance of the heavier alkaline earth atoms for analyzing mixed He-3-He-4 nanodroplets, and in particular, we suggest their use to experimentally probe the He-3-He-4 interface.