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

TL;DR

This study uses density functional calculations to explore the structure and energetics of helium nanodroplets doped with alkaline earth atoms, revealing their preferred positions and experimental spectroscopic evidence for solvation.

Contribution

It provides the first systematic analysis of alkaline earth atom positioning in helium nanodroplets and correlates theoretical predictions with experimental spectra.

Findings

Alkaline earth atoms from Mg to Ba prefer the center of $^3$He drops.

Ca, Sr, and Ba form surface dimples in $^4$He drops.

Spectroscopic data supports the solvation of Sr inside helium nanodroplets.

Abstract

We present systematic results, based on density functional calculations, for the structure and energetics of $^3$He and $^4$He nanodroplets doped with alkaline earth atoms. We predict that alkaline earth atoms from Mg to Ba go to the center of $^3$He drops, whereas Ca, Sr, and Ba reside in a deep dimple at the surface of $^4$He 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 $5s5p\leftarrow5s^2$ 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 $^3$He nanodroplets, supporting the calculations. In the light of our findings, we emphasize the relevance of the heavier alkaline earth atoms for analyzing mixed $^3$He-$^4$He nanodroplets, and in particular, we suggest their use to experimentally probe the $^3$He-$^4$He interface.