  There has been notable uncertainty regarding the degree of solvation of
alkaline-earth atoms, especially Mg, in free He-4 nanodroplets. We have
measured the electron energy dependence of the ionization yield of picked-up
atoms. There is a qualitative shape difference between the yield curves of
species solvated in the middle of the droplet and species located in the
surface region; this difference arises from the enhanced role played by the
Penning ionization process in the latter case. The measurements demonstrate
that Mg, Ca, Sr and Ba all reside at or near the droplet surface.
