Calcium atoms attached to mixed Helium droplets: a probefor the He3-He4 interface

Experimental and theoretical results for mixed helium droplets doped with one calcium atom are reported. The absorption spectrum around the 4s4p <- 4s(2) transition is calculated as a function of the composition of the droplet and compared to the experiment. Our study reveals the distinct feature that for specific (3)He concentrations, Ca atoms sit at the (3)He-(4)He interface. This particular property of Ca, likely not shared with any other atomic or molecular species but the heavier alkaline-earth atoms, might offer the possibility of using it to probe the (3)He-(4)He interface of the droplet