Ca impurity in small mixed He-4-He-3 clusters

The structure of small mixed helium clusters doped with one calcium atom has been determined within the diffusion Monte Carlo framework. The results show that the calcium atom sits at the 4He-3He interface. This is in agreement with previous studies, both experimental and theoretical, performed for large clusters. A comparison between the results obtained for the largest cluster we have considered for each isotope shows a clear tendency of the Ca atom to reside in a deep dimple at the surface of the cluster for 4He clusters, and to become fully solvated for 3He clusters. We have calculated the absorption spectrum of Ca around the 4s4p←4s2 transition and have found that it is blue-shifted from that of the free-atom transition by an amount that depends on the size and composition of the cluster