ATOMIC-STRUCTURE AND COLLECTIVE EXCITATIONS OF MEDIUM-SIZE NANKM CLUSTERS

Density functional theory is used to study the atomic and electronic structure of NanKm clusters with up to seventy atoms. The simplifying approximation has been made of replacing the external potential of the ionic background by its spherical average about the cluster centre in the iterative process of solving the Kohn-Sham equations for each geometry tested. The search for the equilibrium geometry is performed by employing steepest descent and simulated annealing techniques. We have found segregation of K to the surface and when the cluster is large enough, a neat stratification of K and Na shells. Those effects (segregation and stratification) do not perturb the electronic magic numbers well known for pure alkali metal clusters. Our results for the atomic structure are rather similar to those reported earlier for NanCsn clusters. We have also studied in a selected case, Na20Cs20, the dependence of the collective electronic excitation spectrum on the segregation and other geometric characteristics of the cluster.