COMPARATIVE AB-INITIO STUDIES OF SMALL TIN AND LEAD CLUSTERS

We have performed nb initio total-energy pseudopotential calculations on neutral and negatively charged Sn-n and Pb-n (n = 3 - 10) clusters. The lowest energy structures have been determined for all clusters, a nd the stabilities of neutral clusters were investigated by comparing their evaporation energies and stability functions. Clusters with n = 7, 10 were found to be most stable while the clusters with n = 8 and P b-5 were much less stable, in agreement with features of the observed mass spectra. Calculations on Sn-n(-) and Pb-n(-) show that both atomi c and electronic structures of a neutral cluster change substantially upon charging. The densities of states of Sn-n(-) clusters reproduce t he main features of the experimental photoelectron spectra. The agreem ent is poorer for Pb-n(-) clusters where the calculations underestimat e the separation between energy levels which we think is due to the la rger spin-orbit splitting in Pb, which was neglected in the calculatio ns. We found that the differences between Sn and Pb clusters cannot be completely addressed without a more complete accounting of relativist ic effects. The electron affinities of Sn-n and Pb-n clusters have als o been calculated and the results agree fairly well with experimental values. Finally we considered Sn-4(2-) and Pb-4(2-) clusters and relat ed the results to the formation of Zintl anions in liquid alkali-Sn an d alkali-Pb alloys.