Bonding and Stabilities of Small Silicon Clusters: A Theoretical Study of Si sub 7 -Si sub (10).

The study of the structures and properties of small elemental clusters has been an extremely active area of current research. In particular, small silicon clusters have been the subject of a variety of experimental investigations by several groups within the last three years. These studies have included photofragmentation , collision induced dissociation, photoelectron spectroscopy of negative ions, and ion-molecule reactions of both positive and negative ions. However, detailed theoretical understanding of the nature of these clusters is lacking in most cases. In this work, accurate Ab initio quantum chemical calculations have been performed to study the structures and energies of intermediate-sized silicon clusters (Si sub n ,n=7-10). The effects of polarized basis sets and electron correlation have been included in these calculations. Along with our earlier work on Si sub 2 -Si sub 6 ,we now have detailed results on Si sub 2 -Si sub (10) all studied at a uniform level of theory which permits detailed comparisons to be performed between the different clusters. The most stable structure for Si sub 7 is a pentagonal bipyramid and the lowest energy calculated structures for Si sub 8 -Si sub (10) correspond to capped octahedral or prismatic geometric arrangements. The evolution of the cluster geometries with increasing size is discussed. Clusters containing 4, 6, 7 and 10 atoms have been identified as "magic numbers" for small silicon clusters, both theoretically and experimentally. The hybridization and bonding in small silicon clusters is discussed. Our results are used to interpret the recent photoelectron spectra of negative silicon cluster ions.