Theoretical Studies on Carbon and Silicon Clusters: Comparison of the Structures and Stabilities of Neutral and Ionic Forms.

This is a paper to be published in the proceedings of the "International symposium on small particles and inorganic clusters" held in Aix, France, July 4-9, 1988. The field of atomic and molecular clusters has recently witnessed an upsurge of experimental and theoretical work focussed on small clusters of carbon and silicon. Interesting questions such as the relative stabilities of the clusters ("magic numbers") and the nature of the fragmentation processes have been addressed. However, much of the theoretical discussion involving these clusters has been restricted to the neutral molecules while experimentally all the clusters have been detected as ionic species. The correlation between the structures and properties of the neutral and ionic clusters is thus an extremely important question which has to be addressed carefully. Optimized molecular geometries and electronic structures are determined for neutral, positively charged, and negatively charged carbon and silicon clusters containing up to ten atoms. Carbon clusters have linear or monocyclic ground state geometries whereas silicon clusters containing five or more atoms have three-dimensional ground state structures. Neutral C sub 4, C sub 6 and C sub 8 all have linear and monocyclic isomers of comparable stability whereas the ionic forms appear to be generally more stable as linear geometrical arrangements. In the case of neutral and positively charged carbon clusters, the odd-numbered clusters are significantly more stable than the adjacent even-numbered clusters whereas the opposite order of stability occurs for the negative ions. This is due to the large values of the electron affinities of the linear forms of even-numbered clusters such as C sub 4 and C sub 6. The relative stabilities of silicon clusters does not change with the charge state of the clusters.