REACTIONS OF COBALT CLUSTERS WITH WATER AND AMMONIA - IMPLICATIONS FOR CLUSTER STRUCTURE

Reactions of cobalt clusters in the 6 to 150 atom size range with water and ammonia are used to probe cluster geometrical structure. Clusters saturated with ammonia are found in several isomeric forms in much of this size region, including a structure based on icosahedral packing for clusters having 50 to 120 atoms. The variations with cluster size of the binding energy for a single water molecule provide evidence for icosahedral structure of the bare cobalt clusters in the same size range seen for the ammoniated clusters. However, both probes suggest that in many cases the bare clusters also have multiple isomers. The evidence for icosahedral structure in the bare and ammoniated clusters disappears above approximately 120 atoms and is clearly not seen at the third icosahedral shell closing at 147 atoms. Comparison with earlier studies of hydrogenated cobalt clusters and with bare and hydrogenated nickel clusters is made. Reasons for the failure to form the closed-shell 147 atom icosahedral cluster are discussed.