Chemisorption on Size Selected Metal Clusters: Activation Barriers and Chemical Reactions for Deuterium on Aluminum Cluster Ions.

We describe a new approach to investigating chemisorption on size selected metal clusters. This approach involves investigating the collision energy dependence of chemisorption using low energy ion beam techniques. The method provides a direct measure of the activation barrier for chemisorption and in some cases an estimate of the desorption energy as well. We describe the application of this technique to chemisorption of deuterium on size selected aluminum clusters. The activation barriers increase with cluster size (from a little over 1 eV for Al sub (10) sup + to around 2 eV for Al sub (27) sup +), and show significant odd-even oscillations. The activation barriers for the clusters with an odd number of atoms are larger than those for the even numbered clusters. In addition to chemisorption of deuterium onto the clusters, chemical reactions were observed, often resulting in cluster fragmentation. The main products observed were Al sub (n-1) D sup + , Al sub (n-2) sup + and Al sup + for clusters with n