Indication of a size-dependent transition from molecular to dissociative chemisorption on clusters

We report experimental indications for a size-dependent change of the chemical nature of chemisorption on small atomic clusters. We studied chemisorption of atomic hydrogen on negatively charged Ti-n(-) clusters using mass and photoelectron spectroscopy. Our experimental data support the assumption that for clusters with up to four Ti atoms, adsorption of intact H-2 molecules is the energetically preferred configuration. For larger Ti-n clusters with n>4, dissociative hydrogen chemisorption is the most stable configuration, forming clusters with an electronic structure similar to bulk TiH2. [S0163-1829(99)15847-8].