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 ${\mathrm{Ti}}_{n}^{\ensuremath{-}}$ clusters using mass and photoelectron spectroscopy. Our experimental data support the assumption that for clusters with up to four Ti atoms, adsorption of intact ${\mathrm{H}}_{2}$ molecules is the energetically preferred configuration. For larger ${\mathrm{Ti}}_{n}$ clusters with $ng4,$ dissociative hydrogen chemisorption is the most stable configuration, forming clusters with an electronic structure similar to bulk ${\mathrm{TiH}}_{2}.$