Formation of activated diatomic species on mass-selected clusters

Using vibrationally resolved ultraviolet photoelectron spectroscopy, we provide evidence that molecular adsorption of diatomic molecules is often preferred on small nanoclusters. These results are much different from those of the respective metal bulk crystals, on which dissociative chemisorption takes place. The systems we studied include N₂ adsorption on W and Nb clusters, H₂ on Ti clusters, and O₂ on Au and Ag clusters, implying that the non-dissociative chemisorption can be also relevant for many other systems. The less efficient dissociation of the diatomic molecules on these small nanoclusters can be rationalised by the reduced metal to adsorbate charge transfers as a consequence of the less effective screening of the positive holes in these small nanoclusters. Our results suggest that chemical reactions on small nanoclusters can often proceed through activated non-dissociated species.