Dissociation at surfaces: Barriers and fences

Molecular beam studies involving the interaction of (hyper-)thermal O2 with Ag(111) are reported. Several different scattering paths are observed: physisorption followed by desorption, direct-inelastic scattering from both the physisorption and the chemisorption potential, transient trapping-desorption, molecular chemisorption and dissociative chemisorption. The underlying dynamics of all of these processes is reviewed. The results are discussed in terms of simple potential energy surfaces. Dissociative chemisorption on Ag(110) appears to be strongly hindered by the build-up Ag-O- added rows on the surfaces, which act like fences blocking dissociation.