Adsorption of oxygen on Pd(111): Precursor kinetics and coverage-dependent sticking

The adsorption of oxygen on Pd(111) was studied using supersonic molecular beam techniques. The dependences of the initial sticking coefficient (zero coverage) on O2 translational energy (0.083–0.69 eV) and substrate temperature (100–650 K) indicate a precursor dissociation mechanism involving a chemisorbed and a physisorbed precursor state. At low translational energies, ⩽0.13 eV, the chemisorbed (molecular) precursor state is populated either directly or via trapping into the physisorbed precursor state, while the direct molecular chemisorption channel dominates at higher translational energies. No indication of direct dissociation was observed. The oxygen coverage dependence of the sticking coefficient shows that extrinsic precursors assist the sticking at low substrate temperatures. The saturation coverage is independent of O2 translational energy, supporting the proposed precursor dissociation mechanism.