Hydrogen adsorption on clean and oxygen covered Pt(111)

The dissociative adsorption of hydrogen on oxygen covered Pt(111) has been investigated using molecular beam techniques. The D₂-sticking probability has been measured as a function of oxygen coverage (0 < _O < 0.25 ML) and angle of incidence for two incident energies, 14 and 63 meV. In addition, the order of the oxygen layer has been measured using thermal He-scattering. The measurements show clear evidence for the existence of two distinct adsorption processes both on the oxygen covered and on the clean Pt(111) surface, i.e. in the limit were _O approaches zero: an activated process which depends on the total oxygen coverage and a non activated process which is sensitive only to the amount of locally ordered oxygen. The non activated process can be explained in terms of a mechanism involving a short living precursor state. The picture for the activated process is less clear. The dependence of this process on the incident energy seems strong evidence for a mechanism involving a barrier to dissociation directly upon impact, whereas the dependence on the oxygen coverage supports previously reported experiments which seem to be only compatible with a precursor mechanism.