Dynamics of the chemisorption of O2 on Pt(111): Dissociation via direct population of a molecularly chemisorbed precursor at high incidence kinetic energy

We have used the thermal desorption spectroscopy of the O/O2+CO→CO2 system to probe the chemical nature of oxygen that remains on a Pt(111) surface following exposure to a supersonic O2 beam under various conditions. We find that for a surface temperature of 90 K, the resulting CO2 formation thermal desorption spectrum is the same for all beam kinetic energies employed up to 1.1 eV at normal incidence, in all cases resembling that assigned to the O2+CO co-adsorbate system. This spectrum is clearly distinct from the O+CO case, where atomically chemisorbed oxygen is obtained either by thermal dissociation of O2 on the surface or by exposing the 90 K surface to a beam containing O atoms. These results imply that the dissociative chemisorption of O2 on Pt(111) proceeds by way of a molecular precursor even at relatively high incidence kinetic energies, at least as high as 1.1 eV. This interpretation readily accounts for the strong surface temperature dependence associated with dissociation under these conditions but contrasts with previous assignment of a direct (or quasidirect) dissociation process at high energies. We have also reexamined a number of previous observations in terms of this new picture, including the initial decline in dissociation probability with increasing kinetic energy. This falloff is attributed to a decrease in the trapping probability into a physisorption state, as recently suggested by Luntz et al. Considering the present results in the light of other recent studies, it now seems clear that the physisorption state is then a precursor to the molecular chemisorption state which can also be accessed directly at high kinetic energy. In this picture the molecular chemisorption state is then a precursor to dissociation even at high kinetic energy, and the dissociation probability depends on the (temperature-dependent) branching ratio between the dissociation and ultimate desorption of these species.