A MOLECULAR-BEAM STUDY OF THE O2-PT(111) INTERACTION

We have performed scattering experiments for O2 on Pt(111) for translational energies up to 1.7 eV (1.09 eV normal energy). The scattered O2 flux from the surface is measured as function of arrival time and outgoing angle. The experimental results reveal pronounced lobular scattering. The final energy shows for most experimental conditions a maximum near the specular direction. At higher incident energy, however, no such maximum is observed and the final energy is increasing monotonically with final angle. These results suggest a transition from scattering at a "flat" vibrating surface for low energies to scattering from a corrugated surface for the higher energies. The angular width at low incoming energy is comparable to that for Ar scattering, and is quite low compared with O2 scattering from Ag(111) where a broadening of the angular spectrum towards the surface normal is observed. For Pt(111) we see a much reduced broadening despite the fact that the binding energies of the physisorption, chemisorption and dissociation states are similar for both systems. The broadening in the case of both Ag(111) and Pt(111) at higher normal energies can be ascribed to the influence of the chemisorption precursor in the interaction potential. In the case of Pt(111), however, the direct inelastic distributions are additionally modified by the significant probability of dissociation.