Incidence angle dependence of scattering and dissociation of O2 on Al(111): Possible weakly bound molecular precursors

Molecular beams have been used to search for evidence for a weakly bound molecular precursor in the interaction of O2 with Al(111). The experiments are consistent with a precursor whose binding energy is smaller than 0.1 eV. The total reflectivity as a function of incidence angle shows a pronounced dip at 25° for Etrans between 90 and 300 meV. This feature corroborates an earlier observation by Österlund et al. in sticking measurements. Modeling using a reduced dimensionality potential energy surface shows a similar behavior which is caused by steering into a shallow molecular adsorption well located at the same site in the unit cell as the maximum in the barrier towards dissociative adsorption. This effect is not observed if the molecular adsorption well is located at the same site as the minimum energy pathway to dissociative adsorption.