Abstractive chemisorption in the initial oxidation of Al(111) has been experimentally verified using variable incident energy O2. Scanning tunneling microscopy images show a transition between single O-adatom reaction products to more pairs of O-adatom reaction products as the O2 incident energy is raised from 0.025 to 0.8 eV. The ejected O atoms have been detected in the gas phase with resonant enhanced multiphoton ionization. The observations that both abstractive and dissociative chemisorption are activated processes are in contrast to current adiabatic models of the absorption process.

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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.

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Experimental results are presented for the scattering of well-defined beams of molecular oxygen incident on clean Al(111). The data consist of scattered angular distributions measured as a function of incident angle, and for fixed incident angle, the dependence on surface temperature of the angular distributions. The measurements are interpreted in terms of a scattering theory that treats the exchange of energy between the translational and rotational motions of the molecule and the phonons of the surface using classical dynamics. The dependence of the measured angular distributions on incident beam angle and temperature is well explained by the theory. Rotational excitation and quantum excitation of the O(2) internal stretching mode are briefly discussed.