Initial oxidation of aluminum(111): Abstractive chemisorption reaction dynamics

Chemisorption, the formation of chemical bonds at a surface, of diatomic molecules may occur via two fundamental mechanisms: abstraction and dissociation. In abstractive chemisorption, one atom from a diatomic molecule forms a bond to the surface and the other atom is ejected from the surface back into the gas-phase. In dissociative chemisorption, both atoms bond to the surface. Abstractive chemisorption in the initial oxidation of aluminum has been experimentally verified using supersonic molecular beams of O2 and NO to dose the clean Al(111) surface. Scanning tunneling microscopy (STM) 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 eV (thermal) to 0.8 eV. Alone, these observations provide compelling evidence for the abstractive chemisorption mechanism at low O2 incident energies, which also competes with normal dissociation at higher incident energies. In order to probe these ideas, we have studied the reaction between a heteronuclear diatomic molecule, nitric oxide, and the Al(111) surface. Auger electron spectroscopy studies of Al(111) exposed to non-oriented molecular beams of NO show the surface to be oxygen rich for low surface adsorbate coverages. These results offer direct experimental evidence that the reaction between NO and the clean Al(111) surface often leads to the formation of only one bound adsorbate, O-Al(s), while the N-atom is ejected back into the gas phase. However, an oriented NO molecular beam was used to show that a collision between the N-end of the molecule and the surface is preferred in order to initiate the reaction. Together, the experimental results for NO chemisorption imply that a flip of the molecule occurs during the reaction.