Adiabatic potential-energy surfaces for oxygen on Al(111)

The initial stage of oxidation of aluminum, i.e., adsorption and dissociation of the oxygen molecule, is one of the common model systems for molecule-surface interactions and dynamics. It might seem surprising that there are still some unsettled key dynamics issues. To assess how much can be accounted for in an adiabatic description, an extensive first-principles density-functional-theory study of large pieces of the potential-energy hypersurface (PES) of O-2 on the Al(111) surface is performed. Properties calculated for the free-O-2 molecule, the clean Al(111) surface, and atomic O on the Al(111) surface get values close to measured ones and/or earlier calculated ones, which gives confidence in the method. The calculated PES for O-2/Al(111) shows (i) an entrance-channel energy barrier in only one channel having the molecular axis parallel to the surface, out of many; (ii) a molecularly chemisorbed state, an O-2(2-) precursor state, in all the considered channels with nonparallel molecular axis (iii) potential for abstraction, i.e., dissociative decay by emission of one neutral O atom; and (iv) both single and close-paired atomically adsorbed O atoms as possible end products. Furthermore, the calculated diffusion barrier (approximate to0.7 eV) for an O adatom on the surface could rule out the thermal motion of the adsorbed O atoms along the surface at low temperatures. The predicted abstraction channel finds compelling evidence in a recent laser/STM study and resolves a long-standing issue of seemingly huge separations between adsorbed oxygen atoms after dissociation on the Al(111) surface. The predicted metastable molecular state should have great consequences for the dynamics and is stabilized by the very nonequivalent Al-surface field on the oxygen atoms of the nonparallel O-2 molecule. The absence of absolute energy barriers in almost all of the considered adiabatic entrance channels suggests nonadiabatic processes to be required to explain the measured low initial sticking probability and its radical growth with increasing translational energy.