A leed I(E) analysis based on an island model for the interaction of oxygen with Al{111} surfaces

Abstract

The low-energy electron diffraction (LEED) technique has been used to study the structures which appear during the first stages (≤200 L) of the interaction of oxygen with Al{111} surfaces at room temperature (RT). LEED dynamical programs have been used to compute I(E) theoretical curves for different structures (islands, clean surface, oxygen underlayer, oxygen overlayer). An island model obtained by linear combinations of those structures has been tested comparing the theoretical curves to the experimental ones measured at several oxygen exposures using the R -factor technique. The clean Al{111} surface layer, d bulk 12 = 2.338 A, has been found to be dilated by (3 ± 2)%, d 12 = 2.41 ± 0.05 A. An Al{111}1×I-O underlayer structure at 25 L O 2 exposure has been found, providing a consistent explanation for all experimental results at this coverage. During the adsorption of the first 1/3 monolayer (≤30 L), oxygen penetrates through the three-fold fcc hollows of the Al{111} surface, occupying underlayer positions at 0.3 ± 0.1 A below the first Al plane, in islands which have their top Al layer contracted by about 15%. Above this coverage, oxygen starts to occupy fcc hollows in overlayer positions, and at about 75 L the simultaneous formation of oxide-like alumina begins, probably on the borders of the depressed terraces. At 100 L the maximum coverage of the Al{111}1×I-O overlayer structure is reached, with an interplanar distance d 12 = 0.7 (+0.12, −0.04) A. This result agrees with the determination made by surface-extended X-ray-absorption fine structure (SEXAFS), and recent theoretical calculations. At coverages around half a monolayer, at about 50 L, a new d 12 value is obtained, d 12 = 0.4 (+0.10, −0.05) A, and its meaning is discussed.