Initial stages of Al(111) oxidation with oxygen-temperature dependence of the integral reactive sticking coefficient

The initial stages of Al(111) oxidation by oxygen were investigated using X -ray photoelectron spectroscopy (XPS) and high resolution electron energy l oss spectroscopy (HREELS). The integral reactive sticking coefficient (S) w as evaluated by measuring the O(ls) XPS peak area for exposures below 50 L. Experiments were carried out at surface temperatures from 95 to 773 K. The dissociative adsorption of O-2 is proposed to occur by means of a molecula r precursor mechanism leading to the non-monotonic behavior of S with chang ing surface temperature. The kinetics of O-2 adsorption are governed by the strong temperature dependent balance of chemisorbed and oxidic species on the surface. Oxidic phase formation then influences the adsorption kinetics , by lowering the adsorption barrier from the precursor state (since strong er O-Al bonds are formed by oxide formation compared to chemisorption) and transforming the activated O-2 adsorption process into a non-activated one. A rapid onset of the oxidic phase formation was found in the temperature r ange 473-573 K at these low exposures. (C) 1999 Elsevier Science B.V. All r ights reserved.