Activated chemisorption of oxygen on Si(111)-2x1

Results on the chemisorption of molecular oxygen on Si(111)-2x1 at different temperatures are reported. They show that oxygen is initially physisorbed in a precursor state, From which it can be thermally activated into the chemisorbed slate. An activation energy of 20 meV is found. The dependence of the sticking coefficient upon coverage deviates from the classical Langmuir theory. A model of chemisorption fitting the experimental observations is presented. In the frame of this model, the heights of the barriers for chemisorption and desorption are obtained.