The temperature dependence of the initial sticking probability of oxygen on Pt(111) probed with second harmonic generation

The dissociative adsorption of oxygen on Pt(lll) in the temperature range from 220 to 670 K has been investigated by means of second harmonic generation, SHG. The time envelope of the SHG signal was measured while dosing the surface. The sticking data were fitted to a quadratic coverage dependence giving an initial sticking coefficient S-0=0.06(1) at room temperature. Using a model based on two sequential precursors, i.e. physisorbed and molecular chemisorbed oxygen, a good fit of the temperature dependence of the initial dissociative sticking is obtained. A difference between the activation energy for desorption from the physisorbed molecular state and the activation energy for adsorption into the chemisorbed molecular state from the molecular physisorption state is determined to 43.0(5) meV. The fit also gives zero difference between the activation energy for adsorption into the atomic chemisorption state and the activation energy for adsorption into the molecular physisorption state from the molecular chemisorption state. (C) 1997 Elsevier Science B.V.