MECHANISMS AND ENERGETICS OF SITE HOPPING AND CHEMICAL-REACTIONS OF O-2 MOLECULES AT SI(111)-7X7 SURFACES

With continuous-time imaging using a variable-temperature scanning tun neling microscope (STM), we observe the dynamic behavior of a bright s pecies, which we believe to be single O-2 molecules on Si(111)-7 x 7 s urfaces. We have found the hopping motion of this molecular species be tween neighboring adatom sites, which is mediated by two intermediate states. A model is proposed to explain the detailed mechanism of the s ite hopping. Using Arrhenius plots, we determine the activation energi es of different hopping paths and the relative binding energies of dif ferent states. We also observe the chemical reaction process of two ad sorbed O-2 molecules to form an atomic species. Our results reveal tha t the hopping motion and chemical reactions of oxygen molecules depend strongly on the configuration of dangling bonds at the surface and th e oxygen bonding. This study provides a direct view of molecular dynam ics at a solid surface. (C) 1998 Elsevier Science B.V.