AMOUNT AND ORIENTATION OF PI-BONDED ETHYLENE FORMED AT 95-K ON K/PT(111) SURFACES (AND O/PT(111) SURFACES) - A TDS, UPS, XPS, AND NEXAFS STUDY

The variation in amounts of loosely-bonded (usually called pi-bonded) and di-sigma-bonded ethylene species adsorbed onto Pt(1 1 1) at 95 K in the presence of potassium adatoms has been independently followed, using XPS and TDS, as a function of potassium coverage. The obtained results show that, in the neighborhood of potassium atoms, di-sigma ethylene formation is inhibited as well as further reactions leading to ethylidyne formation and dehydrogenation. Di-sigma bonding is replaced by a weaker bonding and reversible adsorption at about 130 K. The maximum amount of pi-bonded ethylene occurs at a coverage where potassium adatoms interact and give a LEED superstructure. The pi-bonded ethylene species formed on the Pt(1 1 1) surface at 95 K in the presence of potassium or oxygen adatoms has been investigated, using UPS, XPS, and NEXAFS. The interpretation of data has been made using symmetry considerations and selection rules. It indicates that in the presence of oxygen atoms the carbon-carbon double bond plane is parallel to the surface while potassium adatoms induce a strong rotation of this plane along the carbon-carbon axis.