PHYSISORBED, CHEMISORBED AND DISSOCIATED O-2 ON PT(111) STUDIED BY DIFFERENT CORE-LEVEL SPECTROSCOPY METHODS

For O-2/Pt(111) we have found four different adsorption phases which a re formed at different substrate temperatures. At about 25 K the oxyge n molecules physisorb on the surface. Two chemisorbed phases are obser ved at 90 and 135 K, respectively. An atomic phase, characterized by a sharp (2 X 2) LEED pattern, exists at a temperature above 250 K. Diff erent spectroscopic techniques have been used to characterize the four different adsorption states: XPS studies of adsorbate and surface cor e level shifts, UPS, NEXAFS, autoionization and Auger spectroscopy. We conclude that oxygen adsorbs in two different molecular chemisorbed s tates which can be considered to be precursors for the thermally activ ated atomization process. The first of these molecular states is weakl y chemisorbed at 90 K. It is adsorbed in a hollow site with a saturati on coverage of 0.23 (molecules per Pt surface atom). We have identifie d this phase as a superoxo-like configuration. The second phase is mor e strongly bonded to the Pt substrate. It is characterized by a longer and weaker molecular a bonding due to more charge transfer from the m etallic substrate to the antibonding molecular 1 pi(g) orbitals than f or the first chemisorbed phase. With a coverage of 0.15 the oxygen mol ecules seem to be adsorbed in hollow or hollow-bridge sites. We have c haracterized this phase as a peroxo-like configuration of the oxygen m olecule. For atomic oxygen on platinum we have found a coverage of 0.2 5 (oxygen atoms per Pt surface atom) and a threefold adsorption site, in agreement with previous studies. We discuss the XAS results accordi ng to a model for the density of states induced by the hybridization o f the 2p atomic orbitals with the 6sp states and 5d band of the metal.