Transformation of molecular oxygen on a platinum surface:: A theoretical calculation of STM images

Using the recently modified electron scattering quantum chemical approach. we have performed simulations of the scanning tunneling microscope (STM) image contrast associated with elementary steps of O-2 dissociation on platinum(111). Two metastable molecular precursors, binding to different adsorption sites, and the stable atomic species have been studied, The two molecular chemisorbed structures have been identified very recently with a low-temperature STM. From density-functional-theory calculations, which allow a full geometry optimization of the adsorbate and the surface, two molecular precursors adsorbed at the fee hollow and bridge sites have very similar chemisorption energy. In the calculated STM image, with a tip ending with a Pt atom, the O-2 molecule and the O atom appear as a depression, which only agrees with the experiment in the case of the O atom. However, the image is found to be strongly dependent on the value of the gap resistance and on the termination of the tip. A tip ending with a CO molecule, which models a tip contaminated with an adsorbate, gives images in good agreement with the experimental results.