STICKING PROBABILITIES FOR CO ADSORPTION ON PT(111) SURFACES REVISITED

The coverage dependence of the sticking probaiblity for carbon monoxide on Pt(lll) surfaces was investigated by using the dynamic method originally devised by King and Wells. The CO uptake was studied as a function of CO beam flux, surface-to-doser distance, and surface temperature. The sticking probability on the clean surface was found to be quite high in all cases, about 0.8, and to remain approximately constant at low temperatures up to coverages close to 0.50 monolayers (ML), at which point a c(4x2) ordered layer forms on the surface. This behavior is explained by a model originally proposed by Kisliuk where the molecules adsorb on a highly mobile extrinsic precursor state before migrating to their final chemisorbed state. Above 0.50 ML the sticking probability then drops suddenly, presumably because the chemisorption energy drops as compressed CO layers start to form on the surface. The saturation coverage varies at low temperatures with CO beam flux because of the induced changes in the adsorption-desorption dynamics at the high coverages by the changes in CO impinging rates. The effect of inhomogeneities in the spatial distribution of the CO beam across the surface was studied by changing the distance between the sample and the doser, which was found to affect the overall shape of the CO uptake curves. Finally, the adsorption kinetics was determined as a function of surface temperature: the uptake was found to change from precursor mediated below 200 K to a more Langmuir type behavior around room temperature.