Low temperature sticking and desorption dynamics of oxygen on Pt(111)

The adsorption/desorption dynamics of oxygen on Pt(111) have been studied by angle resolved thermal programmed desorption (TPD) over the temperature range from 20 to 1000 K. The first monolayer of physisorbed O-2 formed by dosing at 20 K completely converts to the chemisorbed state at 30-40 K. From 100 to 160 K, chemisorbed O-2 either desorbs with a cosine angular distribution or dissociates to form adsorbed O atoms. At low coverages all the molecules thermally dissociate upon heating. It follows that the minimum energy pathway leading to thermal dissociation of adsorbed O-2 lies below the energetic barrier for thermal desorption. Nevertheless, the O-2 angular distribution from associative desorption of adsorbed O atoms at 720 K is sharply peaked around the surface normal (similar to 0.2 cos theta + 0.8 cos(5) theta) for both low and saturation O atom coverages. The emergence of an exit channel barrier in desorption at high temperatures is discussed within the context of transition state theory and the potential of mean force.