Sticking of O/sub 2/ on a Pt(111) surface

This paper reports detailed molecular beam measurements of the sticking coefficient at zero coverage for O/sub 2/ on a Pt(111) surface as a function of initial energy (E/sub i/), angle of incidence (theta/sub i/), and surface temperature (T/sub s/). Under most conditions the sticking coefficient measures the probability for dissociative chemisorption. These results demonstrate that both precursor mediated and quasi-direct dissociation can be observed, depending upon the initial conditions. The quasi-direct process is revealed by a step increase in the sticking with E/sub i/. This feature scales intermediately between E/sub i/ and the normal component E/sub n/, and is weakly dependent on T/sub s/. The precursor mediated sticking is well described by standard precursor kinetic models. At low E/sub i/ and T/sub s/, sticking measures trapping into a molecularly adsorbed state. This trapping decreases more rapidly with E/sub i/ than anticipated from simple models and scales intermediately between E/sub i/ and E/sub n/. The sticking results are discussed in terms of likely dynamic processes occurring on a potential energy surface which contains an intermediate molecularly adsorbed species formed by charge transfer from the metal to the O/sub 2/.