CHARACTERIZATION AND O-2 TITRATION OF CO ISLANDS ON PT(100) - HELIUM ATOM SCATTERING AND REACTIVE STICKING PROBABILITY MEASUREMENTS

Thermal energy atom scattering (TEAS) has been used simultaneously wit h molecular beam adsorption and thermal desorption to probe the latera l distribution of CO on a Pt{100} single-crystal surface. On the initi al hex-R phase at 350 Ii, (1 x 1) island growth occurs at a constant l ocal coverage (theta(CO)((1 x 1))) of 0.4 ML. However, during desorpti on the surface remains wholly in the (1 x 1) state until (theta(CO)((1 x 1))) approximate to 0.25 ML. Subsequent reconstruction to hex occur s al a constant local coverage of 0.19 ML in the remaining (1 x 1) dom ains. Hysteresis is not observed for the pre-prepared (1 x 1) surface, on which a random CO distribution is observed during adsorption. The scattering cross-section Sigma for a single CO molecule on the hex-R a nd (1 x 1) surfaces at 350 K has been determined as 71 +/- 10 and 130 +/- 18 Angstrom(2) respectively. The behaviour during oxidation of (1 x 1)-CO islands on the initial hex-R surface prepared by either adsorp tion or desorption is found to be consistent with the TEAS data. The f irst sticking probabilities for O-2 and CO on the COad-freed and O-ad- freed (1 x 1) phase of Pt{100} are also reported.