Characterisation 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 with molecular beam adsorption and thermal desorption to probe the lateral distribution of CO on a Pt{100} single-crystal surface. On the initial hex-R phase at 350 Ii, (1 x 1) island growth occurs at a constant local coverage (theta(CO)((1 x 1))) of 0.4 ML. However, during desorption 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 occurs al a constant local coverage of 0.19 ML in the remaining (1 x 1) domains. 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 and (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 adsorption or desorption is found to be consistent with the TEAS data. The first 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.