QUANTITATIVE STRUCTURE-ANALYSIS OF A DISORDERED SYSTEM - RHEED STUDY OF THE CO-INDUCED (1X2)-](1X1) STRUCTURE TRANSITION OF PT(110)

Recent progress in the understanding of RHEED has enabled the interpre tation of data from disordered structures. If the correlation length o f the disorder is small, the strong forward scattering at high energie s corresponds to an averaging over the disorder such that the multiple scattering is mainly determined by the two-dimensional periodic struc ture part of the surface layers. Hence, this structure part which is i ndependent of the statistics of the disorder, can be extracted from ro cking curves of the principal reflections with a similar accuracy as f or ordered surfaces. As an application we present a structural study o f the CO adsorption on Pt(110)(1 X 2) and its induced transition into a disordered Pt (1 X 1) phase. We measured several sets of rocking cur ves for different exposures. These experimental rocking curves were fi tted using an automated evaluation scheme. In this way structure data versus CO exposure (0-saturation) were determined for both the Pt and the CO. We found strong evidence that the lift of the (1 x 2) missing- row reconstruction begins with considerable changes of the Pt relaxati ons (top interlayer spacing, pairing, buckling). Besides adsorption on the first layer, adsorption on the second Pt layer takes place, too, as soon as the (1 X 2) ordering decreases because the rearrangement of the first layer atoms provides for additional vacant top sites in the second layer.