THE NO-INDUCED (1X2)-](1X1) STRUCTURAL TRANSITION OF PT(110) - A QUANTITATIVE RHEED INVESTIGATION

The adsorption of NO on Pt(110) and its induced (1 x 2)-->(t x 1) stru ctural transition was studied with quantitative reflection high-energy electron diffraction (RHEED). Time/exposure series of rocking curve s ets were recorded during on-going adsorption processes. A direct evalu ation focusing on the development of reflection intensities and rockin g curve shapes in dependence on the NO exposure allowed dosages to be found for the most rapid structural change and for saturation. 10 sets of rocking curves from room temperature adsorption were quantitativel y evaluated using an automated fitting procedure in conjunction with d ynamic RHEED theory to determine the values of a comprehensive set of structural parameters for the substrate and the adsorbate. We found co nsiderable changes of the Pt relaxations just prior to the transition from the (1 x 2) missing-row reconstruction into the (disordered) (1 x 1) structure. As long as the substrate is (1 x 2) reconstructed, NO a dsorbs on bridge sites on the first Pt layer and on hollow sites on th e (111) microfacets while in the case of the (1 x 1) structure first-l ayer top sites are occupied too. As soon as the transition begins, the rearrangement of the first Pt layer provides for additional vacant br idge and top sites in the second layer. These are then occupied while the hollow sites loose their population. (C) 1997 Elsevier Science B.V .