COMPARISON OF SULFUR AND SELENIUM AS SURFACE MODIFIERS - EFFECT ON THE CO/PT(111) SYSTEM

CO adsorption on Pt(111) modified with a p(2 × 2)Se overlayer has been studied at two adsorption temperatures, 90 K, and 220 K, and compared with the results for the CO/p(2 × 2)S-Pt(111) system. The effect of the two modifier overlayers on the CO adsorption kinetics, desorption energetics, surface ordering and CO vibrational amplitudes has been studied by TPD, LEED and digital ESDIAD. The presence of a p(2 × 2) chalcogen modifier overlayer always causes a reduction of (i) the CO adsorption rate, (ii) the CO maximum coverage, (iii) the CO adsorption binding energy, (iv) the range of polar angles of the ESDIAD patterns of all CO ESD products, and (v) an increase of the ESD products' desorption cross section by a factor of ~ 1.5. At an adsorption temperature of 220 K, CO coadsorption induces a rearrangement of the p(2 × 2)Se overlayer and as a result a new (√7 × √7) R19.1° ordered structure is formed. Under identical starting conditions this CO induced phase transformation is not observed for a S overlayer on Pt(111). Detailed LEED, ESDIAD, and TPD studies at different CO coverages and annealing temperatures have shown that the CO induced phase transformation in the p(2 × 2)Se overlayer occurs at temperatures higher than 130 K, and is stabilized in the CO coverage range 0.04 ML < θco < 0.16 ML. The driving force for the observed overlayer rearrangement and the differences in the CO adsorption state in the Se p(2 × 2) and the Se(√7 × √7) R19.1° overlayer are explained considering the repulsive interactions between the coadsorbates. The absence of a CO-induced phase transformation in the p(2 × 2)S overlayer is attributed to the smaller size of the S adatom which ensures a larger CO-S effective separation in the p(2 × 2)S overlayer compared with the p(2 × 2)Se overlayer. © 1990.