Coverage- and temperature-dependent site occupancy of carbon monoxide on Rh(111) studied by high-resolution core-level photoemission

High-resolution core-level photoemission is used to study structural aspects for the molecular adsorption of CO on the Rh(111) single-crystal surface, and in particular to derive the adsorption sites. The site sensitivity of the core-level binding energy and the fact that the core level photoemission signal is proportional to the adsorbate coverage make it possible to study quantitatively how the occupation of different sites changes with temperature and/or CO coverage. For the CO/Rh(111) adsorption system we find two sites (on-top and three-fold hollow) to be occupied by the CO molecules. At coverages up to 0.33 ML only on-top sites are occupied, whereas at higher coverages a mixture of three-fold hollow and on-top sites are found. The distribution between these two sites is found to depend strongly on temperature. Quantitative studies of these reversible, temperature-dependent site changes have been carried out for a number of CO coverages. For coverages between 0.33 and similar to 0.54 ML, increasing the temperature results in part of the molecules moving from on-top to three-fold hollow sites. This change is strongest for a (4x4) structure formed at 0.5 ML where an order-disorder transition is observed at a temperature of 120 K. For coverages above similar to 0.54 ML, increasing the temperature leads instead to a decrease of the relative occupation of the three-fold hollow sites. For coverages below 0.33 ML, the molecules occupy on-top sites at all temperatures. (C) 1998 Elsevier Science B.V.