COVERAGE-DEPENDENT 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 a spects for the molecular adsorption of CO on the Rh(111) single-crysta l surface, and in particular to derive the adsorption sites. The site sensitivity of the core-level binding energy and the fact that the cor e level photoemission signal is proportional to the adsorbate coverage make it possible to study quantitatively how the occupation of differ ent sites changes with temperature and/or CO coverage. For the CO/Rh(1 11) 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 o n-top sites are occupied, whereas at higher coverages a mixture of thr ee-fold hollow and on-top sites are found. The distribution between th ese 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 or der-disorder transition is observed at a temperature of 120 K. For cov erages above similar to 0.54 ML, increasing the temperature leads inst ead 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.