CARBON-MONOXIDE ADSORBED ON RU(10(1)OVER-BAR0)

The adsorption of CO on the Ru(10 (1) over bar 0) surface has been stu died by means of XPS, LEED and TPD. The CO O1s spectra, including the satellites, were used to obtain information about the CO bonding confi guration and the population of different adsorption sites as a functio n of the CO coverage and adlayer ordered structures. The on-top bonded CO, which is the only species up to one monolayer, is characterized b y an O1s binding energy of 531.7 eV at coverages less than 0.6 ML. The slight shift of the on-top CO O1s binding energy to 531.9 eV at highe r coverages has been ascribed to the compression and displacement of C O from the equilibrium on-top position. Beyond 1 ML the adsorption of extra 0.25 ML CO in a higher coordinated site results in a second O1s peak at 530.9 eV. On the basis of the XPS and LEED data a structural m odel for the arrangement of the CO molecules in the compressed [GRAPHI CS] saturated layer has been suggested. The coverage dependence of des orption energy, E(d), and pre-exponential factor, nu, evaluated by usi ng two methods for fitting the TPD data, have shown almost constant E( d) and nu values of similar to 140 kJ mol(-1) and 10(14) s(-1) up to s imilar to 0.33 ML, a linear decline to similar to 80 kJ mol(-1) and 10 (9) s(-1) at 0.8-1.0 ML and a sharp drop to similar to 50 kJ mol(-1) a nd 10(6) s(-1) at saturation (similar to 1.25 ML). This coverage depen dence has been explained in terms of CO-CO repulsive interactions and reduced CO-substrate coupling in the higher coordinated site.