CO ADSORPTION STUDIES ON A STEPPED CU(111) SURFACE

The adsorption of CO on a stepped Cu(332) surface has been studied by means of TDS, LEED, and work function measurements during adsorption and desorption. As can be deduced from the energy depending LEED spot splitting, a well prepared surface consists of regular (111) terraces with monatomic steps. TDS experiments reveal three different desorption states, two of them deriving from CO molecules originally bound to terrace sites, showing nearly the same behaviour as CO molecules adsorbed on an undisturbed Cu(111) surface. The third desorption maximum is assigned to CO desorbing from step sites showing a higher binding energy than the terrace states. No evidence for dissociative adsorption is found from the TDS experiments. After about 2 L exposure a faint streaky adsorption structure arises, interpreted as c(4 x 2) incoherent across the steps. At higher exposure no compression of this adsorption phase is observed. Measurements of the work function change during adsorption and desorption reveal that for a CO molecule on a step site the contribution to the surface dipole-change is considerably higher than for a CO on a terrace site. Based on this result, a possible explanation is given for the fact that the step sites have a higher adsorption energy than the terrace sites.