STEP SITE ADSORPTION AND ORDERING OF CO ON NI(510) AND PD(510)

Using HREELS and LEED we have observed profound differences in step si te adsorption and ordering of CO on two stepped fee (100) surfaces, Ni (510) and Pd(510). On Ni(510) we find that CO initially decorates the steps. This seems to prohibit the formation of any globally ordered st ructure, although the local structure at high coverages resembles that observed on the hat Ni(100) surface. Two distinct adsorption sites as sociated with the near step region are observed. On Pd(510) adsorption initially occurs on the terraces, and a well-ordered structure corres ponding to 0.5 monolayers is formed. More dense structures also develo p, but are somewhat disordered. A specific step site is occupied at la rge CO coverages and low substrate temperature.