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.