CO ADSORPTION ON NI(551)

CO adsorption on a high indexed Ni surface with nominal (551) orientation was measured with LEED and thermal desorption spectroscopy (TDS). LEED from the clean Ni(551) surface exhibits a (1 x 1) surface layer expected for the ideal atomic arrangement of a fcc (551) surface. TDS spectra show a sequential filling of four desorption states labelled A (T(M) = 450 K), B (420 K), C (320 K) and D (230 K). The desorption states A and B are related to the desorption from (111) and (110) microfacets of the Ni(551) surface, respectively. Desorption states C and especially D, which are missing on "flat" Ni(111) or Ni(110), are attributed to CO absorbed at step sites. With LEED in a definite regime of CO coverage half and one-fourth order streaks in direction perpendicular to the (111) steps of Ni(551) are detected. Near the saturation of states C a weak c(4 x 2) overlayer develops.