THE INFLUENCE OF CRYSTALLOGRAPHIC STEPS ON COADSORPTION - CO-O NI(119)/

On low index nickel surfaces, repulsive interactions between atomic ox ygen and CO drive the phase separation of these species into oxygen-ri ch and GO-rich islands. Because these adsorbates interact differently with crystallographic steps, the size and the structure of these islan ds are modified on stepped surfaces. We have monitored coadsorption-in duced changes in the distribution of CO with IRRAS, observing six diff erent CO stretching bands which are assigned to distinct local chemiso rption environments. When oxygen fully saturates sites along the step edge, the steps are completely blocked from CO adsorption and virtuall y all the CO population on the terraces shifts from atop to bridge sit es. This terrace site shift is similarly accomplished by atomic oxygen chemisorbed at terrace sites. From these coadsorption-induced changes in CO site distributions, constrained by the 10 Angstrom terrace widt h, we conclude the through-metal O-CO interaction responsible for this CO site shift must be operative over a range of 5 Angstrom. At theta( 0) = 0.18 ML, when oxygen occupies, but does not fully saturate the st ep edge, a new CO adsorption site is created, with a characteristic fr equency of 1750 cm(-1). This new site is assigned to CO bonded to kink s along the step edge based upon its low intensity (similar to geometr ic kink density), enhanced binding strength and sensitivity to oxygen coverage. At higher oxygen coverages, compression of the CO adlayer is observed, with CO shifting to asymmetric bridge sites. As saturation coverage is approached, CO occupies weakly bound sites in close proxim ity (similar to 3 Angstrom) to O adatoms, with high characteristic fre quencies of 2100 cm(-1). (C) 1997 Elsevier Science B.V.