DYNAMICAL LEED ANALYSES OF THE PT(111)-P(2X2)-NO AND THE NI(111)-C(4X2)-2NO STRUCTURES - SUBSTRATE RELAXATION AND UNEXPECTED HOLLOW-SITE ADSORPTION

Dynamical LEED analyses of ordered structures of NO molecules chemisor bed on Pt(111) and on Ni(111) conclusively yield threefold-coordinated hollow-site adsorption and reasonable bond lengths. Both metal surfac es exhibit molecular induced substrate relaxations; those in Ni(111) a re greater than those in Pt(111). The Pt(111)-p(2 X 2)-NO structure ha s NO adsorbed in a threefold fcc site with a small buckling of the fir st metal layer. NO of the Ni(111)-c(4 X 2)-2NO structure is adsorbed i n threefold fcc and hcp sites with a 0.16 angstrom buckling of the fir st metal layer. The hollow-site adsorption on both metal surfaces inva lidates long-standing bridge and top-site assignments based on vibrati onal spectroscopy measurements, calling that approach into question. T he threefold hollow-site adsorption determined for these systems also contrasts with the lower coordinated bridge and top sites generally fo und for CO molecules on the same metal surfaces.