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 chemisorbed on Pt(111) and on Ni(111) conclusively yield threefold-coordinated hollow-site adsorption and reasonable bond lengths. Both metal surfaces exhibit molecular induced substrate relaxations; those in Ni(111) are greater than those in Pt(111). The Pt(111)-p(2 X 2)-NO structure has NO adsorbed in a threefold fcc site with a small buckling of the first metal layer. NO of the Ni(111)-c(4 X 2)-2NO structure is adsorbed in threefold fcc and hcp sites with a 0.16 angstrom buckling of the first metal layer. The hollow-site adsorption on both metal surfaces invalidates long-standing bridge and top-site assignments based on vibrational spectroscopy measurements, calling that approach into question. The threefold hollow-site adsorption determined for these systems also contrasts with the lower coordinated bridge and top sites generally found for CO molecules on the same metal surfaces.