CN coordination in the adsorption system Ni(110)c(2x2)-CN: an unexpected geometry

N K-edge NEXAFS and N Is and C Is scanned-energy photoelectron diffraction have been used to determine the orientation and local adsorption geometry of CN adsorbed on Ni(110) in the c(2 x 2) phase. The NEXAFS results show that the C-N molecular axis lies approximately parallel to the surface in agreement with several prior studies of CN adsorbed on metal surfaces, but also show that it is approximately aligned in the [001] azimuth, perpendicular to the close-packed rows of the surface, in contradiction to earlier conclusions based on angle-resolved valence level photoemission data. Photoelectron diffraction shows the CN to lie approximately atop a second-layer Ni atom such that the C atom has similar distances to this second-layer Ni atom and two top layer Ni atoms, whereas the N atom bridges two top layer Ni atoms in the next close-packed surface atom row. This geometry appears to have no analogue in previous structure determinations of adsorbed diatomic species. In view of the intense fractional order LEED beams seen for this phase, several possible substrate restructuring models have been considered. (C) 1998 Elsevier Science B.V. All rights reserved.