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 diffrac tion 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 r esults 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 valen ce level photoemission data. Photoelectron diffraction shows the CN to lie approximately atop a second-layer Ni atom such that the C atom ha s 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 c lose-packed surface atom row. This geometry appears to have no analogu e in previous structure determinations of adsorbed diatomic species. I n 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.