Absorption-edge resonances, core-hole screening, and orientation of chemisorbed molecules: CO, NO, andN2on Ni(100)

Near-edge x-ray-absorption fine-structure spectra are reported above the $K$ absorption edges of diatomic low-$Z$ molecules (CO, NO, and ${\mathrm{N}}_2$) chemisorbed on Ni(100). It is shown that the $K$-edge fine structure is dominated by intramolecular resonances which arise from a sharp bound-state transition to an unoccupied molecular orbital of $\pi$ symmetry and a broader $\sigma$ shape resonance in the continuum. For dissociated molecules the $K$ absorption-edge structure is found to be distinctly different, and large (∼3 eV) shifts of the edge position are observed. The position of the molecular resonances is discussed in comparison with gas-phase absorption data and the $1s$ binding energies determined by photoemission. The question of ligand $2\pi$ and metal $d$ charge transfer is discussed for the neutral and ionic (core-hole) chemisorption complex. For ${\mathrm{N}}_2$ on Ni(100) the giant satellite structure in the N $1s$ photoemission spectrum due to dynamic screening of the core hole has been measured at $h\nu =500\mathrm{}$ eV, and comparison is made with the absorption case. A theoretical expression is derived for the polarization dependence of the resonance structures which allows the precise determination of the molecular orientation on the surface. Comparison is made to the theory governing angle-resolved photoemission spectra. The molecular orientation for CO, NO, and ${\mathrm{N}}_2$ on Ni(100) at saturation coverage is determined. All molecules are found to stand up on the surface with the molecular axis along the surface normal (±10°).