On the adsorption site of ethylene at the Ni(110) surface: A combined experimental and theoretical study involving the unoccupied band structure

The adsorption of ethylene on Ni(110) was investigated by angle resolved inverse photoemission (ARIPE) spectroscopy as well as by detailed density functional model cluster and slab model band structure calculations to clarify the preferred adsorption site. Cluster model calculations both at the local density as well as at the gradient corrected level of theory gave a slight preference for the di-σ over the π coordinated geometry on top of the ridges, but no or, at best, a very weak binding over the troughs. A dispersionless band in the ARIPE spectra about 1.8 eV above E_F is assigned to the band derived from the lowest unoccupied ethylene orbital, 1b_3g(π*). The surface state feature of the clean Ni(110) surface connecting the image state at Γ̄ and the d_yz state S₂ at Ȳ is lowered almost uniformly by 0.8 eV through the interaction with the adsorbate. For adsorption on top of the ridges in the so-called "half-bridge" position intermediate between the short bridge (di-σ) and the top site (π), the symmetry requirements imposed by the surface state band are ideally met by the second lowest unoccupied band of the adsorbate monolayer which changes its character from ethylene 4a_g at Γ′ to 2b_3u at Y′. A similarly strong and uniform interaction is not possible when ethylene adsorbs above the troughs. Taking all experimental and theoretical evidence together, the adsorption site in the densely packed c(2 × 4) C₂H₄/Ni(110) adsorption system is identified as the half-bridge position on top of the ridges.