Nature of the surface chemical bond inN2on Ni(100) studied by x-ray-emission spectroscopy andab initiocalculations

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The electronic structure of the system ${\mathrm{N}}_{2}/\mathrm{N}\mathrm{i}(100)$ has been studied by means of angle-resolved x-ray-emission spectroscopy (XES) and ab initio calculations. XES allows a symmetry-resolved decomposition of the $2p$ density of states projected on each N atom. The calculations reproduce the experimental spectra well. Our results show that it is necessary to use an atom-specific description rather than treating the molecule and substrate as separate units. Hence a model of the surface chemical bond for this system is presented, in which the N${}_{2}$ $1\ensuremath{\pi}$--Ni $3d$ interaction is important for the bond of N${}_{2}$ to the Ni(100) surface. The weakening of the internal $\ensuremath{\pi}$ is seen as the appearance of a nonbonding orbital whose character is essentially Ni $3d$ with a contribution of N $2p$ lone pair on the outer nitrogen atom. The $\ensuremath{\sigma}$ system strongly polarizes in order to minimize the Pauli repulsion with the Ni $4sp$ states in the substrate. The traditional picture of the Blyholder model, which in a frontier orbital framework involves $\ensuremath{\sigma}$ donation and $\ensuremath{\pi}$ backdonation with more or less unperturbed orbitals, is not in agreement with the experimental data and is not supported by the calculations. In order to create the adsorbate orbitals we need to involve the whole original $\ensuremath{\pi}$ system of the free molecule, i.e., both the $1\ensuremath{\pi}$ and $2{\ensuremath{\pi}}^{*}$ orbitals.