Nature of the surface chemical bond in N₂ on Ni(100) studied by x-ray-emission spectroscopy and ab initio calculations

The electronic structure of the system N₂/Ni(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 N₂ 1π–Ni 3d interaction is important for the bond of N₂ to the Ni(100) surface. The weakening of the internal π 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 σ 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 σ donation and π 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 π system of the free molecule, i.e., both the 1π and 2π^* orbitals.