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 N-2/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 N-2 1 pi-Ni 3d interaction is important for the bond of N-2 to the Ni(100) surface. The weakening of the internal 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 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 sigma donation and pi backdonation with mon 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 rr system of the free molecule,i.e., both the 1 pi and 2 pi* orbitals. [S0163-1829(98)04415-4].