NATURE OF THE SURFACE CHEMICAL-BOND IN N-2 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 init io calculations. XES allows a symmetry-resolved decomposition of the 2 p density of states projected on each N atom. The calculations reprodu ce the experimental spectra well. Our results show that it is necessar y to use an atom-specific description rather than treating the molecul e and substrate as separate units. Hence a model of the surface chemic al bond for this system is presented, in which the N-2 1 pi-Ni 3d inte raction is important for the bond of N-2 to the Ni(100) surface. The w eakening 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 pol arizes in order to minimize the Pauli repulsion with the Ni 4sp states in the substrate. The traditional picture of the Blyholder model, whi ch in a frontier orbital framework involves sigma donation and pi back donation with mon or less unperturbed orbitals, is not in agreement wi th the experimental data and is not supported by the calculations. In order to create the adsorbate orbitals we need to involve the whole or iginal rr system of the free molecule,i.e., both the 1 pi and 2 pi or bitals. [S0163-1829(98)04415-4].