Adsorption of molecular nitrogen on nickel. I. Cluster-model theoretical studies

Abstract

Hartree-Fock calculations on a linear cluster, NiN2, have been performed. Self-consistent-field calculations on the ground and hole states of the cluster and the free N2 molecule have been used to determine the shifts in valence ionization potentials (IP) of N2 when it is bound to a Ni atom. The understanding of these shifts is an important contribution to the analysis of the valence-level photoemission spectrum of N2 adsorbed on Ni, even though additional effects are likely here because of the extended metal surface. We can explain, on the basis of the bonding characteristics, why the 2fu-3fgN2 IP separation increases when N2, is bonded to a Ni atom, whereas the equivalent IP separation in a NiCO cluster decreases compared with free CO. We demonstrate also that one of the extra effects of a real surface is that significant backbonding from Ni 4p levels to the empty * N2 orbital is possible. Such a bonding contribution in a NiN2 cluster can only be included by choosing a separated Ni atom configuration containing 4p character. When this is done, the self-consistent calculations of the Ni atom in this configuration bonded to an N2 molecule reveal significant 4p-* backbonding in addition to the approximately equal f-bonding contributions from the 2fu and 3fg orbitals of N2. The inclusion of the * backbonding reduces the N2-derived valence IP's by some 2.5 eV. The theoretical results for NiN2 are compared with those for NiCO in order to facilitate the comparison of the photoemission results for the Ni(100)/N2 and Ni(100)/CO systems, which is discussed in detail in the following paper. © 1981 The American Physical Society.