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

Hartree-Fock calculations on a linear cluster, Ni${\mathrm{N}}_2$, have been performed. Self-consistent-field calculations on the ground and hole states of the cluster and the free ${\mathrm{N}}_2$ molecule have been used to determine the shifts in valence ionization potentials (IP) of ${\mathrm{N}}_2$ 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 ${\mathrm{N}}_2$ 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 $2{\sigma }_u-3{\sigma }_g{\mathrm{N}}_2$ IP separation increases when ${\mathrm{N}}_2$, 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 $\mathrm{Ni}4p\pi$ levels to the empty ${\pi }^{*}{\mathrm{N}}_2$ orbital is possible. Such a bonding contribution in a Ni${\mathrm{N}}_2$ cluster can only be included by choosing a separated Ni atom configuration containing $4p\pi$ character. When this is done, the self-consistent calculations of the Ni atom in this configuration bonded to an ${\mathrm{N}}_2$ molecule reveal significant $4p\pi -{\pi }^{*}$ backbonding in addition to the approximately equal $\sigma$-bonding contributions from the $2{\sigma }_u$ and $3{\sigma }_g$ orbitals of ${\mathrm{N}}_2$. The inclusion of the ${\pi }^{*}$ backbonding reduces the ${\mathrm{N}}_2$-derived valence IP's by some 2.5 eV. The theoretical results for Ni${\mathrm{N}}_2$ are compared with those for NiCO in order to facilitate the comparison of the photoemission results for the Ni(100)/${\mathrm{N}}_2$ and Ni(100)/CO systems, which is discussed in detail in the following paper.