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Abstract

The bonding of molecular N₂ has been investigated with angle resolved photoelectron spectroscopy and inelastic electron scattering. The spectra obtained from N₂ chemisorbed onto a Ni(110) surface are compared to CO chemisorbed onto Ni(110) and to N₂ physisorbed onto Pd(111). The N₂ molecular axis was found to be normal to the crystal surface for the chemisorbed state on Ni(110) and random for the physisorbed state on Pd(111). The N-N and Ni-N₂ stretching frequecies indicate that the N₂ molecule is terminally bonded to a single Ni atom on Ni(110). The binding energies of the two outer σ states and one π state of chemisorbed N₂ were measured, indicating that the bonding of N₂ to a metal surface is different than CO. Both σ states drop in energy compared to the π level due to the fact that both of them are involved in the N₂ substrate bond. The symmetry of the gas phase N₂ molecule is reduced upon adsorption. The consequences of this are seen in the dipole active N-N vibrational mode, the large intensity of the Ni-N₂ vibrational mode and the coupling of the adsorbate 4σ(2σ_u) level to the final state resonance which is forbidden by symmetry in the gas phase. Many electron excitation satellite lines are observed in the valence spectra of both the chemisorbed and physisorbed N₂. The physisorbed satellite lines are nearly identical to those seen in gas phase N₂, while the chemisorbed N₂ spectra has new satellite structure, due to the interaction with the substrate.