New mechanism for screening in core-level photoemission of adsorbates: Model studies

As shown previously, the two-peak structure observed in core-level ionization from CO and N₂ adsorbed on Ni surfaces can be explained by different final hole states and a screening mechanism involving the lowest unoccupied orbital (2π*) of the molecule which is only partially filled in the chemisorbed state. In the ground state of free NH₃ the lowest empty orbitals are 4a₁ and $2e$; they are quite diffuse Rydberg-type orbitals. It might be expected that these orbitals would not lead to structure in the N $1s$ core-hole photoemission spectra. However, model calculations on NiNH₃ clusters show that in the presence of the N $1s$ core hole the Rydberg orbitals lead to a screening mechanism similar to the one proposed by Sch\"onhammer and Gunnarsson for adsorbed CO and N₂. In NiNH₃ the different final states are similar in intensity and are separated by only 1-1.5 eV. The x-ray photoemission spectroscopy data for the NH₃-Ni(110) system show---in the N $1s$ energy region---a broad asymmetric peak which, from the present calculations, might be interpreted as a superposition of at least two peaks originating from unscreened and Rydberg screened final states of the core hole. Preliminary cluster calculations on different N $1s$ hole states of linear NiN₂ suggest that a similar Rydberg screening effect, involving the 3π orbital of N₂, can occur for N $1s$ ionization in the N₂-Ni adsorbate system in addition to the well-known 2π* screening. Here Rydberg screening could account for part of the observed asymmetry and broadening of the high-binding-energy peak in the N $1s$ photoemission spectrum.