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

Abstract

As shown previously, the two-peak structure observed in core-level ionization from CO and ${\mathrm{N}}_2$ adsorbed on Ni surfaces can be explained by different final hole states and a screening mechanism involving the lowest unoccupied orbital ($2{\pi }^{*}$) of the molecule which is only partially filled in the chemisorbed state. In the ground state of free N${\mathrm{H}}_3$ the lowest empty orbitals are $4a_1$ 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 NiN${\mathrm{H}}_3$ 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 Schönhammer and Gunnarsson for adsorbed CO and ${\mathrm{N}}_2$. In NiN${\mathrm{H}}_3$ 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 N${\mathrm{H}}_3$-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 Ni${\mathrm{N}}_2$ suggest that a similar Rydberg screening effect, involving the $3\pi$ orbital of ${\mathrm{N}}_2$, can occur for N $1s$ ionization in the ${\mathrm{N}}_2$-Ni adsorbate system in addition to the well-known $2{\pi }^{*}$ 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.