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 N2 adsorbed on Ni surfaces can be explained by different final hole states and a screening mechanism involving the lowest unoccupied orbital (2*) 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 Schönhammer 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 showin the N 1s energy regiona 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 3 orbital of N2, can occur for N 1s ionization in the N2-Ni adsorbate system in addition to the well-known 2* 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. © 1983 The American Physical Society.