Rydberg Screening in Core Level Photoemission of Ammonia Adsorbed on Nickel Surfaces

The lowest empty orbitals of the ground state of free ammonia, 4a1 and 2e, are quite diffuse Rydberg-type orbitals. Thus, one might expect that involvement of these orbitals in the final hole states would not lead to additional structure in the N1s core hole photoemission spectra of the adsorbed molecule. Model calculations on NiNH3 clusters show, however, that in the presence of the N1s core hole Rydberg orbitals can lead to a screening mechanism similar to the one proposed by Schonhammer and Gunnarsson for adsorbed CO and N2. In NiNH3 the different final states are similar in intensity and are separated by 1-1.5 eV. The XPS data for the NH3/Ni (110) system show a broad asymmetric N1s peak which, from the present study, can be interpreted as a superposition of at least two peaks stemming from unscreened and Rydberg screened final states of the core hole.