Many-body calculation of the valence photoemission spectrum of PdN2

Abstract

We have performed ab initio two hole one particle (2h1p) CI and algebraic diagrammatic construction [ADC(3)] Green’s function calculations of the valence photoemission spectrum of PdN2. The spectral features obtained by two different reliable methods are similar. However, they disagree with the experimental spectrum of N2/Pd(111) system. The 1π and 5σ ionization peaks are interpreted to be so well separated (1.6 eV) according to the experimental assignment, but the present theoretical result shows that despite a much weaker metal–ligand bonding, they are almost degenerate as in the case of the CO/Ni, CO/Pd, and N2/Ni systems. The basic spectral features and the relative positions of the 1π and 5σ levels do not change much with an increase of the metal–ligand bond length (weaker bonding strength). This shows that further detailed experimental study of the valence photoemission spectrum of the N2/Pd system is needed. The many-body effects such as the breakdown of the quasiparticle picture for the ionization is much stronger in NiN2 that in PdN2, despite the widely held assumption that weak coupling leads to increased many-body effects. It is shown that the change of the valence spectral features of the adsorbate depends on a subtle change of the electronic structure of the substrate metal atom rather than the metal–ligand bond strength.