Many-body calculations on the valence photoemission of NiCO and Ni(CO)4

The valence electron spectra of NiCO and Ni(CO${)}_4$ are calculated by the third-order algebraic-diagrammatic-construction Green’s-function method. We obtain fairly good agreement with experiment. It is shown that the quasiparticle picture of 1π breaks down due to the one-hole/two-hole–one-particle (1h/2h1p) charge-transfer (CT) coupling in the bonded system. The dominant role played by the intraligand relaxation in free CO is replaced by the CT relaxation because of a drastic decrease of the intraligand 1h/2h1p coupling strength. The CT relaxation is much stronger because of much smaller 1h/2h1p energy separations. When the Ni-CO bond length becomes larger (weak coupling), the intensity increase of the satellite and breakdown of the quasiparticle picture also occur for 4σ ionization. This is not due to the change in the CT 1h/2h1p coupling strength but due to the smaller CT 1h/2h1p energy separations. The present work emphasizes the importance of 1h1h and 1h1p interactions in the 2h1p states. It is shown that, in the case of 5σ and 4σ levels, the 1h state is more stable than the 2h1p states. Consequently the main line is interpreted as the 1h state, where the screening charge resides on the bonding orbital, which is more polarized toward the ligand than in the ground state. The satellites are dominated by 2h1p configurations, where the bonding to antibonding shakeup excitations occur. For 1π ionization the strong mixing of 1h and 2h1p configurations leads to the breakdown of the quasiparticle picture of the ionization, and thus a distinction between main line and satellite line becomes meaningless. The effects of the relaxation and screening in the 2h1p states are also discussed. We show the usefulness of the molecular-orbital Green’s-function calculations for understanding ionization in adsorbates.