Many-body calculation of the valence photoemission spectrum ofNiN2

The valence photoemission spectrum of ${\mathrm{NiN}}_2$ is calculated by the third-order algebraic-diagrammatic-construction [ADC(3)] Green’s-function method. It is shown that the quasiparticle picture of both 1π and 4σ breaks down due to the one-hole (1h) and two-hole–one-particle (2h1p) charge-transfer (CT) coupling in the bonded system. In contrast to NiCO, the π CT excitation in ${\mathrm{NiN}}_2$ is not so significant, and the σ excitation becomes more substantial. It is shown that in the case of the 5σ level, the 1h state is still 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 towards the ligand than in the ground state. The satellites are dominated by 2h1p configurations, where the bonding to antibonding shake-up excitations occur. For 1π and 4σ ionizations the strong mixing of 1h and 2h1p configurations leads to the breakdown of the quasiparticle picture of the ionization, and thus a distinction between the main line and satellite line becomes meaningless. When the Ni-${\mathrm{N}}_2$ bond length becomes larger (weaker coupling), the intensity increase of the satellite and breakdown of the quasiparticle picture also occurs for 5σ and local metal ionizations. This is not due to the change in the CT 1h-2h1p coupling strength but to the smaller CT 1h-2h1p energy separations.