On the role of relaxed charge-transfer excitations: Ni 3s hole states in NiO

In this Letter a non-orthogonal configuration interaction study is presented for the high-spin coupled final states involved in the Ni 3s X-ray photoelectron spectra of NiO. Charge-transfer effects play an important role in the interpretation of this spectrum. These effects are incorporated in a conceptually attractive way by a non-orthogonal configuration interaction description, in which the wave functions are expressed as linear combinations of the reference configuration and relaxed charge-transfer configurations. The energy separation and relative intensities of the high-spin coupled final states were calculated in good agreement with experiment. Both final states have considerable charge-transfer character.