Breakdown of Koopmans' theorem and strong shake-up bands in the valence shell region of N2 photoelectron spectra

Abstract

Several ionized states of N2 are studied using ab initio Cl methods. The large corrections to the Koopmans values for the three lowest ionization potentials (IP's) are analyzed by setting up the effective hamiltonian matrix for each ionized state. The most important correction terms to the IP of an orbital, v, result from the type of excitations, ij → k∞ (ij ≠v), where ij → k∞ denotes that an electron in one of the occupied orbitals, either φi or φj, is excited to the vacant orbital, φk and simultaneously an electron in the other orbital is ionized. The excitation ij → k∞ can be shown to be an intramolecular charge-transfer excitation in terms of the valence bond picture. It is this large mixing of the v → ∞ and the ij → k∞ states that is responsible for the strong shake-up bands in the region of 24-32 eV in the photoelectron spectra. The calculated IP's for the three lowest single hole states and for the five shake-up states are in good agreement with experimental values. For comparison, the IP's of CO are also studied.