Interpretation of the photoelectron spectrum of N₂O₄

Many-body Green's function calculations on the ionization spectrum of N₂O₄ are presented. The ordering of the five ionization potentials below 15 eV is found to be 4a_g, 3b_2g, 1a _u(π), 1b_1g(π), with increasing binding energy. This ordering differs from that obtained by the application of Koopmans' theorem. Above 15eV binding energy the molecular orbital picture of ionization ceases to be valid. Some of the lines between 15 and 21eV disintegrate because of final state correlation effects. Above 21 eV the intensity associated with each orbital is distributed over numerous lines, leading to a dense line structure extending from about 21 to about 44eV. The energies and spectral intensities of >2300 cationic states have been computed.