Breakdown of the One-Electron Picture of Ionization for Hydrocarbon n Systems

The photoelectron spectra of a number of characteristic hydrocarbon n systems are analyzed in terms of Koopmans' and non-Koopmans' contributions within a< limited configuration interaction · (CI) framework. The results for several compounds, including quinodimethanes, fulvenes, annulenes, and related systems, suggest frequent invalidity of the one-electron picture of ionization even in the outer valence region. A similar situation may apply for the corresponding anion spectra, as indicated by the electron transmission spectrum of p-benzoquinone. This and severa'l recent investigations demonstrate a general need for the introduction of a more realistic excited state concept than the one implied by Koopmans' approximation.