Core-valence double ionization of carbon suboxide

We have measured the core-valence double ionization spectra of carbon suboxide above both the O 1s and C 1s edges. Following several core-valence cases known in the literature, to begin with the spectra are compared with the well-known single ionization valence photoelectron spectrum of this system, from which they surprisingly differ quite strongly. This motivates a comparison to electronic structure calculations carried out within the sudden approximation where the overlap between the core and valence orbital is included, while still assuming decoupling of the core and valence ionization events. The substantially improved agreement indicates that this more complex description is needed to model the core-valence double ionization process adequately in the present case. Accordingly, assignments of spectral features are made by comparison of our experimental and numerical spectra. Auger decay following C 1s hole formation at the chemically distinct central and outer C atoms shows strong selectivity in the final multiply charged states produced, for initial core-valence ionization, being consistent with the Auger decay from single core ionization. Comparison to calculations allows for the identification of the initial core ionization site for the Auger decay following core-valence ionization.