Evidence for atomic processes in molecular valence double ionization

Complete molecular valence-electron spectra were measured for CO. Unexpectedly, discrete lines at low kinetic energies were found, superimposed on a continuous energy spectrum representing direct double-ionization processes. The appearance of these lines is discussed in the context of the formation of the ${\mathrm{C}}^{+}$+${\mathrm{O}}^{+}$ ion pair near its associated threshold at 38.4 eV. It is ascribed to valence-excited repulsive (${\mathrm{CO}}^{+}$ ${)}^{\mathrm{*}}$ states, which dissociate to a large part rapidly into atomic fragments before electronic relaxation takes place. From our spectra, partial cross sections for the different processes leading to dissociative valence double ionization are derived.