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 C+ + O+ ion pair near its associated threshold at 38.4 eV. It is ascribed to valence-excited repulsive (CO+)* 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.