Negative-Fragment-Ion Formation by Photon Excitation of Molecules in the Vicinity of Their Core-Ionization and Direct-Double-Ionization Thresholds

By using S${\mathrm{O}}_2$ as a prototype molecule, negative fragment ions are produced by photon excitation in the vicinity of its sulfur $2p$ core-ionization (≃175 eV) and direct-double-ionization (≃34.1 eV) thresholds. This new class of processes is shown to be at least partly related to the existence of superexcited electronic states of the singly charged parent cation lying above the double-ionization-threshold energy. The use of these mechanisms as a specific tool for probing the nature and the relaxation of core-excited states of molecules is also illustrated.