Highly selective mechanisms for the production of N2 and O2 dications by electron impact

Different mechanisms for the dication production of molecular nitrogen and oxygen are reported here. It is shown that the N-2 dication is produced by direct double ionization, or TS-2, while the O-2 dication results primarily from an inner valence shell single ionization followed by a postcollisional Auger decay. In order to unveil these different characteristics of both production pathways, absolute cross-sectional measurements for the homoisotopic N-2(2+) and O-2(2+) by electron impact with energy ranging between 200 and 900 eV were measured. The separation of the doubly ionized parent molecules from the singly ionized fragments N+ and O+ was possible by employing the delayed extraction time-of-flight technique. The reasons for the two pure different pathways are discussed in the light of the present data, showing that while the single-ionization dication production process is impossible for nitrogen due to the energy levels limitations of its molecular orbitals, the direct double-ionization channel is strongly inhibited for oxygen due to geometrical constraints of its molecular orbitals orientation.