MECHANISMS OF ANION FORMATION IN O2, O2/AR AND O2/NE CLUSTERS - THE ROLE OF INELASTIC ELECTRON-SCATTERING

Measurements are reported on the formation of negative ions in O2, O2/Ar and O2/Ne clusters aimed at establishing the mechanisms of anion formation and the role of inelastic electron scattering by the cluster constituents on negative ion formation in clusters. In the case of pure O2 clusters the main anions we detected are of two types: O-(O2)n greater-than-or-equal-to 0 and (O2)n greater-than-or-equal-to 1-. The yields of O-(O2)n showed maxima at 6.3, 8.0 and almost-equal-to 14.0 eV and the data suggest O- as their precursor; the maxima at 8 and almost-equal-to 14 eV are due to the production of O- via symmetry forbidden dissociative attachment processes in O2 at these energies which become allowed in clusters. The yields of (O2)n- showed a strong maximum at near-zero energy (almost-equal-to 0.5 eV) and also at 6.3, 8 and almost-equal-to 14 eV. With the exception of the near-zero energy resonance, the (O2)n- anions at 6.3, 8 and almost-equal-to 14 eV are attributed to nondissociative attachment of near-zero energy "secondary" electrons to O2 clusters. The slow secondary electrons result predominantly from scattering via the O2- negative ion states of incident electrons with energies in their respective regions. Similar results were obtained for the mixed O2/rare gas clusters except that now a feeble and distinctly structured contribution in the yields of O-(O2)n, (O2)n- (and Ar(O2)n-) was observed at energies > 10 eV. These anions are believed to have the lowest negative ion states of Ar*-(Ne*-) as their precursors.