Threshold electron attachment and electron impact ionization involving oxygen dimers

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Using two different crossed-beams machines we have carried out the first quantitative study of threshold electron attachment and electron impact-induced ionization and fragmentation involving oxygen dimers (O2)2. In the electron attachment experiment we study electron transfer from state-selected Ar**(20d) Rydberg atoms to O2 molecules and dimers in a skimmed supersonic beam at variable nozzle temperatures (T0) and stagnation pressures (p0). The relative dimer density is determined through measurements of Penning ionization by metastable Ne*(3s3P2,0) atoms and used to estimate the absolute cross-section for O2− formation in collisions of Ar**(20d) Rydberg atoms with O2 dimers to be nearly 10−17 m2, almost four orders of magnitude larger than that for O2− formation in collisions of Ar**(20d) Rydberg atoms with O2 monomers. The fragmentation of the oxygen cluster beam is quantitatively characterized by the transverse helium beam scattering method which allows us to spatially separate different clusters. It is shown that in 70 eV electron impact of (O2)2 only 3.6(4)% of the dimers are detected as dimer ions (O2)2+. In additional experiments involving SF6 clusters we show that SF6 dimers fragment nearly completely upon 70 eV electron impact, yielding SF5+ ions (probability for (SF6)·SF5+ production at most 0.3%).