Electron attachment to SO2 clusters

Electron attachment to SO₂ clusters formed by nozzle expansion was investigated (n up to 8) in a molecular-beam electron-ionization mass-spectrometer system. Electron ionization of SO₂ clusters was also studied (n up to 18) showing no pronounced structure in the mass spectra and no strong dependence on electron energy, the dominant positive ion being the (SO₂) _n ⁺ series. Also present but less abundant are the fragment ion series (SO₂) _n SO⁺, (SO₂) _n S⁺ and (SO₂) _n O⁺ with decreasing intensities in that order. The dominant negative ion is (SO₂) _n ^? . The homologous series (SO₂) _n O^? and (SO₂) _n SO^? are less abundant, the series (SO₂) _n S^? has not been observed at all (except SO₂·S^? produced in the background gas via secondary processes). The negative ion mass spectra show a strong dependence on electron energy due to a rich resonance like structure of the attachment cross sections involved. These attachment cross sections have been determined (up to 40 eV) and they show significant differences for the different homologous series. The most striking feature is that for instance (SO₂) _n SO^? ions do not show any signal at the first main resonance of SO^? from SO₂. Likewise (SO₂) _n O^? ions only show a strongly diminished signal at the first main resonance of O^? from SO₂. This is in contrast to results in O₂, CO₂ and N₂O. Conversely, (SO₂) _n ^? ions show — besides peaks at the position of the first and second O^? resonance — additional resonances below and above these peaks. In addition, (SO₂) ₅ ^? and larger ones show a zero energy peak consisting of stoichiometric SO₂ cluster ions similar to observations in O₂, CO₂ and H₂O. The attachment cross section of S^? from SO₂ has been found to show an additional previously undetected peak at ～1 eV. Moreover, the present study revealed the existence of a S _n ^? (n up to 8) series being produced with nearly zero energy electrons via volume and surface processes in and around the ion source.