Negative Ion Formation by Thermal Surface Ionization of Sulfur Dioxide

The generation of negative ions from SO₂ in the gas phase was studied using the thermal surface ionization method. Six anion types were measured: O^- , S^- , SO^- , and SO₂^- and anions with m/z=96 and m/z=128. The most abundant anion formed was S^- and the formation routes are discussed for each of the six anions. O^- , S^- , and SO^- are formed via dissociative electron attachment to the molecule, whereas the generation of SO₂^- and anions with m/z=96 and m/z=128 are probably associated with the formation of H₂ SO₄ in the gas inlet system and the ion source. Using statistical thermodynamics the dissociation temperatures of SO₂ and SO in the gas phase are calculated and values of above 1800 °C are obtained for both molecules. We also estimated the optimal filament temperatures for the formation of all anions measured, indicating that for SO₂ the optimal temperature is related to the electron affinity of the molecules: the optimal temperature increases with decreasing value of the electron affinity for the molecule corresponding to the respective anion.