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.