The supersonic expansion was intersected by a beam of electrons emitted from a thoria-coated iridium filament (e-Filaments, LLC). The filament was kept at a variable 200 V to 500 V potential and resistively heated by an approximately 5 A current from a floated DC supply. The plasma created by electron bombardment of the neutral precursor gas was cooled in the supersonic expansion. Negative pulses (about 700 to 900 V) applied to an extraction plate positioned downstream from the ionization region were used to separate the negative ions from the cations and neutral species and extract them into the acceleration region of a Wiley-McLaren time-offlight mass-spectrometer. After passing through a ~2 m long flight tube, the anions were separated according to their masses. In the detection region of the instrument, kept at a pressure of ~10 9 torr, the SO  ion packets were intersected by a pulsed laser beam.