Electron ionisation of sulfur dioxide.

Relative precursor-specific partial ionisation cross sections for the fragment ions formed following electron ionisation of sulfur dioxide (SO2) have been measured for the first time, from 30 to 200 eV, using time-of-flight mass spectrometry coupled with two-dimensional ion coincidence detection. These data quantify the yields of O(2+), O(+), SO(2+), S(+), O2(+), and SO(+) ions, relative to the formation of SO2(+), via single, double, and triple electron ionisation of SO2. Formation of O(2+), following electron-SO2 collisions, has been quantified for the first time. The data allow a first experimental estimate of the triple ionisation potential of SO2 (69.0 ± 3.6 eV), an energy in good agreement with a value derived in this study via computational chemistry. The triple ion combination S(+) + O(+) + O(+) is clearly detected following electron collisions with SO2 at electron energies markedly below the vertical energy for forming SO2(3 +). This observation is accounted for by the operation of a stepwise pathway to the formation of S(+) + 2O(+) which does not involve the formation of a molecular trication.