Electron impact ionization of the SO₂ molecule

We report measurements of the absolute cross sections for the electron-impact ionization of SO₂ from threshold to 200 eV. Absolute cross sections for the formation of the SO₂^- parent ions and of the SO⁺, S⁺, O⁺, and O₂ ⁺ fragment ions were obtained independently in two different laboratories using two different experimental techniques with uncertainties ranging from ±18% to ±25%. The level of agreement between the absolute cross sections (at 70 eV) obtained by the two techniques ranges from about 10% for SO₂⁺ and SO⁺ to 20% for (S ⁺+O₂⁺) and O⁺, which in all cases is well within the combined error margins of the two measurements. The high resolution capability of the mass spectrometer employed in one experiment enabled the separation of the S⁺ and O₂⁺ fragment ions, which are separated by only 0.017 76 atomic mass units (amu), for the first time. The single positive ion formation is the dominant process for all observed product ions. The total single SO₂ ionization cross section obtained by the two techniques agreed to within 8%. A comparison of the experimentally determined total SO₂ single ionization cross sections with calculated cross sections based on a modified additivity rule revealed agreement to within 20%.