Ultraviolet and visible fluorescence produced by controlled electron impact on SO2

Abstract

The emission spectrum in the ultraviolet and visible region of the optical spectrum (2000–8000 Å) produced by electron impact on SO2 has been studied under single-collision conditions. Two major features have been observed: the A3Π → X3Σ− system of the SO fragment (2400–2650 Å) and a broad, unstructured emission extending from 2500 to 4600 Å, which has been identified as arising from the excitation–de-excitation of the SO2 molecule. Absolute emission cross sections for the SO system have been measured for incident electron energies from threshold to 500 eV, and a value of 5.2 × 10−18 cm2 has been found at 100 eV with an uncertainty of approximately ± 18%. The maximum in the emission cross section is at 30 eV (6.5 × 10−18 cm2). The measured threshold of 11.5 eV is about 1 eV above the minimum energy required for partial fragmentation of SO2 into SO (A3Π) and a ground-state oxygen atom, indicating that very little translational kinetic energy is carried away by the dissociation fragments in the near-threshold region. A complex excitation–de-excitation scheme involving three different excited states of the SO2 molecule, e.g., the 8a1 → 3b1, 1.3B1 states and the 5b2 → 3b11A2 state, produces the broad 2500–4600 Å emission feature. At 100 eV, an emission cross section of 12.2 × 10−18 cm2 has been measured for the SO2 system. Several O1 and S1 emission lines have also been identified, yet they are found to be extremely weak with estimated emission cross sections smaller than 10−21 cm2 at 100 eV impact energy.