Electron attachment to sulphur dioxide in high pressure gases

Abstract

The attachment of slow (≲10 eV) electrons to SO₂ in N₂(300–25 000 Torr), C₂H₄(200–15 000 Torr) and Ar (500–2000 Torr) gases has been investigated. At electron energies ≲ 0.5 eV, SO^–₂ is formed via a nuclear-excited Feshbach resonance mechanism. The rate for its formation is strongly dependent both on the density and the nature of the gaseous medium and the corresponding cross section possesses two maxima at 0.06 eV and ∼0.28 eV, attributed to electron capture into vibrational levels of SO^–₂. Reaction schemes consistent with the observed density dependences of the attachment rate are presented and discussed. On the basis of these the probability of stabilization of SO^–*₂ in collisions with C₂H₄ and N₂ is found to be ∼60 times larger for C₂H₄ than for N₂. The autodetachment lifetime of SO^–*₂ at thermal energies is estimated to be 2 × 10^–10 s. Experimental evidence is provided suggesting that SO^–*₂ is radiatively stabilized with a rate of ∼1.8 × 10⁸ s^–1, in addition to being stabilized collisionally. At higher energies (∼4–10 eV) SO₂ attaches electrons dissociatively. The cross section for dissociative attachment to SO₂ has a primary maximum at 5 eV and a secondary one at ∼8 eV. At 5 eV the cross section is ∼5.5 × 10^–18 cm².