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. 2013 Jul 8;110(44):17697–17702. doi: 10.1073/pnas.1306979110

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Fig. 1. — Photochemistry of SO₂ in the 240–400-nm region. (A) UV absorption cross-section for SO₂ between 240 and 400 nm (from refs. 33 and 34). (B) Vertical lines showing the spectral ranges for the different optical filters tested. (C) Schematic of the photochemistry of the experiments. SO₂ initially is excited into the coupled ¹B₁/¹A₂ states. It vibrationally relaxes (VR) via collisions with the bath gas. At low vibrational levels of the ¹A₂ state, intersystem crossing may lead to irreversible crossing to the ³B₁ state as the result of vibrational relaxation of the ³B₁ state below the origin of the ¹A₂ band. Isotope effects due to near-resonant spin–orbit coupling between the singlet and triplet states in this region may lead to mass-independent isotope anomalies in the resulting triplet-state SO₂. The triplet-state SO₂ finally reacts with acetylene (C₂H₂) to form the organosulfur products analyzed in this study. (D) Potential energy surfaces for the singlet and triplet states of SO₂ as a function of the O'-SO Jacobi distance with the S-O distance and the Jacobi angle optimized.

Inline graphic — Photochemistry of SO₂ in the 240–400-nm region. (A) UV absorption cross-section for SO₂ between 240 and 400 nm (from refs. 33 and 34). (B) Vertical lines showing the spectral ranges for the different optical filters tested. (C) Schematic of the photochemistry of the experiments. SO₂ initially is excited into the coupled ¹B₁/¹A₂ states. It vibrationally relaxes (VR) via collisions with the bath gas. At low vibrational levels of the ¹A₂ state, intersystem crossing may lead to irreversible crossing to the ³B₁ state as the result of vibrational relaxation of the ³B₁ state below the origin of the ¹A₂ band. Isotope effects due to near-resonant spin–orbit coupling between the singlet and triplet states in this region may lead to mass-independent isotope anomalies in the resulting triplet-state SO₂. The triplet-state SO₂ finally reacts with acetylene (C₂H₂) to form the organosulfur products analyzed in this study. (D) Potential energy surfaces for the singlet and triplet states of SO₂ as a function of the O'-SO Jacobi distance with the S-O distance and the Jacobi angle optimized.