The photolysis of SO2 at 3080 Å in the presence of cis- and trans-1,2-difluoroethylene

The photolysis of SO₂ at 3080 Å, FWHM = 150 Å, and 22°C has been investigated in the presence of cis- and trans-C₂F₂H₂. Quantum yield measurements for the photosensitized isomerization of cis-C₂F₂H₂ to trans-C₂F₂H₂ have been made for a variation in the [SO₂]/[cis-C₂F₂H₂] ratio from 0.992 to 253. The results fit a mechanism which is consistent with the SO₂(³B₁) state being the reactive excited state of sulfur dioxide. A mechanism employing only the SO₂(¹B₁) and SO₂(³B₁) excited states is quite satisfactory to rationalize the data. A value for the SO₂ collisionally induced intersystem crossing efficiency from SO₂(¹B₁) to SO₂(³B₁) of 0.35 ± 0.14 was estimated while the cis-C₂F₂H₂ efficiency was found to be 0.030 ± 0.012. The rate constant at 22°C for the removal of SO₂(³B₁) molecules by cis-C₂F₂H₂ was found to be (1.43 ± 0.13) × 10¹⁰1./mole · sec. A photostationary composition, [cis]/[trans] = 1.0 ± 0.1, was found from prolonged irradiations of SO₂ in the presence of the cis and trans isomers.