Intersystem crossing in sulphur dioxide

Abstract

Excitation spectra of the fluorescence of SO₂ and of the SO₂ photosensitised phosphorescence of biacetyl have been recorded between 250 and 325 nm, and the rates of decay of the fluorescence and the generation and decay of the biacetyl phosphorescence have been monitored following pulsed laser excitation at (295–300 nm). Rate coefficients for the fluorescence quenching by SO₂, N₂, CF₄, SF₆ and cyclohexane and for triplet energy transfer to biacetyl have been determined, together with relative rate coefficients for collisionally induced intersystem crossing by N₂, CF₄, SF₆ and SO₂; SF₆ and SO₂ are far more efficient than CF₄ or N₂.

The results indicate a single fluorescent species at pressures 2 N m^–2(emitting to wavelengths 500 nm) and a common precursor to both the fluorescent ¹B₁ state and the triplet states populated through collisionally induced intersystem crossing. Arguments against the postulate of a very low-lying ³B₂ state are discussed, and evidence for the population of non-emitting triplet states is presented.