Flash Photolysis of Sulfur Dioxide

The body of information presented in this paper is directed to photochemists and air pollution scientists interested in species which result from the interaction of SO₂ and light. When SO₂ at low pressures is subjected to an intense photolysis flash, the characteristic, very structured SO₂absorption spectrum disappears immediately after the flash and is replaced by a continuous absorption. The continuous absorption gradually decays and the normal SO₂ absorption spectrum returns. The initial absorbance of the continuous absorption is proportional to the square of the SO₂ pressure and the square of the flash irradiance. From these facts we propose the formation of a metastable dimer of SO₂ formed by the collision of two excited molecules. Some properties of this dimer are: natural lifetime = 2 sec; energy above separated monomers = 4 kcal; lifetime at atmospheric pressure = 1 sec (quenching coefficients with several foreign gases = 10^-20 cm³/sec molecule); absorption of ultraviolet light results in photode-composition of the dimer into monomeric SO₂. The long lifetime of this species and its low quenching cross section may make it an important intermediate in photochemical reactions of SO₂. The relatively low excitation energy of the metastable species indicates it may also be an intermediate in thermally excited reactions and perhaps an important component of smoke stack effluent.