Vacuum ultraviolet photodissociation of sulfur dioxide and its implications for oxygen production in the early Earth's atmosphere.

The emergence of molecular oxygen (O 2 ) in the Earth's primitive atmosphere is an issue of major interest. Although the biological processes leading to its accumulation in the Earth's atmosphere are well understood, its abiotic source is still not fully established. Here, we report a new direct dissociation channel yielding S( 1 D) + O 2 (a 1 Δ g /X 3 Σ g - ) products from vacuum ultraviolet (VUV) photodissociation of SO 2 in the wavelength range between 120 and 160 nm. Experimental results show O 2 production to be an important channel from SO 2 VUV photodissociation, with a branching ratio of 30 ± 5% at the H Lyman-α wavelength (121.6 nm). The relatively large amounts of SO 2 emitted from volcanic eruptions in the Earth's late Archaean eon imply that VUV photodissociation of SO 2 could have provided a crucial additional source term in the O 2 budget in the Earth's primitive atmosphere. The results could also have implications for abiotic oxygen formation on other planets with atmospheres rich in volcanically outgassed SO 2 .