A new class of oxygen isotopic fractionation in photodissociation of carbon dioxide: potential implications for atmospheres of Mars and Earth

Abstract

Photodissociation of CO₂ by ultraviolet light (λ = 185 nm) generates CO and O₂, which are unusually enriched (more than 100‰) in ¹⁷O. The dissociation takes place through a spin forbidden process during transition from a singlet to a triplet state, the latter lying on a repulsive potential energy surface. The ¹⁷O isotopic enrichment is a primary process associated with this transition and could be due to near resonant spin-orbit coupling of the low energy vibrational levels of the ¹⁶O¹²C¹⁷O molecule in the singlet state with those of the triplet state near the zone of transition. In contrast, photodissociation at shorter wavelengths (λ < 160 nm) involves no spin violation and produces CO and O₂ which are fractionated in a conventional mass dependent fashion. The proposed explanation is further supported using ¹³C enriched CO₂; in this case the products are enriched in both heavy isotopes but about 100‰ more in ¹⁸O. The ¹⁷O enrichment in CO and O₂ generated by CO₂ photolysis in a range of UV wavelengths may be a useful tracer in delineating processes in the atmospheres of Earth and Mars.