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

Photodissociation of CO 2 by ultraviolet light (λ = 185 nm) generates CO and O 2 , which are unusually enriched (more than 100‰) in 17 O. 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 17 O 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 16 O 12 C 17 O 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 O 2 which are fractionated in a conventional mass dependent fashion. The proposed explanation is further supported using 13 C enriched CO 2 ; in this case the products are enriched in both heavy isotopes but about 100‰ more in 18 O. The 17 O enrichment in CO and O 2 generated by CO 2 photolysis in a range of UV wavelengths may be a useful tracer in delineating processes in the atmospheres of Earth and Mars.