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

Photodissociation of CO2 by ultraviolet light (lambda = 185 nm) generates CO and O-2. which are unusually enriched (more than 100 parts per thousand) in O-17. 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 O-17 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 (OCO)-O-16-C-12-O-17 molecule in the singlet state with those of the triplet state near the zone of transition. In contrast, photodissociation at shorter wavelengths (lambda < 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 C-13 enriched CO2; in this case the products are enriched in both heavy isotopes but about 100 parts per thousand more in O-18. The O-17 enrichment in CO and O-2 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.