Anomalous enrichment of ¹⁷O and ¹³C in photodissociation products of CO₂: possible role of nuclear spin

Abstract

Oxygen and carbon isotope fractionation associated with products (CO and O₂) of gas phase photodissociation of CO₂ have been studied using photons from Hg lamp (184.9 nm) and Kr lamp (123.6 and 116.5 nm). In dissociation by Hg lamp photons both CO and O₂ are enriched in ¹⁷O by about 81‰ compared to the estimate based on a kinetic model. Additionally, CO is enriched in ¹³C by about 37‰ relative to the model composition. In contrast, in dissociation by higher energy Kr lamp photons no such anomaly was found in O₂. The observed isotopic enrichments in case of Hg lamp dissociation are proposed to be due to a hyperfine interaction between nuclear spin and electron spins or orbital motion causing enhanced dissociation of isotopologues of CO₂ containing ¹⁷O and ¹³C. The ¹⁷O enrichment is higher than that of ¹³C by a factor of 2.2±0.2 which can be explained by the known magnetic moment ratio of ¹⁷O and ¹³C due to differing nuclear spins and g-factors. These results have potential implications in studies of the planetary atmospheres.