Temperature dependence of isotopic fractionation in CO2 photolysis

The effect of temperature on the heavy isotope ratios of the products (CO and O2) of photolysis of CO2 by ∼185 nm photons was studied. The isotope ratios 18O/16O and 17O/16O in O2 and CO and 13C/12C in CO increase with temperature by ∼1‰ per K. The increase in 17O/16O in O2 is related linearly to the increase in 18O/16O with a slope 0.8 contrary to the mass dependent value of 0.5. Variations in factors related to the transition dipole moment and Boltzmann distribution of the vibrational state populations are probably responsible for the observed change. Temperature dependence of the gas production rate agrees with the reported cross-section variations.