| Stratospheric CO2 is enriched in the stable heavy isotopes 17O and 18O. The unusual enrichments, which cannot be explained by standard kinetic isotope effects, are thought to be transferred from ozone, which also exhibits anomalous enrichments due to unconventional kinetic isotope effects in ozone formation. Ozone is photolyzed to O(1D) in the stratosphere followed by quenching of O( 1D) to O(3P) by CO2 via a CO3* complex in which oxygen atom exchange can occur. Whether this scheme fully explains the stratospheric CO2 enrichments or whether an additional anomalous kinetic isotope effect exists in the reaction O(1D) + CO2 → CO3* → O(3P) + CO 2 is not known. To address these uncertainties, the reaction dynamics of O(1D) + CO2 have been studied using two crossed molecular beam instruments, one using velocity map ion-imaging and the other a universal detector. The universal detector experiments at two collision energies, 4.2 and 7.7 kcal/mol, revealed that oxygen isotope exchange between O(1D) and CO2 occurs via two channels: one involving electronic quenching to O(3P) and one resulting in an O( 1D) product. Previous studies have assumed that all O(1D)--CO 2 isotope exchange resulted in electronic quenching, but this work shows that a significant fraction (16% at 4.2 kcal/mol) does not. Both channels observed proceed through a CO3* complex which is long-lived relative to its rotational period. Along with recent ab initio and statistical calculations performed by A. Mebel of Florida International University motivated by this work, these long CO3* lifetimes suggest that, neglecting zero point energy effects, isotope exchange in CO3* is statistical---i.e., that the incoming oxygen atom has a 2/3 chance of being incorporated into the product CO2. A small dynamically-driven unconventional isotope effect, however, cannot yet be completely ruled out. These new results may help achieve a quantitative understanding of the stratospheric CO2 oxygen isotope anomaly, which in turn may allow atmospheric measurements to be used as a unique tracer of stratospheric chemistry and transport on timescales of several years and a means to quantify gross carbon fluxes to and from the biosphere on annual to millennial time scales. |
