Energy Dependence of Oxygen Isotope Exchange and Quenching in the O(¹D) + CO₂ Reaction:  A Crossed Molecular Beam Study

Abstract

The dynamics of the 18 O( 1 D) + 44 CO 2 oxygen isotope exchange reaction has been studied using a crossed molecular beam apparatus at collision energies of 4.2 and 7.7 kcal/mol. At both collision energies, two reaction channels are observed: isotope exchange in which quenching to O( 3 P) occurs and isotope exchange in which the product oxygen atom remains on the singlet surface . Electronic quenching of O( 1 D) is the major channel at both collision energies, accounting for 84% of isotope exchange at 4.2 kcal/mol and 67% at 7.7 kcal/mol. Both channels proceed via a CO 3 * complex that is long-lived with respect to its rotational period. Combined with recent ab initio and statistical calculations by Mebel et al., the long complex lifetimes suggest that statistical isotope exchange occurs in the CO 3 * complex (apart from zero-point energy isotope effects), although the existence of a small, dynamically driven unconventional isotope effect in this reaction cannot yet be ruled out. These new molecular-level details may help provide a more quantitative understanding of the heavy isotope enrichment in CO 2 observed in the stratosphere.