Orienting reactants using van der Waals precursors: OCO ⋅ HBr+hν → {OCO ← H}+Br → CO+OH+Br

Abstract

We report the results of an experimental study in which the reaction of atomic hydrogen with carbon dioxide is examined under conditions wherein the reagents are highly oriented relative to one another. Orientation is achieved using nozzle expansions to prepare a precursor of the form CO2 ⋅ HBr, which is best described as a weakly bound van der Waals or hydrogen bonded complex. The weak bond insures that the CO2 and HBr moieties are separable insofar as electronic excitations and HBr dissociation are concerned. Translationally hot H atoms are produced by the 193 nm photolysis of the HBr constituent of the nearly linear CO2 ⋅ HBr complex. Dissociation is direct, and the initial H-atom velocity is directed primarily along the HBr axis. Because the HBr is complexed with the CO2, the initial conditions for the ensuing reaction are quite restricted relative to cases in which there is no selective orientation, i.e., bulk or molecular beam conditions. OH(X2Π) is detected under experimental conditions which minimize effects due to uncomplexed bimolecular processes and/or contributions from higher clusters, and vibration, rotation, spin-orbit, and Λ-doublet populations are determined. The OH(X 2Π) deriving from the complexed reaction appears instantly on the time scale of the measurement, in contrast to the clear rise time observed under bulk conditions, and the rotational population distributions differ measurably from those obtained under bulk conditions. This difference can be due to the short lifetime of the HOCO intermediate, and/or interactions of the OH fragment with the nearby Br atom from the CO2 ⋅ HBr complex.