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Investigation of the reaction of OH and CO to form H and CO₂ by photoelectron-photofragment coincidence spectroscopy in a cryogenic ion beam trap


The HOCO radical plays a crucial role in a wide variety of chemical processes, including atmospheric CO₂ regulation and combustion chemistry, as an intermediate in the elementary reaction OH + CO [right arrow] H + CO₂. However , scant information exists on this species due to the difficulties in studying it. Previous photoelectron- photofragment coincidence (PPC) studies performed in this laboratory have identified key processes occurring on the HOCO potential energy surface, but are complicated by the presence of internal excitation in the precursor anions, leading to uncertainties in product energies and dynamics. To address this, a new instrument has been constructed which incorporates a cryogenically cooled linear electrostatic storage device, providing a cold source of anions for dissociative photodetachment studies by PPC spectroscopy. The enhanced resolution and well- characterized energetics provided by this instrument have allowed the fundamental energetics and processes occurring on the HOCO potential energy surface to be studied in unprecedented detail. New data shows unambiguous confirmation of the presence of tunneling in the reaction HOCO [right arrow] H + CO₂. Careful study of this product channel has led to the generation a model one-dimensional potential barrier describing this process directly from experimental tunneling data, and tunneling lifetimes over a range of relevant internal energies to be predicted. High resolution photodetachment experiments provide a reassignment of the electron affinities of both cis- and trans-HOCO and the determination of several normal mode frequencies not previously measured in the gas phase, each with the support of high-level ab initio quantum chemical calculations. Further details on the previously-unknown isomer well depths and the process of isomerization have been extracted using this information. Finally, nonresonant two-photon photodetachment studies of NO₂⁻, a species with striking electronic structure similarities to HOCO⁻, reveal a highly asymmetric angular distribution of photoelectrons. A model for reconstructing these photoelectron angular distributions is proposed, showing that interference between atomic emissions in the linear combination of atomic orbitals - molecular orbital picture is sufficient to explain the asymmetry

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