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Open Access Publications from the University of California

Dynamics of gas-phase transient species studied by dissociative photodetachment of molecular anions


Gas-phase transient species, such as the CH₃CO₂ and HOCO free radicals, play important roles in combustion and environment chemistry. In this thesis work, the dynamics of these two radicals were studied by dissociative photodetachment (DPD) of the negative ions, CH₃CO₂-С and HOCO⁻, respectively. The experiments were carried out with a fast-ion-beam photoelectron-photofragment coincidence (PPC) spectrometer. Mass-selected molecular anions in a fast ion beam were intercepted by a linearly polarized laser pulse. Three dimensional velocities of the resulting photoelectron and photofragments were measured in coincidence by time- and position-sensitive detectors, providing complete kinematic descriptions of the DPD dynamics. The energetics and dynamics of CH₃CO₂were studied by the DPD of acetate anion at photon energy of E̳hv̳ = 3.49 eV and 4.82 eV. Evidence for several low-lying electronic states of the CH₃CO₂ radical were observed. While a stable CH₃CO₂ channel (̃10%) was found, most CH₃CO₂ radicals (̃90%) lead to a dissociation channel forming CH₃ + CO₂ products with ̃72% of the available energy partitioned into translational degrees of freedom at E̳hv̳ = 3.49 eV. The potential energy surface of HOCO and the dynamics of the OH + CO-> H + CO₂ reaction were explored by DPD of HOCO⁻ and DOCO⁻ at a photon energy of E̳hv̳ = 3.21 eV. The PPC experiments allow the identification of photodetachment processes leading to the production of stable HOCO free radicals and both the H + CO₂ and OH + CO dissociation channels. The PPC spectra and branching ratio analysis are consistent with a role for a tunneling mechanism in the production of H + CO₂. Evidence was also shown that vibrationally excited HOCO⁻ anions play a role in the observed DPD process. The near-threshold one- and two-photon photodetachment dynamics of HOCO⁻ at E̳hv̳ = 1.60 eV was also studied. An anisotropic photoelectron angular distribution (PAD) consistent with a strong molecular alignment was observed in the two-photon process. A quantitative analysis of the PAD reveals this is associated with a temporary anion formed by a p-wave shape resonance and the PAD in the two-photon signal is a result of interfering s- and d-partial waves within the atomic approximation. Finally, the photoelectron-photofragment angular correlations in the one- and two-photon DPD of HOCO⁻ and DOCO⁻ were investigated. The results show a transition dipole transition in the plane of symmetry for the DPD of HOCO⁻. The HOCO and DOCO lifetimes were examined as a function of the orientation of transition dipole and were estimated to be less than 9x10⁻¹³ sec and 1.3x10⁻¹² sec, respectively. Measurement of the two-photon photofragment angular distribution in the OH + CO channel confirmed the alignment of HOCO⁻ inferred from the two- photon PADs

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