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Dissociation Dynamics of Transient Molecules Studied by Photoelectron-Photofragment Coincidence Spectroscopy


As the simplest carboxyl free radical the formyloxyl radical, HCO2, serves as a prototype for more complex carboxyl free radicals such as CH3CO2, C6H5CO2 and HC2CO2. It is also an intermediate in the OH + CO ⟶ H + CO2 reaction, of increasing interest for both atmospheric and combustion chemistry. The predissociation dynamics for the DCO2¯ + h? → D + CO2 + e¯ reaction were studied by DPD of DCO2¯. Photodetachment near threshold (hν = 4.27 eV) accesses all three of the lowest-lying electronic states (2B2, 2A1, and 2A2) of the formyloxyl radical. The photoelectron spectrum indicates vibrational excitation dominated by the O–C–O bending mode in the 2A1. In the PPC spectra for DCO2¯, each vibrational state of the nascent neutral is coupled to bending excitation in the resulting CO2 product observed in the eKE-gated state-resolved translational energy distribution. Each CO2 product-state distribution exhibits a substantial level of bending excitation, more than predicted by recent quantum-dynamics calculations on an ab initio potential energy surface.

Extending our work on carboxyl free radicals, the dissociation dynamics of the propiolyl radical, HCCCO2, and its related isomer CCCO2H were studied by DPD of their respective anions using PPC spectroscopy. At hν = 4.27 eV, the HCCCO2¯ anions is found to readily dissociate to HCC + CO2. Similar to the formyloxyl radical, dissociation to HCC + CO2 results in a series of resolved peaks in the translational energy release spectrum corresponding to the O–C–O bending mode in the CO2 fragment. The isomeric CCCO2H were not observed. High level ab initio calculations and Franck-Condon simulations were carried out to reproduce the stable photoelectron spectrum.

Extending the recent work on the F + H2O benchmark F initiated hydrogen abstraction reaction, the effects of vibrational excitation of the precursor anion was explored. To carry out this experiment, an IR laser was coupled into the existing PPC spectrometer using a mirror on the existing neutral beam block. Excitation of the internal hydrogen bond F—H—OH of the anion causes a noticeable change in dissociation dynamics, increasing access to the HF + OH products. Finally, the effects of intramolecular vibrational relaxation of the excited anions was explored by looking at how the anions aged during confinement in the electrostatic ion beam trap.

Finally, the F + HOCH3 → HF + OCH3 reaction was studied; expanding our work on F initiated hydrogen abstraction reactions. The HF product is formed with considerable excitation while the OCH3 product behaves as a spectator to the reaction. Both the metastable reactant channel van der Waals complex and stable product hydrogen bonded complexes were observed, along with a series of long-lived metastable vibrational Feshbach resonances.

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