State-resolved predissociation dynamics of the formyloxyl radical

Dissociative Photodetachment Dynamics of the OH-(C2H4) Anion Complex

Photoelectron-photofragment coincidence (PPC) measurements on OH^-(C₂H₄) anions at a photon energy of 3.20 eV revealed stable and dissociative photodetachment product channels, OH-C₂H₄ + e^- and OH + C₂H₄ + e^-, respectively. The main product channel observed was dissociation to the reactants (>67%), OH + C₂H₄ (v = 0, 1, 2) + e^-, where vibrational excitation in the C-H stretching modes of the C₂H₄ photofragments corresponds to a minor channel. The low kinetic energy release (KER) of the dissociating fragments is consistent with weak repulsion between the OH + C₂H₄ reactants near the transition state as well as the partitioning of energy into rotation of the dissociation products. An impulsive model was used to account for rotational energy partitioning in the dissociative photodetachment (DPD) process and showed good agreement with the experimental results. The low KER of the dissociating fragments and the similarities in the photoelectron spectra between stable and dissociative events support a mechanism involving the van der Waals complex formed upon photodetachment of OH^-(C₂H₄) as an intermediate in the dominant OH + C₂H₄ + e^- dissociative channel.

Photoelectron photofragment coincidence spectroscopy of carboxylates

Photoelectron–photofragment coincidence (PPC) spectroscopy is a powerful technique for studying the decarboxylation dynamics of carboxyl radicals. Measurement of photoelectron and photofragment kinetic energies in coincidence provides a kinematically complete measure of the dissociative photodetachment (DPD) dynamics of carboxylate anions. PPC spectroscopy studies of methanoate, ethanoate, propanoate, 2-butenoate, benzoate, p-coumarate and the oxalate monoanion are reviewed. All of the systems studied undergo decarboxylation via a two-body DPD channel i.e., driven by the thermodynamic stability of CO₂. Additionally, decarboxylation is observed via a three-body ionic photodissociation channel for p-coumarate. In some cases photodetachment also results in a stable carboxyl radical (RCO₂). The branching ratio for DPD, the threshold detachment energy and the peak of the kinetic energy release spectrum are compared for different carboxylates, as a probe of the character of the potential energy landscape in the Franck–Condon region.

Photoelectron photofragment coincidence spectroscopy studies of a range of carboxylate anions are reviewed, revealing details of the decarboxylation dynamics of carboxyl radicals.

Dissociative photodetachment dynamics of the oxalate monoanion

The dissociative photodetachment (DPD) dynamics of the oxalate monoanion are studied using photoelectron-photofragment coincidence (PPC) spectroscopy. Following photodetachment of C₂O₄H^- at 4.66 eV HOCO + CO₂ products are observed, indicating the facile decarboxylation of the radical driven by the thermodynamic stability of CO₂. No evidence is seen for photodetachment to stable C₂O₄H or ionic photodissociation to produce HOCO^-. Calculations indicate the stabilizing presence of an intramolecular hydrogen bond in the anion via the formation of a strained five-membered ring. No intramolecular hydrogen bond is predicted in the radical due to the lower charge density on the oxygen atom. The PPC spectrum is consistent with a single direct two-body DPD channel that results in fragments of similar mass and is characterized by a large kinetic energy release (KER) and a broad photoelectron spectrum. The large KER is indicative of substantial repulsion in the radical following photodetachment. The form of the photoelectron spectrum is dominated by the bound to continuum Franck-Condon factors (BCFCF) and is suggestive of photodetachment to a repulsive potential energy surface. A lower bound for the electron affinity of C₂O₄H is reported as 4 eV. BCFCF calculations allow an approximate functional form of the repulsive surface along the C-C stretch coordinate to be extracted from the experimental photoelectron spectrum. PPC spectroscopy of the deuterated analogue (C₂O₄D^-), at higher anion beam energies is used to increase the detectability of any possible D atom products, but none are observed.

Dynamics of transient speciesviaanion photodetachment

Recent experimental and theoretical advances in transient reaction dynamics probed by photodetachment of polyatomic anions are reviewed.

Spectroscopy and dynamics of the HOCO radical: insights into the OH + CO → H + CO2 reaction

Photoelectron–photofragment coincidence experiments coupled with quantum chemistry and dynamics calculations have significantly enhanced our understanding of the reactive intermediate HOCO.