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.