Short-Time Water Caging and Elementary Prehydration Redox Reactions in Ionic Environments

Electron Photodetachment from Aqueous Anions. 3. Dynamics of Geminate Pairs Derived from Photoexcitation of Mono- vs Polyatomic Anions

Photostimulated electron detachment from aqueous inorganic anions is the simplest example of solvent-mediated electron transfer reaction. As such, this photoreaction became the subject of many ultrafast studies. Most of these studied focused on the behavior of halide anions, in particular, iodide, that is readily accessible in the UV. In this study, we contrast the behavior of these halide anions with that of small polyatomic anions, such as pseudohalide anions (e.g., HS(-)) and common polyvalent anions (e.g., SO(3)(2-)). Geminate recombination dynamics of hydrated electrons generated by 200 nm photoexcitation of aqueous anions (I(-), Br(-), OH(-), HS(-), CNS(-), CO(3)(2-), SO(3)(2-), and Fe(CN)(6)(4-)) have been studied. Prompt quantum yields for the formation of solvated, thermalized electrons and quantum yields for free electrons were determined. Pump-probe kinetics for 200 nm photoexcitation were compared with kinetics obtained at lower photoexcitation energy (225 or 242 nm) for the same anions, where possible. Free diffusion and mean force potential models of geminate recombination dynamics were used to analyze these kinetics. These analyses suggest that for polyatomic anions (including all polyvalent anions studied) the initial electron distribution has a broad component, even at relatively low photoexcitation energy. There seems to be no well-defined threshold energy below which the broadening of this electron distribution does not occur, as is the case for halide anions. The constancy of (near-unity) prompt quantum yields vs the excitation energy as the latter is scanned across the lowest charge-transfer-to-solvent band of the anion is observed for halide anions but not for other anions: the prompt quantum yields are considerably less than unity and depend strongly on the excitation energy. Our study suggests that halide anions are in the class of their own; electron photodetachment from polyatomic, especially polyvalent, anions exhibits qualitatively different behavior.