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

Abstract

Photostimulated electron detachment from aqueous inorganic anions is the simplest example of solvent-mediated electron transfer. As such, this photoreaction became the subject of many ultrafast studies. Most of these studied focussed 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., 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 seem to be no well-defined threshold energy below which the broadening of the distribution does not occur, as is the case for halide anions.Direct ionization to the conduction band of water is the most likely photoprocess broadening the electron distribution. The constancy of (near-unity) prompt quantum yields vs. the excitation energy as the latter is scanned across the lowest charge-transferto-solvent band of the anion is observed for halide anions and Fe(CN) 6 4-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; photodetachment from polyatomic, especially polyvalent, anions follows a different set of rules.

Electron detachment from anions in aqueous solutions studied by two- and three-pulse femtosecond spectroscopy

“…Femtosecond pump-probe spectroscopy on various CTTS systems supports the predicted formation of pair species [9][10][11][12][13][14][15][16][17]. The extracted depth of the potential well is typically in the order of a few k B T [15][16][17][18][19].…”

UV/Nitrilotriacetic Acid Process as a Novel Strategy for Efficient Photoreductive Degradation of Perfluorooctanesulfonate

“…whereas it is only 1 ns in UV/sulfite process 38 and lower than 2 ns in UV/KI process 40 . In order to verify our speculation, DMPO was used as an ESR spin-trap to 319 identify possible short-lived radicals produced in the UV/NTA process.…”

Ultrafast dynamics for electron photodetachment from aqueous hydroxide

“…[7] The formation of such a cage is not prerequisite: it has been observed for halide anions in water [3][4][5][6][7] and sodide anions in ethers [8,9], whereas electron photodetachment from polyvalent anions, such as ferrocyanide, sulfite, and sulfate does not seem to involve cage formation, but rather long range ejection of the electron. [45,46] The fast exponential regime observed in the geminate dynamics of "caged" pairs corresponds to their escape from and recombination in the cage, whereas the power "tail" is due to slow re-encounters of the electrons that escape from the cage and their geminate partners.…”