arXiv subjects

David M. Bartels

Publications and source records attributed to David M. Bartels.

3 recordsLinked to original sources

arXiv2004

Electron Photodetachment from Aqueous Anions. II. Ionic Strength Effect on Geminate Recombination Dynamics and Quantum Yield for Hydrated Electron

In concentrated solutions of NaClO4 and Na2SO4, the quantum yield for free electron generated by detachment from photoexcited anions (such as I-, OH-, ClO^4-, and [SO3]^2-) linearly decreases by 6-12% per 1 M ionic strength. In 9 M sodium perchlorate solution, this quantum yield decreases by roughly an order of magnitude. Ultrafast kinetic studies of 200 nm photon induced electron detachment from Br-, HO- and [SO3]^2- suggest that the prompt yield of thermalized electron does not change in these solutions; rather, the ionic strength effect originates in more efficient recombination of geminate pairs. Within the framework of the recently proposed mean force potential (MFP) model of charge separation dynamics in such photosystems, the observed changes are interpreted as an increase in the short-range attractive potential between the geminate partners. Association of sodium cation(s) with the electron and the parent anion is suggested as the most likely cause for the observed modification of the MFP. Electron thermalization kinetics suggest that the cation associated with the parent anion (by ion pairing and/or ionic atmosphere interaction) is passed to the detached electron in the course of the photoreaction. The precise atomic-level mechanism for the ionic strength effect is presently unclear; any further advance is likely to require the development of an adequate quantum molecular dynamics model.

physics.chem-ph↗

arXiv2004

Ultrafast Dynamics for Electron Photodetachment from Aqueous Hydroxide

Charge-transfer-to-solvent (CTTS) reactions of hydroxide induced by 200 nm monophotonic or 337 nm and 389 nm biphotonic excitation of this anion in aqueous solution have been studied by means of pump-probe ultrafast laser spectroscopy. Transient absorption kinetics of the hydrated electron, eaq-, have been observed, from a few hundred femtoseconds out to 600 ps, and studied as function of hydroxide concentration and temperature. The geminate decay kinetics are bimodal, with a fast exponential component (ca. 13 ps) and a slower power "tail" due to the diffusional escape of the electrons. For the biphotonic excitation, the extrapolated fraction of escaped electrons is 1.8 times higher than for the monophotonic 200 nm excitation (31% vs. 17.5% at 25 oC, respectively), due to the broadening of the electron distribution. The biphotonic electron detachment is very inefficient; the corresponding absorption coefficient at 400 nm is < 4 cm TW-1 M-1 (assuming unity quantum efficiency for the photodetachment). For [OH-] between 10 mM and 10 M, almost no concentration dependence of the time profiles of solvated electron kinetics was observed. At higher temperature, the escape fraction of the electrons increases with a slope of 3x10-3 K-1 and the recombination and diffusion-controlled dissociation of the close pairs become faster. Activation energies of 8.3 and 22.3 kJ/mol for these two processes were obtained. The semianalytical theory of Shushin for diffusion controlled reactions in the central force field was used to model the geminate dynamics. The implications of these results for photoionization of water are discussed.

physics.chem-ph↗

arXiv2004

Recombination of Geminate (OH,eaq-) Pairs in Concentrated Alkaline Solutions: Lack of Evidence For Hydroxyl Radical Deprotonation

Picosecond dynamics of hydrated electrons and hydroxyl radicals generated in 200 nm photodissociation of aqueous hydroxide and 400 nm (3-photon) ionization of water in concentrated alkaline solutions were obtained. No deprotonation of hydroxyl radicals was observed on sub-nanosecond time scale, even in 1-10 M KOH solutions. This result is completely at odds with the kinetic data for deprotonation of OH radical in dilute alkaline solutions. We suggest that the deprotonation of hydroxyl radical is slowed down dramatically in concentrated alkaline solutions.

physics.chem-ph↗