32 research outputs found
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.Comment: 40 pages, 10 figures + supplement 2 pages, 9 figures; will be
submitted, in a modified form, to J. Phys. Chem