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Electron Photodetachment from Aqueous Anions. 1. Quantum Yields for Generation of Hydrated Electron by 193 and 248 nm Laser Photoexcitation of Miscellaneous Inorganic Anions †

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Electron Photodetachment from Aqueous Anions. 1. Quantum Yields for Generation of Hydrated Electron by 193 and 248 nm Laser Photoexcitation of Miscellaneous Inorganic Anions †
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  1 J. Phys. Chem. Aversion 4; 4/12/2004 Electron Photodetachment from Aqueous Anions.II. Ionic Strength Effect on Geminate Recombination Dynamics andQuantum Yield for Hydrated Electron. 1 Myran C. Sauer, Jr., Ilya A. Shkrob, * Rui Lian,Robert A. Crowell, David M. Bartels, a) Chemistry Division , Argonne National Laboratory, Argonne, IL 60439 Stephen E. Bradforth  Department of Chemistry, University of Southern California, Los Angeles, CA 90089 The submitted manuscript has been created by the University of Chicago asOperator of Argonne National Laboratory ("Argonne") under Contract No.W-31-109-ENG-38 with the U. S. Department of Energy. The U. S.Government retains for itself, and others acting on its behalf, a paid-up,nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publiclyand display publicly, by or on behalf of the Government. Abstract In concentrated solutions of NaClO 4  and Na 2 SO 4 , the quantum yield for freeelectron generated by detachment from photoexcited anions (such as I - , OH - , ClO 4- , andSO 32- ) linearly decreases by 6-12% per 1 M ionic strength. In 9 M sodium perchloratesolution, this quantum yield decreases by roughly an order of magnitude. Ultrafast kineticstudies of 200 nm photon induced electron detachment from Br  - , HO -  and SO 32-  suggestthat the prompt yield of thermalized electron does not change in these solutions; rather,the ionic strength effect srcinates 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 interpretedas 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 themost likely cause for the observed modification of the MFP. Electron thermalizationkinetics 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  2 presently unclear; any further advance is likely to require the development of an adequatequantum molecular dynamics model.  ___________________________________________________________________________________  1 Work performed under the auspices of the Office of Science, Division of Chemical Science, US-DOE under contract number W-31-109-ENG-38. * To whom correspondence should be addressed: Tel   630-252-9516,  FAX   630-2524993, e-mail:  shkrob@anl.gov.a) present address: Radiation Laboratory, University of Notre Dame, Notre Dame,Indiana 46556, e-mail:   bartels.5@nd.edu.  3 1. Introduction. In Part I of this series, [1] estimates for absolute quantum yields (QYs) and crosssections for electron photodetachment from miscellaneous aqueous anions were given.With a single exception of perchlorate, these measurements were carried out in dilute (<50 mM) solutions of these CTTS (charge transfer to solvent) active anions. In this work,we explore the electron dynamics for photoexcited anions in high ionic strengthsolutions.The motivation for such a study is provided by the recent ultrafast kinetic studiesof electron photodetachment from halide [2,3,4] and pseudohalide [4,5,6] anions. Thesestudies suggest the existence of a weak attractive mean force potential (MFP) betweenthe residual radical (such as HO or a halogen atom) and the ejected electron that localizesin its vicinity. This attractive potential causes bimodality of the electron decay kinetics: inthe first 10-50 ps these kinetics are exponential (due to the fast escape and recombinationof the electrons situated near the bottom of the potential well); [7] these rapid kinetics aresucceeded by a slower t  -1/2  decay due to the diffusional escape and recombination of hydrated electrons which are thermally emitted from this potential well. [2,5,7] For  polyvalent anions, only these slow kinetics are observed; [4,8] the fast exponentialcomponent is lacking. This is not surprising since long-range Coulomb repulsion betweenthe electron and a radical anion (derived from the parent polyvalent anion) should bemuch stronger than short-range, weak attraction between the electron and a neutralradical (derived from the parent monovalent anion). For the latter type of geminate pairs,the MFP is thought to srcinate mainly through the polarization of the residue (e.g.,halide atom) by the electron, [9,10] though electron - dipole interaction may also besignificant for pairs generated by electron photodetachment from polyatomic anions.According to Bradforth and coworkers, [9] for geminate pairs derived from monovalentanions, the strength of the interaction (as estimated from MFP model kinetic fits)increases with the polarizability of the radical/atom, being maximum for iodine atoms.For chloride, the MFP has been obtained theoretically, using quantum molecular dynamics simulations and umbrella sampling. [10] Unfortunately, these simulations do  4not point to any  specific  interactions that are responsible for the MFP: the latter emergesas a sum total of many interparticle interactions. Still, it is reassuring that the strength of this simulated MFP (which is a few kT   units) is comparable to the strength of the MFPthat was extracted by Bradforth and coworkers from the experimental kinetics for aqueous iodide. [2]These MFP models were found to quantitatively account for (i) the temporal profile of the geminate recombination kinetics, [2,4,5] (ii) the changes observed in thesekinetics with increasing photoexcitation energy (direct ionization that occurs at higher energies broadens the spatial distribution of photoejected electrons), [11] (iii) temperatureeffects on the decay kinetics of electrons generated in 200 nm photoexcitation of hydroxide. [5] The MFP model also qualitatively accounts for the trends in the kineticsobserved for homologous anions (e.g., the correlation between the potential well depthand the polarizability of the radical), [9] as well as some trends observed for nonaqueousand mixed solvents. [3a,b]Impressive as this record may appear, there is no direct, unambiguous way inwhich the MFP profile can be extracted from the experimental data, and this leaves roomfor fudging the MFP parameters and the initial electron distribution to accommodate anyof the trends observed. E. g., to “explain” temperature effects on the geminate kinetics for  OH e aq , − ( )  pairs generated in electron photodetachment from hydroxide it was postulatedthat the MFP for this pair becomes shallower and more diffuse with the increasingtemperature. [5] While the entire set of 8 o C to 90 o C kinetics can be consistently fit withthese assumptions, there is currently no way of demonstrating experimentally that theMFP behaves as postulated or that this attractive potential even exists. This uncertainty pertains to other studies: in the end, it is not yet clear whether the MFP model is a correct(though simplified) picture that captures peculiarities of electron dynamics for  photoexcited CTTS anions or merely a recipe for producing kinetic profiles that resemblethose observed experimentally.We believe that the MFP model does capture the reality, albeit incompletely. E.g.,the existence of a short-range interaction that temporarily stabilizes close pairs is  5suggested by experimental [12] and theoretical studies [13] of Na -  CTTS by Schwartzand coworkers. These authors showed that electrons in the close pairs (observed on theshort time scale) responded differently to IR photoexcitation than the electrons in moredistant pairs. The short-term electron dynamics observed in the sodide photosystem had been viewed as the dissociation of these close pairs (in which the electron and sodiumatom reside in a single solvent cavity) to solvent separated pairs (in which the species areseparated by 1-2 solvent molecules) that subsequently decay by diffusional migration of the partners to the bulk.The purpose of the present study was to validate the use of the MFP model for aqueous anions. The approach was to modify this potential in a predictable way, throughionic atmosphere screening of electrostatic charges. It was expected that for geminate pairs derived from monovalent anions, the screening of the electron would weaken theelectrostatic interactions of this electron with its neutral partner and the survival probability Ω ∞  of electrons that escape geminate recombination would thereby increase.Conversely, for polyvalent anions, this probability would decrease because the ionicatmosphere would screen the Coulomb repulsion between the electron and its negativelycharged partner, the radical anion. As demonstrated below, these expectations were notrealized.The experimental results show that both the survival probability and the quantumyield of free electron decrease  with the ionic strength for all of the systems studied,whereas the prompt QY for the solvated electrons does not change. The magnitude of thisdecrease is the same for monovalent and divalent anions and changes little from one photosystem to another; it also varies little with the salt that is used to change the ionicstrength. While similar magnitude of the decrease in the survival probability Ω ∞  of theelectron with ionic strength was attained for all photosystems, the rate for the attainmentof this decrease varied considerably between different photosystems.Though we cannot presently suggest a detailed model that accounts for theseobservations in their totality, it is certain that ion screening cannot account for theseresults even at the qualitative level. We suggest that the effect srcinates through the
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