On the reactions of hydrated electrons with OH⋅ and H3O+. Analysis of photoionization experiments

Abstract

With the help of Monte Carlo simulation techniques, we study the recombination kinetics of hydrated electrons (e−aq) with H3O+ and OH⋅ which results from the photoionization of pure water with femtosecond pulsed lasers. A full description of the simulation procedure is given and various comparisons are made with analytical formulations of the reaction kinetics. Particular attention is given to the reaction of e−aq with H3O+, which is only partially diffusion controlled and which involves a Coulombic interaction with dielectric saturation effects. We find that the probability of reaction per e−aq –H3O+ encounter is small (∼6%) and that the encounter duration can be of the order of a few picoseconds. The competition between the reaction of e−aq with H3O+ and with OH⋅ is analyzed with the simulations and with the independent reaction times method. Both approaches indicate that the e−aq decay is largely dominated by the reaction of e−aq with OH⋅. The effect of neighboring ionization sites on the e−aq decay kinetics is also included in the simulations to account for different possible densities of ionization sites. The initial separation between the reactants is found to be about 1 nm, in agreement with previous determinations. The significance of this last value and the constraints that it puts on the initial kinetic energy of the photoelectrons is discussed.