Comparison of Stochastic and Deterministic Methods for Modeling Spur Kinetics

Stochastic and deterministic kinetic methods have been used to model the temporal evolution of spatially nonhomogeneous clusters of reactants resulting from the dissociation of one to six water molecules into either H₃ O⁺, OH, and$e_{{\rm aq}}^{-}$, or H atoms and OH radicals. When the ionic fragmentation initially producing H₃ O⁺, OH, and$e_{{\rm aq}}^{-}$ is considered, the stochastic and deterministic methods predict similar time dependences for the decay of the reactive species; however, the two methods suggest very different product yields. For a two-dissociation spur, the deterministic treatment overestimates both the H₂ and the H₂ O₂ yields by about 70%. The error decreases to less than 15% for a spur with six water dissociations. For a distribution of spurs representing a high-energy electron track, the differences in the predicted yields of reactants are less than 6% at 0.1 μs, but the stochastic and deterministic predictions for the yields of H₂ and H₂ O₂ differ by about 50%. The kinetics of spurs produced by the fragmentation of water to H atom and OH radical shows discrepancies in both the reactant and the product yields. The size of the discrepancy decreases as the number of H/OH pairs increases, and the predictions of the two techniques are almost the same for clusters of six water dissociations.