Stochastic models of multi-species kinetics in radiation-induced spurs

Diffusion-limited kinetics in multi-species radical clusters, typical of those resulting from the passage of high-energy radiation through a liquid, have been analysed using several theories. A Monte Carlo (MC) model, in which the trajectories of the diffusing reactants are modelled by time-discretised random flights and in which reaction occurs when particles undergo pairwise encounters, is proposed as a measure of reality. The random reaction times (RRT) model, an efficient stochastic simulation technique based on the approximation that interparticle distances evolve independently, is generalised to multi-species spurs and validated by comparison with the full Monte Carlo model. Further comparisons with other analytical models are less favourable and demonstrate the importance of a stochastic rate equation in predicting product ratios. The RRT model is also extended to include reactive products in three different ways, each of which provides an acceptable approximation to the observed (MC) kinetics in all systems tested so far.