MONTE-CARLO SIMULATION OF DIFFUSION AND REACTION IN WATER RADIOLYSIS - A STUDY OF REACTANT JUMP THROUGH AND JUMP DISTANCES

In Monte Carlo simulations of water radiolysis, the diffusion of react ants can be approximated by ''jumping'' all species randomly, to repre sent the passage of a short period of time, and then checking their se parations. If, at the end of a jump, two reactant species are within a distance equal to the reaction radius for the pair, they are allowed to react in the model. In principle, the possibility exists that two r eactants could ''jump through'' one another and end up with a separati on larger than the reaction radius with no reaction being scored. Igno ring this possibility would thus reduce the rate of reaction below tha t intended by such a model. By making the jump times and jump distance s shorter, any error introduced by 'jump through' is made smaller. Thi s paper reports numerical results of a systematic study of 'jump throu gh' in Monte Carlo simulations of water radiolysis. With a nominal jum p time of 3 ps, it is found that more than 40% of the reactions of the hydrated electron with itself and of the H atom with itself occur whe n reactions during 'jump through' are allowed. For all other reactions , for which the effect is smaller, the contributions of 'jump through' lie in the range 1%-16% of the total. Corrections to computed rate co nstants for two reactions are evaluated for jump times between 0.1 and 30 ps. It is concluded that jump-through corrections are desirable in such models for jump times that exceed about 1 ps or even less. In a separate study, we find that giving all species of a given type the sa me size jump in a random direction yields results that are indistingui shable from those when the jump sizes are selected from a Gaussian dis tribution. In this comparison, the constant jump size is taken to be t he root-mean-square jump size from the Gaussian distribution.