Reactive ion scattering from ice in a molecular dynamics perspective

Abstract

This is a study into the scattering dynamics of the alkaline ions Cs +, K +, Na +, and Li + from an ice surface, and the process of abstracting water molecules by the scattered ions to form ion–water clusters as a result of the ion–dipole attraction. In a classical molecular dynamics computer simulation a semi-empirical ion–water interaction potential and a modified version of the TIP3P ice model are employed. The thickness of the ice structure at the surface greatly affects the abstraction efficiency. From a thin ice overlayer all alkaline ions exhibit similar scattering probabilities, but Cs + abstracts water molecules most efficiently; its lower speed facilitates a mechanism where the Cs + in its outgoing trajectory pulls water molecules out of the ice structure. From a thick ice structure the scattering probabilities decrease dramatically due to an effective energy transfer to the ice structure. A more grazing angle of incidence reduces the energy transfer and enhances the scattering probabilities for the lighter alkaline ions. The deprived formation of ion–water clusters in the simulations confirms that from thick ice the cluster formation probability is reduced by at least three orders of magnitude.