A multi-scale ab initio theoretical study of the production of free radicals in swift ion tracks in liquid water

Using a multi-scale theoretical approach from first principles, we show that the production of HO2 radicals in liquid water can be understood from the initial Coulomb explosion of doubly ionized water molecules. Based on the separation of time scales, we used three different theoretical models, each one associated with a specific time scale. The initial similar to 1 fs of water radiolysis is taken care of with a Monte Carlo code whose basic ingredients are cross-sections. These have been calculated in the present work using the continuum distorted wave eikonal initial state (CDW-EIS) model framework. Our calculated cross-sections nicely demonstrate that double ionization of water molecules is one major event compatible with the experimental HO2 molecular rate production. The subsequent tens of fs following the double ionization of one water molecule of the liquid medium have been described with microscopic ab initio Car-Parrinello molecular dynamics simulations. Dynamics shows that the water Coulomb explosion leads to the formation of two H3O+ ions and an atomic oxygen atom. The final stage of the Coulomb explosion ( up to the ms timescale) has been modelled with a chemical Monte Carlo code, assessing that the production of HO2 results from the O + OH -> HO2 reaction in the liquid phase.