The existence of Young-type interference patterns, recently measured ͓N. Stolterfoht et al., Phys. Rev. Lett. 87, 023201 ͑2001͔͒ in double-differential cross sections for single-electron ionization of H 2 molecules by ion impact, is theoretically supported by calculations obtained using a molecular distorted-wave model introduced here. The importance of adequately describing the target as a molecule is emphasized. The contributions from direct and interference terms are separated.

We present a theoretical study of H 2 ionization by photons of a few hundreds of eV. Accurate bound and continuum states are obtained by using B-spline basis functions. In agreement with a simple model by Cohen and Fano [6], we observe interference effects that can be interpreted as a result of the coherent emission from both molecular centres. Nevertheless, significant discrepancies are found at low electron energies, in agreement with recent experimental observations. The origin of this 'anomalous' behaviour at low electron energies is discussed in detail.

Using a multi-scale theoretical approach from first principles, we show that the production of HO 2 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 ∼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 HO 2 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 H 3 O + 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 HO 2 results from the O + OH → HO 2 reaction in the liquid phase.