In the first stage of the project, we simulated the process of ice irradiation with protons at space conditions via Ehrenfest MD, describing the electronic structure of the systems at the DFT level. We studied nine values for the kinetic energy of the proton projectile, from 1 keV to 1 MeV, in order to better compare with the experimental values. The resulting estimation for the electronic stopping power of water ice (i.e. the extent to which the proton projectiles are slowed down as a consequence of its interaction with the electrons in the system) was in complete agreement with the experimental results, improving previous theoretical results of other groups. This demonstrates that the methodology is now perfectly able to predict experimental results of radiation damage in water systems, which will be very useful for a wide number of applications where radiation damage of solvated systems is relevant. This is particularly true for biological systems, since biological tissues are mostly composed of water, hence the results are relevant for radiation damage of biological systems, such as astronauts subject to cosmic radiation, and also for cancer research and radiation therapy (in particular, the increasingly popular proton therapy).
In addition, the previous simulations provide information about what is the energetic distribution of the electronic excitations in the system, which informs us about the likelihood of an ionization event occurring, and about which electrons are most likely to be “ejected” of the system, i.e. which water molecules are most likely to get ionized. With this starting information, we have been able to model the dynamics of an ice system where one or two water molecules have been ionized, successfully observing the splitting of a water molecule into an OH· radical and an H+ that migrates through the water network in the first case, and when two neighboring waters are ionized, observing the splitting of both of them and the subsequent reaction of the two OH· radicals to form a molecule of hydrogen peroxide. These results open the door to efficient and accurate simulations of post-ionization processes of chemical systems, which can again be useful for a very wide number of scientific/technological problems, from radiation therapy to materials processing.
Finally, we carried out simulations of the proton irradiation of water ice containing molecules such as CO or NH3. These simulations again provide information about the energy absorbed by the system due to the irradiation event, and about what would be the most probable ionized states that would be obtained after the irradiation.
