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Proton-Irradiated Ice: Dynamics and Chemistry from First Principles

Periodic Reporting for period 1 - PROIRICE (Proton-Irradiated Ice: Dynamics and Chemistry from First Principles)

Reporting period: 2017-10-01 to 2019-09-30

In the PROIRICE project we have employed first-principles simulations in order to study the process of proton irradiation on water ice. Our main motivation is to understand the physics and chemistry occurring when the ice present in different space bodies (such as cosmic dust, comets, asteroids, and icy satellites) is irradiated with the high-energy protons present in solar wind, cosmic rays or strong ionospheres. However, our simulations can be also relevant for a wide number of applications on Earth, such as materials processing and especially cancer research and radiation therapy. All these aspects confer this project a high scientific, technological and societal impact.

The precise objectives of the project are:
- To obtain a theoretically accurate description, at the level of the dynamics of electrons and nuclei, of the process of proton irradiation on pure water ice and its effects on the ice structure, with particular emphasis on the influence of the proton energy and trajectory on the effects of proton irradiation.
- To obtain an equally accurate description of the chemical reactions triggered by proton irradiation of water ice, be it pure or mixed (i.e. containing simple molecules such as CO or NH3)
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.
We have gone beyond the state of the art by considering the actual trajectory change of the projectile, which is usually neglected in most of the simulations of radiation damage. In addition, we have been able to simulate the post-ionization dynamics of the system, observing chemical reactions, which is usually not done in this kind of studies due to the huge computational cost of these simulations. Therefore, we expect a high impact at different levels thanks to the insights obtained from the simulations carried out in addition to the thorough and careful setup of the methods needed to simulate the post-ionization dynamics. These are not only immediately relevant for astrochemistry/physics and radiation therapy, but they will also allow us to perform further radiation damage simulations in the future targeting different systems and problems such as materials processing and cancer treatment.
Water ice sample irradiated with a proton at space conditions.