First-principles molecular dynamics simulations of the interaction of ionic projectiles with liquid water and ice

We first present results of first-principles molecular dynamics simulations of the passage of Carbon projectiles through water in the liquid state in the adiabatic regime, where the electrons are always in the instantaneous ground state. We study a range of projectile velocities up to the estimated upper limit for the adiabatic approximation and analyze the different types of collision events. We show that for high projectile velocities collisions are mostly binary, but at lower velocities most trajectories exhibit a continuous energy loss to the medium.. which cannot be properly described as a sequence of independent binary collisions. For the slowest projectiles we observe the formation of new chemical species such as hydronium, H5O2+ and hydrogen peroxide. When C-atoms are completely stopped, then we also see the formation of species like formic acid. By analyzing the generation of secondary fragments, we observe that these are mostly hyperthermal and their spatial rate of generation increases with decreasing projectile energy. The two most numerous species are H and OH. In the second part we study., via electronic dynamics with fixed nuclei, the opposite regime of very fast protons producing only electronic excitation in ice, under channeling conditions. We observe the existence of a threshold velocity for electronic excitation of about 0.2 a.u. By monitoring the rate of increase of the total energy, we calculate the electronic stopping power.