Electronic stopping of protons in anisotropic weakly bound materials

Ions shooting through condensed matter dissipate their kinetic energy by transferring it to the target's electrons and nuclei. At high velocities (above 1% of the spped of light) the stopping is mostly electronic, in a highly non-equilibrium, non-adiabatic process. First-principles simulations of such processes have been quite successfully performed in the last decade for varied systems. Here we present results for electronic stopping power for protons in graphite and ideal crystalline polyethylene, anisotropic systems with strong bonding in two dimensions and weakly bound in the other dimension, and vice-versa, respectively. They are based on time-dependent density-functional theory in real time, and using a basis of atomic orbitals (LCAO) within the SIESTA program. Results of the effect of trajectory orientation and impact parameter will be presented, displaying a transition from electronic-structure dependence at low velocity, to a regime at higher velocities in which the particle density along the projectile's path dominates. A reformulation of the TD-LCAO formalism and its implications for numerical simulations will be also presented.