Swift heavy ions (SHI) (E>1 MeV/amu, and masses more than 20 proton masses) lose the most part of their energy by the excitation of the electronic subsystem of irradiated targets. The energy density deposited during 10-17 s in the vicinity of 1nm from the ion trajectory can achieve extremely high values up to 50-70 keV/nm. Subsequent energy transfer to the lattice stimulates nanometric structure transformations along the trajectory. Elastic recoils produce orders of magnitude lower damage to provide the observed structure modifications.
Having the energy up to 50 keV, δ-electrons generated due to ionization of target atoms by a projectile can produce ionization cascades resulting in non-equilibrium ionization states of deep atomic shells in the nanometric vicinity of the ion trajectory. Peculiarities of the kinetics of spatial expansion and relaxation of the electronic excitations determine the energy transfer to the lattice. We made MC simulations for gold and quartz in order to investigate this kinetics.
We observed ballistic expansion of excited electrons in the perpendicular direction from the projectile trajectory up to 10-14 s from the projectile pass when the front position moves away up to 200nm. Such expansion can not be described in the framework of usually used thermodiffusion model.
It was discovered that more than one third of the deposited energy is converted to long living ionization states which life times are much larger than the cooling down time of the delocalized electrons. The most part of this conserved energy is concentrated in the nearest ten nanometers from the ion trajectory.
It was observed that considerable energy is transferred to the lattice during times much shorter than that of the atomic oscillations. At such times the solid dynamics can not be described by collective atomic oscillations. This results in deviation of the energy relaxation mechanism from that predicted by the electron-phonon coupling.
Our simulation obviously indicated that models based on the conceptions of the local equilibrium, temperature and thermo-diffusion can be hardly applied to the subpicosecodn temporal scale of electronic plasma relaxation in nanometric SHI tracks.
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