Stopping power beyond the adiabatic approximation: application to polymeric materials


Pere Alemany


Departament de Ciència dels Materials i Química Física and Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona.



Abstract:

Cosmic radiation is a crucial issue for aerospace industry since any spacecraft or satellite beyond low earth orbits will be exposed to an intense flux of highly energetic radiation consisting mainly of charged particles originating from beyond the solar system: 83% protons, 13% alpha particles, and 3% electrons. The effectiveness of a potential shielding material is measured by its ability to attenuate the radiation intensity as it traverses the material. Several studies have shown that lightweight polymeric materials, such as polyetheramide, with a notable radiation attenuation power, may be suited for aerospace applications. However, the effects of radiation on polymeric materials have not been studied thoroughly, and for most of them we even don't know a critical parameter, the electronic stopping power.

Swift ions traveling in solids loose their kinetic energy in a variety of ways, being nuclear and electronic stopping the two channels in which dissipation is usually treated. This separation between electrons and ions relies on the adiabatic approximation in which ions interact via forces derived from the instantaneous electronic ground state. In a more detailed view in which non-adiabatic effects are explicitly taken into account, electronic excitations alter the atomic bonding, which translates into changes in the interatomic forces. In this work, we use time dependent density functional theory
and forces derived from the equations of Ehrenfest dynamics that depend instantaneously on the time-dependent electronic density to calculate the electronic stopping power for H+ projectiles interacting with polymeric targets such as crystalline polyethylene and polyacetylene.