Effect of chemical bonding on ionization of nanoscale carbon and DNA upon proton irradiation

Abstract

The separation of positive and negative charges under ion irradiation leads to ionization of the material. This ionization mechanism is closely related to DNA or tissue damage during ion beam therapy, enhancement of plasma oscillations driven by high-energy particles and the ion beam material modification. Here, we explore the effect of chemical bonding on the initial physical stage of ionization of target materials by building material models with different bonding states and using real-time time-depended density functional theory. By building the double layer carbon nanotubes, self-assembly bilayer graphene and DNA double helix, and tracking the evolution of nonadiabatic energy transfer, electronic stopping power, atomic force and charge populations on an attosecond scale, we find that the occupied covalent bond can rapidly conduct the excited electrons and the π-π interaction between molecular layers effectively blocks the backflow of excited electron, resulting in the ionization. In particular, the H-bond transfers a small number of excited electrons and suppresses the charge oscillations during ionization, ultimately leading to the ebb-and-flow ionization mechanism in DNA molecule. These results provide an understanding of the ionization mechanism of materials under ion beam irradiation for ionization control in radiotherapy, ion beam material modification and plasmon-enhanced applications.