Mean Excitation Energies and Their Directional Characteristics for Energy Deposition by Swift Ions on the DNA and RNA Nucleobases†

As the mean excitation energy of a molecule is the materials parameter that describes the energy transfer from a swift ion to a molecule, knowledge of it and its properties are necessary to an understanding of ion−molecule interactions. A particularly important case is the interaction of fast ions with biological material. In this contribution we calculate the mean excitation energies of the nucleobases using the polarization propagator formalism. The mean excitation energies and their directional components are calculated, as are the influence of hydrogen bonding and nucleoside formation. We find that the mean excitation energies of the five nucleobases are remarkably similar, but sensitive to the orientation of the target base with respect to the ion beam direction. The mean excitation energies are also very stable with respect to hydrogen bonding and nucleoside formation.