Cross sections for positron and electron collisions with an analog of the purine nucleobases: Indole

Quantitative information about positron and electron collisions with nucleobases is required in charged-particle track simulations to accurately model and assess any radiation damage at the subcellular level in biological systems. However, scattering experiments have so far been restricted to the pyrimidine nucleobases. In this paper we report on total-cross-section measurements for positron impact on indole, a parent molecule of the purine nucleobases, at impact energies between 1 and 25 eV. We also present theoretical cross sections for elastic and total scattering, positronium formation, electronic excitations, and direct ionization between 1 and 500 eV, as calculated with the independent-atom model with the screening-corrected additivity rule. Rotational excitation cross sections are additionally calculated within a Born framework over that same energy range. A significant discrepancy is found between the measured and computed total cross sections, which cannot be entirely accounted for by the lack of forward-angle-scattering discrimination in the experiment. The present results are also compared to the available theoretical cross sections for the purine nucleobases. In addition, total cross sections for electron-indole collisions computed within our theoretical formalism are provided.