Annihilation dynamics of positrons in molecular environments: Theoretical study of low-energy positron-C2H4 scattering

We present a theoretical study of low-energy positron-C2H4 scattering using the Schwinger multichannel method. We calculated integral cross sections and the annihilation parameter Z(eff) for positron impact energies from 0.001 to 6 eV. We have used up to 23 112 configurations in a fully a initio calculation. Our results are in excellent agreement with available experimental data. As seen experimentally, at low energies, our calculated Z(eff) and cross sections are very large. We attribute this behavior to virtual positronium formation.