We present a theoretical study of low-energy positron- ${\mathrm{C}}_2{\mathrm{H}}_4$ scattering using the Schwinger multichannel method. We calculated integral cross sections and the annihilation parameter $Z_{\mathrm{eff}}$ for positron impact energies from 0.001 to 6 eV. We have used up to 23 112 configurations in a fully ab initio calculation. Our results are in excellent agreement with available experimental data. As seen experimentally, at low energies, our calculated $Z_{\mathrm{eff}}$ and cross sections are very large. We attribute this behavior to virtual positronium formation.

• Received 17 January 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.1028