Total (elastic plus inelastic) cross sections for positron-methane (helium) collisions at low, intermediate, and high energies

Theoretical calculations on the total (elastic plus inelastic) cross sections σ/sub t/ for positron (e+)-methane collisions are reported at 2--600 eV. We evaluate a complex optical potential (COP) for the e+-CH4 system and treat it exactly in a partial-wave analysis to obtain the S matrix. The real part is composed of an accurate repulsive static potential plus a parameter-free attractive correlation polarization potential. The imaginary part of the COP, V/sub abs/(r), is derived semiempirically in order to reproduce the sharp rise in σ/sub t/ just above the positronium-formation threshold (E/sub Ps/). This form of V/sub abs/ is a function of target charge density, static plus polarization potential, local kinetic energy of the projectile, Fermi momentum, and the mean excitation energy of the system. Our final e+-CH4 σ/sub t/ compare very well with measurements in the E/sub Ps/-600-eV region. Below E/sub Ps/, the present results are in good accord with close-coupling calculations of Jain and Thompson and the measurements of Charlton and co-workers. We also test this absorption potential for the e+-He system and find qualitative agreement with measurements. It is also possible from this model to extract approximate values of the Ps-formation cross sections in the ore gap