Espalhamento de pósitrons por moléculas de metano, fluoretos de metano e tetrafluoreto de carbono

Abstract

Abstract: In this work we present cross sections for low energy positron elastic scattering by molecules methane (CH4), fluoromethane (CH3F), difluoromethane (CH2F2), trifluoromethane (CHF3), and tetrafluoromethane (CF4), for energies up to 10 eV, using two different basis set in the description of the collision process. The calculations were performed using the Schwinger multichannel method to calculate the scattering amplitude and, in cases where it was required, the Born closure procedure was used to including the permanent dipole moment effects. We made a comparison of the results obtained between the different basis set for each molecule in order to determine the best choice between the size of the basis set needed for a good description of the scattering process and the computational cost of its implementation. Initially, we made a comparison between the cross sections of positron scattering by CH4 and CF4. We have seen that, at low energies, the integral cross section for CH4 is much larger than for CF4 due the presence of a virtual state. We also observed that, again for low energies, comparing the other three molecules, which permanent dipole moment, the absolute value of dipole moment is more important that the geometry or the amount of fluor atoms in the molecule, when we analyze the integral cross section.