Polarization and Forward Scattering Effects in Low Energy Positron Collisions with H2

Positron physical-chemistry has been one important focus of scientific investigation of the last decades, however their low energy scattering by atoms and molecules still offers many questions to be answered, as the low angle scattering effects on the measured cross sections and how the degree of target polarization manifest in the comparison between theoretical and experimental results. In this work, we investigate low energy positron collisions by H2 molecules, with particular attention to the convergence of the polarization contribution on the scattering potential. The interaction between positron and molecule was represented by a model potential conceived from the composition of a free electron gas correlation term with an asymptotic polarization potential, obtained from perturbation theory. In particular, we investigated how polarization effects beyond the second order perturbation affect the scattering observables. Our results show that the model which includes up to the quadrupole polarization contribution presents better agreement to the recent experimental data when corrected for forward scattering effects, since they were measured from a transmission beam technique. The angular distributions were also examined through the comparison between our results to the folded differential cross sections measurements available in the literature. We propose a simple correction scheme to the experimental folded differential cross sections for energies below 1 eV which then, as we argue, favorably compares to the quadrupole polarization model. Finally, the comparison between our phase shifts and scattering lengths with recent full many body ab initio results that explicitly include virtual positronium effects suggests that these are intrisically included in the adopted model correlation potential.